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Martin’s Ten Rules of Roof Design

For the best performance, build a simple roof shape over a vented unconditioned attic

Posted on Dec 9 2011 by Martin Holladay

Lots of things can go wrong with roofs: bad flashing can cause leaks, a poorly designed valley can turn into a slow-moving glacier, and misplaced gutters can do more harm than good. Experienced roofers see a lot of stupid roofs.

Soon after I dropped out of college in 1974, I got my first construction job. I was hired by Edward J. Thornton Roofing Company in Newtonville, Massachusetts. The company paid me $3.50 an hour. For the next 12 months, I installed countless bundles of asphalt shingles and mopped acres of tar-and-gravel roofs with hot asphalt. Every now and then, I also helped Ed, the company’s sheet-metal worker, to install copper valleys and copper-lined cedar gutters on slate roofs.

Most of the time, I was installing asphalt shingles, back in the days before nail guns and portable compressors. We used heavy wooden extension ladders. My staging consisted of 2x12 planks laid on hardwood shingle brackets; each bracket was secured to the roof with three 16d nails. We never had any fall protection.

My tools were simple: a 16-oz. straight-claw Estwing hammer, a Stanley utility knife, a cat’s paw, a chalkline, a measuring tape, a pair of metal snips, and a cotton nail bag. My tool box was so light that I was able to commute to the roofing shop on my bicycle; I strapped my tool box and my lunch box on the rack over the rear wheel. (Fortunately, my boss delivered the ladders, staging, and shingles to the job site.)

I really enjoyed shingling. I still remember the satisfaction I experienced every time I nailed the last few cap shingles on the end of a ridge — especially when the weather was sunny.

Once a roofer, always a roofer. I still shake my head when I drive by a house and see a classic, obvious mistake, like a brick chimney in the middle of a valley. But my eye also catches errors that others miss, like a shingle roof with a badly woven valley between roofs with different slopes. (When the roofer isn’t paying attention, these woven valleys tend to drift to one side.)

I also hate to see asphalt shingle roofs where the slots don’t line up, or a roof without drip-edge at the rakes, or badly planned shingle courses. (A classic error happens when a ridge isn’t parallel to the eave; an inexperienced roofer is surprised by the discrepancy at the end of the job, and the lack of parallelism shows. An experienced roofer snaps lines to gradually correct the problem over 10 or 12 courses.)

I have strong opinions about roofs. Without apology, I hereby present my opinions.

1. Avoid valleys
If you are designing the roof of a new house, try to design a roof without any valleys. Valleys concentrate water and often clog with ice. It’s far more common to have leaks or ice dam problems near valleys than in the middle of a simple sloped roof.

Many valleys exist because of a designer’s conceit rather than necessity. Often, these valleys trace back to the mistaken belief that a chopped-up, complicated, multi-plane roof looks better than a simple gable. It doesn’t.

2. Just say no to dormers and skylights
There’s no reason for a new house to have a dormer. When I see a dormer, I conclude that the designer or the architect made a mistake. They didn’t include enough interior space, and the homeowner was forced to cut a hole in the roof because the ceiling was too low to stand up.

If you want to build a multi-story house, that’s fine. If you want two floors, build two floors. If you want three floors, build three floors. Then build a roof over the top floor. This roof shouldn’t have any deliberate holes in it. The “no holes” rule covers both dormers and skylights.

3. An unconditioned vented attic is better than an insulated roof
It makes more sense to put insulation on the attic floor than to try to insulate a sloped roof, for several reasons:

  • Rafters usually aren’t deep enough to hold a thick layer of insulation; on the other hand, it’s usually easy to add a deep layer of insulation to the attic floor. Insulating the attic floor is also cheaper.
  • If you leave your rafter bays uninsulated, it will be easier to locate roof leaks.
  • It’s easier to air seal the attic floor than a cathedral ceiling.
  • Damp roof sheathing will dry out quicker if it faces an attic than if it is part of a cathedral ceiling.

4. The best roof shape is a simple gable or hipped roof
In a cold climate, the ideal roof is a simple gable. Since gables don’t have any valleys or hips, they are easy to vent. It’s a straight shot from the soffits to the ridge. That’s good.

Chopped-up roofs with a variety of intersecting planes are hard to frame, hard to keep watertight, and hard to vent. Every nook and cranny creates somewhere for pine needles and ice to accumulate. You don’t want any nooks and crannies on your roof.

In a hot climate, a hipped roof makes more sense than a gable, because a hipped roof makes it easier to provide shade on all four sides of the house. In a hot climate, shade is good. Fortunately, people in hot climates rarely have to worry about ice dams — in Florida, it doesn’t really matter if you choose a roof shape that is hard to vent.

In all climates, make overhangs generous. (Roof overhangs help shade south-facing windows in summer, and help keep siding dry on all orientations. Remember: every exterior door needs to be protected by a roof overhang or its own roof.)

If you're building a gable roof, don't forget the rake overhangs; most rake overhangs are too stingy. If necessary, frame the rake overhang with full-depth ladder-style outriggers.

5. Don’t reduce the slope of your roof halfway between the ridge and the eave
A good roof plane has a consistent slope from the ridge to the eave. A roof that changes slope at midpoint is disturbing. Especially disturbing is a steep roof that suddenly switches to a shallow pitch (for example, when a porch with a shallow-pitched roof is affixed to a house with a steep roof). Such roofs hold snow and are susceptible to leaks.

6. Asphalt felt makes more sense than synthetic roofing underlayment
Unless you plan to leave your roofing underlayment exposed to the weather for several weeks, there’s no reason to buy synthetic roofing underlayment, a product that costs much more than old-fashioned asphalt felt. I like to use #30 felt, which is heavier than #15 felt.

Besides being more expensive than asphalt felt, most brands of synthetic roof underlayment are vapor-impermeable, so they don’t allow the roof sheathing to dry to the exterior. According to the manufacturers of synthetic roofing underlayment, these products should never be used on unvented roof assemblies.

7. Plumbing vent pipes should penetrate the roof near the ridge
Like chimneys, plumbing vents should penetrate a roof near the ridge rather than near the eave, for two reasons:

  • While ridges are dry, eaves are wet. Eaves see much more water over the course of a year than ridges, so any defect near an eave will leak more water than a defect near a ridge.
  • If you live up north, snow and ice can tear your plumbing vent right off your roof, especially if it is located near your eave. It’s much safer higher up the roof.

In a house with a vented unconditioned attic, it’s easy to install a couple of 45° ells in the vent pipe so that the pipe penetrates the roof near the ridge. The same approach is also possible in a house with a cathedral ceiling, although the rafter bay in which the vent pipe is run will not be as well insulated as the other rafter bays.

8. Choose metal roofing or asphalt shingles
I’m just expressing my opinion here. Clay tiles and slate are expensive. Concrete tiles are fragile and tricky to walk on.

Cedar shingles are beautiful, but they are time-consuming to install and (because of their flammability) are illegal in some jurisdictions. Imitation slate and imitation wood shingles look like they belong on a Howard Johnson’s restaurant.

EPDM and roll roofing, if visible, are ugly.

My favorite type of roofing is ordinary through-fastened steel roofing. It’s available in a wide variety of colors and can be ordered cut to any length. It goes on fast, lasts a very long time, and is recyclable. It costs less than standing-seam metal roofing.

My second favorite type of roofing is good old-fashioned asphalt shingles. They have their downsides, of course — they are made from petroleum, are susceptible to algae, and don’t last very long. But they are affordable, easy to install, integrate well with all types of flashing, and adapt easily to new penetrations or changes to the roof. Asphalt shingle roofing is easier to repair than other types of roofing.

In most areas of the country, it makes sense to order algae-resistant shingles. Otherwise, install a galvanized steel or copper ridge cap; leachate from the ridge cap will keep your shingles algae-free.

9. Get flashing details right
Step flashing should be generously sized; the vertical leg should be at least 6 inches high, although 8 inches is better. Remember, you aren’t going to be bringing your siding down to the roof, so at least 3 inches of step flashing will remain visible under your siding. Each piece of flashing should be bent from a piece of sheet metal measuring at least 8 inches by 12 inches; crease the flashing so that it has two 6-inch-wide legs.

Each piece of step flashing only gets one nail into the roof. Never nail step flashing to the wall — that only complicates the job of replacing the step flashing in the future. If your step flashing begins at the eave, don’t forget to install kick-out flashing at the eave.

When I install step flashing on an asphalt shingle roof, I like to install a sideways course of cedar shingles under the step flashing, installed at 90° to the usual shingle orientation, with the butt end of each cedar shingle facing the sidewall and the tapered edge blending into the field of the roof. (The cedar shingles are later hidden by the asphalt shingle roofing.) These imperceptible shims direct water away from the vulnerable sidewall flashing, and lighten the load of water that the kickout flashing has to deal with.

Chimneys always get two types of flashing to allow the roof to settle without breaking the flashing. I was taught to flash chimneys with 16-ounce copper flashing and lead counterflashing. These days, however, many roofers are avoiding lead because of its toxicity; it’s possible to counterflash chimneys with copper instead of lead, but the copper isn’t as flexible.

Unless the chimney bisects a ridge, every chimney needs a cricket. Make the cricket oversized, so that the two cricket valleys terminate away from the chimney.

Installers of steel roofing often do a sloppy job with flashing. When I install steel roofing, I always plan carefully for any roof penetrations like vent pipes, chimneys, or skylights. Ideally, you want to lap the steel panels at the penetration. One sheet of metal roofing runs from the eave to a few inches above the penetration; then the penetration is flashed. Then a second sheet of metal roofing is installed from the ridge down to a few inches below the penetration, so that the steel roofing laps at the penetration.

10. Anticipate ice dams
If you’re building in a climate that gets snowy winters, your roof should include details to minimize the likelihood of ice dams:

  • Frame your roof with raised-heel trusses.
  • Make sure your ceiling is as airtight as possible.
  • Install a very deep layer of insulation on your attic floor. The insulation needs to cover the top plates of the home's exterior walls.
  • Make sure there is adequate blocking between your trusses to keep the insulation from spilling into the soffit and to prevent wind-washing.
  • Install ventilation baffles to maintain a ventilation channel from your soffit to the attic.
  • Install two or more courses of self-adhering rubber roof membrane, so that the membrane extends from the eave to a point at least 3 feet higher than the plane of your exterior wall.
  • If possible, make sure your roof has no valleys.
  • If possible, don’t install gutters; if gutters are necessary, make sure that they are installed below the plane of the roofing so they won't prevent ice from sliding off the roof.

A preemptive comment directed at indignant designers

At this point, many readers are itching to comment on my arbitrary rules. Before firing off an e-mail or posting a comment in all caps, however, you should hear me out.

  • Yes, I know that it isn't that hard to install roofing and flashing details that keep valleys and dormers leak-free.
  • Yes, I know that my worries about ice dams and roof glaciers only apply in certain climates.
  • Yes, I know that skylights can provide welcome daylightingUse of sunlight for daytime lighting needs. Daylighting strategies include solar orientation of windows as well as the use of skylights, clerestory windows, solar tubes, reflective surfaces, and interior glazing to allow light to move through a structure. to dark interior spaces.
  • Yes, I know that many home buyers think that dormers are charming.
  • Yes, I know one reason that designers include dormers is because zoning height restrictions preclude unconditioned attics.
  • Yes, I know that design imperatives sometimes prevent chimneys from penetrating the roof at the ridge.

I have provided design rules from the perspective of a roofer. These rules, of course, are not set in stone, but they are useful principles to keep in mind. Break the rules if you must, but break them consciously, and only for good reasons.

If there is any takeaway to this list of rules, it's this: Designers who gussy up their roofs with flourishes and do-dads are often insecure. Apparently, they think that a few more Christmas ornaments will wow their clients. In contrast, classic Japanese and Shaker designers had the self-confidence and restraint to recognize that there is no shame in choosing simple, elegant shapes. In my opinion, these Zen or Shaker principles should govern roof design.

Last week’s blog: “Books on Insulation and Energy-Efficient Building.”

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Image Credits:

  1. Image #1: Top: Ezioman; bottom: Geoffrey Wheeler
  2. Image #2: DB King
  3. Image #3: Martin Holladay
  4. Image #4: Jon Christman
  5. Image #5: Trulia Inc.
  6. Image #7: Trulia Inc.
  7. Image #8: J.P. Bush Homes
  8. Image #9: Martin Holladay
  9. Image #10: Martin Holladay
  10. Image #11: Allison Bailes
  11. Image #12: Fine Homebuilding

Dec 12, 2011 4:51 PM ET

Roof in Finland
by James Morgan

I think we can assume that traditional roofing techniques in a severe climate such as this will have evolved over time with a combination of materials, roof pitch and assembly details to be both effective and durable in the prevalent conditions of use. Constructional Darwinism - the failures got rooted out generations ago. I've seen similar roofs around here on sheds and outbuildings and I'm sure similar techniques have been used worldwide where there was a good supply of suitable lumber and the climate was not too wet for a wood roof. Couple of thoughts on why this assembly could be expected to work well:

First, you wouldn't need to through-nail in the way you do for metal: the boards would be nailed near their edges into the purlin and the batten nailed to the board to cover the first set of nails. No single nail would go all the way through the roof system from exterior to interior.

Further, the nails would be largely self-sealing to the wood in which they were fastened.

And finally, while I think this assembly would be pretty weathertight as long as it was maintained in good condition, if it began to deteriorate over time I suspect the interior materials of this traditional Finnish home would likely be quite tolerant of a few minor leaks. No sheetrock, no FG batts. Just more boards, and possibly thick lime plaster for a ceiling finish. Old farmhouses hereabouts used through-fastened 5V tin with loose nails and no gaskets for years without serious problems because there was nothing much on the inside to hurt if it got a little damp. Though obviously the format is especially unsuited to the hips, valleys and slope changes that Martin abhors I'd certainly be ready to try this on a simple outbuilding with a non-critical interior. By the way, I'm guessing that the trough-shaped boards in Martin's last post might be a relatively recent adaptation to facilitate a lower slope - this has to be a machine-sawn profile, and perhaps copied from Roman tile?

Dec 12, 2011 4:53 PM ET

Edited Dec 12, 2011 4:53 PM ET.

RE William
by Keith Gustafson

Exactly as Martin stated. The ones at the local roofing supply were 30x30" with the double stick EPDM style tape on the back. I cut them in thirds and bonded them at the edge of the roof. the surface is an array of ~1/2" circles ~1/4" high, allowing water to drain by but providing a pretty good 'key' for the glacier. I also added a few in the open field of the roof. Only ones who see it are the deer and coyotes.

I think it would work on a modest pitch metal roof, and would worst case eventually peel off. I do so hate putting holes in a perfectly good roof...........

Dec 12, 2011 4:58 PM ET

re leaks
by Keith Gustafson

I think our ancestors were a little more tolerant of a bit of 'weeping ' in their roofs. Their progeny no doubt went to work in the British motor trade designing side curtains and canvas roofs......

Dec 12, 2011 10:09 PM ET

Finish/Russian roofs
by donald mallow

Thanks for all your input...It has been a help....
I will keep researching....
I know someone who is contemplating such a roof here, but I have my doubts.

Dec 13, 2011 3:25 AM ET

Reason for the roof framing
by Nicholas Migliaccio IV

I read your article and definitely liked the points of view that you had brought up. I only disagree partly with a statement you had made. The reason a roof is cut up is because the designer didn't stick to a square box (based on their ego Lol!) but I think that is only a small part of the eqaution. The number of valleys and hips (and any other intersections) is directly based on how many corners a house has. A square box has 4 corners, a typical home today depending on room sizes and foot print has many more. I've framed many homes with cut up roofs and look forward to the challenge... I think the days of square houses went away with the frontier days, and if you have a competent roofer (such as yourself) and not a bunch of migrant workers looking to produce as much work in a day as possible (without thinking of the mechanics and logistics of roofing) thats when you have problems. Anything can be water proofed, if it's made by man it can be solved by man

Dec 13, 2011 5:09 AM ET

Metal roofing and SIPs
by Steve P

I've got a cottage in Maine that was built post & beam with SIP walls and roof. The roof is a simple 12/12 with gable ends, but to get the height for stairs and an upstairs bathroom we added two shed dormers that start at the ridge (so no changes in slope). The roof itself is SIP panels with standing seam metal roofing. I prevailed on the plumbers to put the vent stack close to the ridge after a friend had the experience of ice and snow decapitating his PVC vent each winter, and I've had no problems. The cottage has a heat recovery ventilator, so there have never been internal moisture issues. It sits on a slab (on footings pinned to ledge) so runoff has never been a problem, but the amount of snow coming off the roof does build up along the sides. There is nothing on the ground there to damage (entry is on the gable end) but there is so much snow that eventually it builds up to the pint that it bounces off and breaks the vinyl siding!

I've not quite decided what to do about this yet - the holes are small and I have plenty of siding for replacement, but I expect short-term to make up some protective panels to slide in for the winter.

I suppose one solution for those who need gutters is to use plastic gutters and just unclip them in the fall. It's a job, but not that difficult if the heights are reasonable. If the clips are left in place some may be damaged, but they are cheap and easy to replace in the spring.

I'd love to have more light upstairs, but the thought of puncturing the roof for a skylight is off-putting. More likely I will add windows to the gable ends - the beauty of P&B and SIPs is the ease of adding windows and doors.

I once owned a house with a hip roof (Why? Who knows?) where a 6' wide chimney was installed along one wall, resulting in a 6' long ice dam in the winter. It only took me one winter to figure out what a "cricket" was and build and install one. The builder had also neglected a ridge vent. On a hip roof!

Dec 13, 2011 7:02 AM ET

Response to Nicholas Migliaccio IV
by Martin Holladay

You wrote, "The reason a roof is cut up is because the designer didn't stick to a square box ... The number of valleys and hips (and any other intersections) is directly based on how many corners a house has. A square box has 4 corners, a typical home today depending on room sizes and foot print has many more. I've framed many homes with cut up roofs and look forward to the challenge... I think the days of square houses went away with the frontier days."

I also enjoy framing challenges, and you're right -- it can be fun to frame a complicated shape. However, all those bump-outs raise several challenges you didn't mention. In addition to complicating efforts to build a waterproof roof -- the topic of this blog -- bump-outs complicate efforts to build an airtight envelope and drastically lower the home's energy performance. If a designer is trying to meet the Passivhaus standard, these penalties quickly become obvious. Reverting to a rectangle solves many problems at once. If the house is compact, heat loss is significantly lowered, because the area of the envelope is smaller.

In some cases, a small bump-out (for example, a bay window) can be accommodated without introducing valleys and hips. All the designer has to do is include wide roof overhangs -- wide enough to extend beyond the bay window -- and everything can fit under a roof with a simple shape.

You wrote, "Anything can be waterproofed. If it's made by man it can be solved by man." That's true. But some roofs are trouble-free for 35 years, while others suffer ice dams almost every winter. The complicated, chopped-up roofs tend to be the ones with chronic problems.

Dec 13, 2011 7:18 AM ET

Response to Steve P
by Martin Holladay

You wrote, "There is nothing on the ground there to damage (entry is on the gable end) but there is so much snow that eventually it builds up to the point that it bounces off and breaks the vinyl siding!"

It's unclear whether this is a roofing problem or a siding problem. Good luck -- but I can't help thinking that your experience is a good argument for installing fiber-cement siding.

You also wrote, "I suppose one solution for those who need gutters is to use plastic gutters and just unclip them in the fall." Well, that's an interesting idea, but I doubt whether your suggestion will attract many eager homeowners. I can't imagine adding this chore to my list of twice-yearly tasks.

Dec 13, 2011 10:01 AM ET

Re: square boxes
by James Morgan

Ignoring for now the snarky reference to designers' egos, I'll comment that one of the many practical reasons for moving away from the square or rectangular box is related to the increasing size of the average dwelling. Speaking from long experience, it becomes increasingly difficult to design a home with a simple rectangular shape once the footprint becomes much greater than around 1800 s.f. - if, that is, you place any value on good daylighting and efficient space planning. And of course there are many other potential advantages to an L or other more complex footprint such as the generation of positive outdoor space and the opportunity to work sensitively with compact urban lots.

Regarding house size, I would be very pleased if I could make a living designing only smaller houses below that 1800 s.f. threshold but the reality is that in the last ten years or so most of my new-home projects have been 2,500 s.f. or above. That said, I have two homes in construction right now both of which have a footprint below 1,000 s.f and they both have clean trouble-free gable roof forms that would make Martin very happy. Maybe there's some good coming out of this recession after all.

Dec 13, 2011 10:09 AM ET

Response to James Morgan
by Martin Holladay

You're right, of course, that there are many reasons that houses get complex and often have a shape other than a simple rectangle. My own house is now L-shaped, due to an addition, and the roof includes two valleys.

As I wrote in the original blog, "These rules, of course, are not set in stone, but they are useful principles to keep in mind. Break the rules if you must, but break them consciously, and only for good reasons."

Dec 13, 2011 10:36 AM ET

Edited Dec 13, 2011 10:39 AM ET.

Eaves, gutters and snow management
by James Morgan

A while back a Danish client requested a roof edge detail which I have seen referred to as a flared eave, in which a 9/12 or higher roof slope breaks to a lower pitch at the wall line, enabling a wide eave to a steep roof without intruding on the window head. I'd previously used this detail on a couple of projects to allow flexible control of solar overhangs - it can be an elegant look though a little fussy in execution, and of course not the only option for that purpose. Framers may complain about it but I've had positive feedback from roofers that it gives them a more comfortable working platform and staging area at the starting edge of a steep pitch. My client told me it was a traditional detail in Denmark, and I wonder if it has some relevance to the roof-avalanche, gutter and ice dam issues mentioned in various posts above, of which I'm happy to say I have no direct experience.

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Dec 13, 2011 11:37 AM ET

snow staying on steep metal roof
by Robert Swinburne

I have Noticed here in VT ( Halifax at 1700 feet ) that sometimes a snowstorm will start with a little freezing rain which turns the metal roof into sandpaper which in turn causes subsequent snow to stay put

Dec 13, 2011 2:41 PM ET

Edited Dec 13, 2011 3:16 PM ET.

Tile Roof - Our 30+ year experience and druthers
by Jeff Holmes

“Put Your Last Roof on First” That was the roofing material supplier’s slogan. We have lived under a concrete tile roof for over 30 years in western Washington, north of Seattle. There are lots of things I would change about this house (2x4 construction and leaks air like a sieve) but roofing material, that was a good decision. In those days the roofing material of choice was cedar shakes and we have outlasted many of those roofs. The major advantage might be longevity. Very little can damage it. Moss builds up over time but has no effect. Maybe 5 to 10 years, rake it off. The occasional tree limb has given a problem on an out building but easily repaired. Wood cleats and gravity hold the tiles in place so it is just a matter of slipping out the broken tile and replace with the new. We have settled on a 4/12 pitch so easy to access. The building code now might require some nailing which would make repair harder but we weathered 90+ mph winds in 1990 (Artic Express) that toppled 40 or 50 trees getting out our road but moved nary a tile. In fact we used the cedars that fell for board and batt siding for a two-story addition, rough cut with aid of a Volkswagen sawmill and sawyer. The roofing material? Concrete tile.

Nope, not a salesman for roofing tile but would like to see it more mainstream for all its advantages. And maybe Monier will start making the “Classic” again. Who would have thought that would drop that style, Dang It. Easily installed (valley’s are tough, made that mistake only once), fire proof and lasts for at least a long time. Silent in the rain, not the pinging of metal. And has a natural airflow underneath the tile so it would work well with the non-vented roof. It doesn’t build up heat in the summer or cause ice dams in the winter. If fact our roof is the last place that the snow melts.

If I could add one more consideration to your list, it would be to stress the roof to carry maximum snow load and then design the roof to hold the snow. The last place I want the snow is stacked up against the side of the house or in the walkways. A European concept that I first noticed in the alpine villages. Comments too long I know. Martin, certainly enjoy your energy musings and other. If you have an email to share would send a picture.

Dec 15, 2011 12:29 PM ET

by Tony Stephenson

Haven't had the opportunity to read every comment yet, so apologies if this is already covered. I'm based in Texas and while we don't get much snow here, foamed attics have been getting progressively more popular over the last 6 or 7 years. I'm regularly repairing roofs and skylights and installing solar attic fans. I have been sceptical of foamed attics for some time on asphalt shingle roofs. I see too many holes in the shingles from wear, hail, hawks/owls and other birds perching on ridgelines for my liking. However, I am starting to get questions from AC contractors that revolve around, "Why do foamed attics smell moist and mouldy?" I am trying to keep an open mind on foamed attics, but it appears apparent that the author too likes well ventilated attics.

Dec 15, 2011 12:38 PM ET

Response to Tony Stephenson
by Martin Holladay

First of all, if an indoor space "smells moist and moldy," that usually means that the indoor relative humidity is too high. In Texas, the solution to that problem is not to introduce more ventilation. Ventilating a space with hot, humid outdoor air only brings more humidity indoors. It doesn't reduce the humidity level.

If these conditioned attics really do smell moist and moldy, then it's probable that the relative humidity is too high in the whole house. This is common in Houston in well-sealed homes; the main reason is that the cooling load has been reduced to the point that the air conditioner doesn't run for enough hours to adequately dehumidify the house. The best solution to this problem is to install and use a stand-alone dehumidifier.

Finally, it's possible that the smell actually comes from the spray foam -- especially if the smell reminds you of rotten fish. Here's more information on the lingering odor problem: Spray Foam Jobs With Lingering Odor Problems.

Dec 15, 2011 1:03 PM ET

by Tony Stephenson

Appreciate the reply. I'll pass the recommendations along and bookmark the website.

Dec 15, 2011 1:15 PM ET

Florida roof, Mansard design
by Jane Brown

I'm goaling to build a LEED home in northern Florida and am trying to get up to green speed as quickly as possible. Your blogs are very helpful. Curious to know how you'd modify your rules for my climate? And what do you think of Mansard design, dormers and all? Thanks.

Dec 15, 2011 1:22 PM ET

Response to Jane Brown
by Martin Holladay

You probably saw that I advised hot-climate builders to include a hipped roof, because a hipped roof makes it easier to provide shade on all four sides of the house. The inclusion of a porch, especially on the west side of a house, provides further protection from the sun. It's easier for roof overhangs to shade a one-story house than a two-story house, so many hot-climate houses are only one story tall.

I'm not a fan of dormers, because they reduce the thermal performance of the envelope (like any bump-out) and provide more opportunities for air leaks, thermal bridging, and roof leaks.

For more hot-climate design ideas, see my article, Hot-Climate Design.

Dec 16, 2011 1:55 PM ET

Great article
by Gavin Farrell

Enjoyed the read Martin. There definitely is something beautiful about a simple, uninterrupted gable roof.

Dec 18, 2011 9:24 AM ET

Pithy goodness.
by James Morgan

"For the best performance, build a simple roof shape over a vented unconditioned attic."

I've just realized the echo in my head from Martin's subtitle is the memorable line with which Michael Pollan introduces and pithily summarizes his book 'In Defense of Food':

"Eat food. Not too much. Mostly plants." In this nuanced world. sometimes, just sometimes, the best advice comes in small packets.

Dec 18, 2011 2:45 PM ET

venting closed soffitts in a unvented roof assembly
by steve asher

Is it required/recommended to vent an enclosed soffit/eave when you have an unvented roof assembly? The eave line is air sealed with rigid foam and caulk. The overhang is four inches thick by three feet wide covered with stucco over densshield soffit board.

Dec 18, 2011 3:46 PM ET

Response to Steve Asher
by Martin Holladay

If your roof assembly or attic is unvented, there is no reason to include soffit vents.

Dec 26, 2011 5:29 PM ET

Note to Indignant Designers
by Timothy Smith

You wrote: "If there is any takeaway to this list of rules, it's this: Designers who gussy up their roofs with flourishes and do-dads are often insecure. Apparently, they think that a few more Christmas ornaments will wow their clients. In contrast, classic Japanese and Shaker designers had the self-confidence and restraint to recognize that there is no shame in choosing simple, elegant shapes. In my opinion, these Zen or Shaker principles should govern roof design." As a residential designer I couldnt agree more. My problem is clients who INSIST on features such as dormers because it makes the house "cuter" and some realtor has told them it will make the house more "sellable" in the future...ugh! PS: I confess to loving skylights - but only after careful consideration and other alternatives unsuitable...

Dec 27, 2011 8:01 AM ET

Response to Timothy Smith
by Martin Holladay

I agree that it is hard to find a good solution when a client says, "Please build me an ugly house."

Feb 8, 2012 6:35 PM ET

I think #5 is a very attractive roof shape!
by Barbara Tomlinson

I approve of all of these opinions, except one. I changed my roof pitch just like in #5. I had a really good reason though. I was using white industrial sheet metal that was leftover from a bigger job so it was only 8' long. My enclosed space is a 12x12 box with 6' added to the back for a bathroom and 6' on the front to cover the front door. I wanted the middle part to have room for a sleeping loft but with a house this small if I had kept up that slope it would have looked very uninviting, like a spear. I disagree that this profile is unattractive. I like it a lot! Especially in a smaller house.

Of course I don't live where it snows. And I don't know how you make that transition but I did it by turning up the back of the bottom sheet before I put the top sheet on and then I sealed it from below with spray foam. I don't see how water could get in there. I've been through several hurricanes and nothing went wrong. Maybe I have a better technique because I did mine with purlins and not sheathing. If you can't get underneath there I don't know how you'd seal it. I understand in some parts of the world they put plywood under sheet metal but here in Georgia it's acceptable to screw heavy gauge corrugated metal directly to purlins.

One of my requirements was to build my whole house with no plywood because I'm sensitive to the chemicals in it and I can't lift those sheets by myself. My small sheets of metal were fine. Anything bigger would have been a deal breaker.

I'll add something about hip roofs in Florida -- you get a discount on your homeowners insurance with a hip roof. They perform better in hurricanes. I love a hip roof but it's hard for a novice to build one. I also don't like the wasted sheet metal because of the angle cuts.

Sep 4, 2012 1:00 AM ET

getting to R-60 on my roof
by tim desmond

Hi Martin, I love this site and just signed up for a pro-account. I'm about to start building my first house. I would love to hear you recommendations for a the most affordable way to get to R-60 in a new roof (I'm on a very tight budget). My design is a basic 4/12 gable using engineered 2x4 trusses. I'm assuming I should blow in a thick bed of cellulose over the bottom chords and cover it somehow to prevent wind washing and vent above it. Is that right? How high should I raise the raise the heel of the trusses at the bearing ends to get a good R value? What do I use to separate the cellulose from the vented space near the bearing ends (where it would want to come right up to the top chords)? Thanks so much!

Sep 4, 2012 4:49 AM ET

Response to Tim Desmond
by Martin Holladay

Q. "I would love to hear you recommendations for a the most affordable way to get to R-60 in a new roof."

A. Cellulose insulation on the attic floor.

Q. "I'm assuming I should blow in a thick bed of cellulose over the bottom chords and cover it somehow to prevent wind washing and vent above it. Is that right?"

A. Most builders don't cover the top of cellulose insulation on an attic floor. However, you need a dam near the soffit to prevent wind-washing in that location (and to keep the insulation from spilling into the soffit).

Q, "How high should I raise the raise the heel of the trusses at the bearing ends to get a good R-value?"

A. If you are aiming for R-60, you need 16.5 inches of cellulose insulation. If you want a 2-inch deep ventilation channel above the top of the cellulose, that means that you need at least 18.5 or 19 inches of height at the heel of your truss.

Q. What do I use to separate the cellulose from the vented space near the bearing ends (where it would want to come right up to the top chords)?"

A. You can use a commercial product like AccuVent, or you can install rigid foam or plywood between the rafters. If you use rigid form or plywood, you'll need to first install some 1.5 x 1.5 inch sticks to have something to support the rectangles of foam or plywood.

If all this is new to you, Tim, my guess is that you are going to have a lot of questions as you build your house. You may want to hire a builder.

Feb 21, 2013 8:12 PM ET

Recycled rubber shingles
by Horatiu Vornica


What is your opinion about recycled rubber shingles roofing?

Feb 22, 2013 9:34 AM ET

Response to Horatiu Vornica
by Martin Holladay

That type of roofing is rarely installed. I have no experience with it, unfortunately.

Oct 15, 2013 7:07 PM ET

kick out roof flashing
by Rainer Semsch

referring to the last picture in the write-up where you suggest not to nail the flashing to the wall: where do you nail the stucco starter strip?

Oct 15, 2013 7:57 PM ET

Response to Rainer Semsch
by Martin Holladay

In the case of stucco cladding, I assume that the starter-strip nails will need to penetrate the flashing. There are exceptions to every rule.

Mar 6, 2014 11:42 AM ET

One problem I see with an
by Donald F. Mallette

One problem I see with an exposed fastener steel roof that has not been mentioned is expansion of the steel when it gets hot. I'm told that a 30' sheet has the potential to grow an inch in hot weather, in which case the fasteners would be compromised. Once those fasteners pull through the wood nailers they must be replaced with oversized screws. That's alright the first time, but what do you use when the oversized screws become compromised?
Say Martin, how is that coil in your wood stove plumbed into your domestic water system and how do you protect the system from overheating.

Mar 6, 2014 12:45 PM ET

Green roof?
by Kent James

I certainly understand and appreciate your advice for regular roofs, but on a blog with "Green" in the title, I'm curious as to how you feel about "green" roofs (those that are flat and planted). It seems that they makes sense on many levels (reducing storm water surges, allowing flat roofs to diminish unused building space, reduce heat absorption, reduce carbon monoxide, etc.), but they do present the construction/maintenance issues associated with flat roofs. Are they worth considering, or does your omission suggest that they have no role in residential construction (or even any other construction?)?

Mar 6, 2014 5:05 PM ET

Response to Kent James
by Martin Holladay

Vegetated roofs are expensive to install. Soil is a lousy insulator. Vegetated roofs are a maintenance nightmare. What you want to install on your roof is insulation, followed by roofing. In some cases, you may want to install PV modules. Leave the grass where it belongs -- on the ground.

For more information on this issue, I recommend Seeing Red Over Green Roofs by Joe Lstiburek.

Apr 8, 2014 11:48 AM ET

Edited Apr 8, 2014 11:51 AM ET.

Dormers = traditional design
by Eric Whiting

I appreciate your article and agree with your primary stance, but with all things in life, opinions are based on perspective.
Dormers are not mistakes. From a purely energy conscious and ease of construction sense, they certainly are more of a challenge than a straight gable roof. However, the reason architects have included dormers in their home designs for as long as there have been houses are:

1. Dormers take advantage of living space in the roof area of a house. Sure you can get natural light through the gable ends, but even in shorter runs this can create a tunnel feeling, and a not very inviting or comfortable space for many people.

2. Dormers create shadow lines and interest.

3. Dormers lower the scale of a roof to a more human size.

Most folks will never invest more in any one thing more than the dollars they spend on their homes. And a large part of "home" for many people is a comfortable sense of scale, which can be enhanced in many people's views by cozy spaces tucked into roofs. And while there is no question that a straight roof line with no breaks is clearly the energy and cost winner, quality of life issues such as aesthetic interest, natural day lighting and human scaled design will more often trump energy concerns taken from a check-box approach to design.

My opinion is that single-window width dormers are useless because of the reasons you mention, but wider, room-width ones are some of the nicest spaces experiencially, and not a mistake, but an intentional design tool for many reasons that cannot be measured.

Apr 8, 2014 12:00 PM ET

Response to Eric Whiting
by Martin Holladay

I wrote, "I have provided design rules from the perspective of a roofer. These rules, of course, are not set in stone, but they are useful principles to keep in mind. Break the rules if you must, but break them consciously, and only for good reasons."

You are a designer who likes dormers, and you are choosing to break my rules consciously. That's fine; you are the designer.

As in all matters of design, tastes differ. Personally, I am unconvinced that "creating shadow lines and interest" are much of a benefit.

Finally, you point out that "Dormers = traditional design ... Architects have included dormers in their home designs for as long as there have been houses."

It's true that dormers are traditional. But if we look back 200 years, we would have to report that roof leaks are also traditional. Not all traditions are worth preserving.

Apr 9, 2014 4:13 PM ET

Response to Martin
by Eric Whiting

I saw your disclaimer, but felt the need to respond because I believe in a balanced approach, and that all (good) designs succeed from a balance of structure, aesthetic and function. Sometimes the design calls for simplicity, and sometimes complexity. And with every design decision there are trade-offs.

As once eloquently stated by another expert on building envelopes...."There are two kinds of roofs. Ones that leak, and ones that are going to leak.".

Thanks for your input.


Apr 29, 2014 3:28 PM ET

Thanks for nuthin'
by Chris Hayward

Thanks for your entirely sensible, and fairly depressing list of guidelines. I recognized their clear common-sense with the sinking feeling in my gut that I was going to have to revisit the roof design for the small house I'm building for myself in central Ontario. I'm a carpenter; an attraction to complicated roof lines is something like an acquired occupational illness.

Apr 29, 2014 3:43 PM ET

Response to Chris Hayward
by Martin Holladay

You're welcome.

I'm sorry to hear that my "entirely sensible" guidelines, characterized by "clear common-sense," were so depressing. Good luck with your project!

Dec 1, 2014 7:30 PM ET

Cathedralizing the attic makes sense
by François Lévy

I have to take issue with item 3, especially in hot climate zones where HVAC ductwork is frequently located in the attic. It makes much more sense to insulate the envelope and keep R-5 or R-8 ducts within the insulated volume, rather than have them run in a 120° attic. Moreover, it's not too difficult to get adequate insulation in 2x6 rafters with open-cell foam, and 2x8 framing is quite common.

Dec 1, 2014 8:59 PM ET

Response to François Lévy
by Martin Holladay

As I have written many times, installing ducts in a vented unconditioned attic is a major mistake. If you live in a home where this mistake was made, then you have my sympathy.

Homeowners who live in homes where this mistake was made should certainly consider transforming their vented unconditioned attic into an unvented conditioned attic, as you suggest. That's what I advised in this 2010 artilce on the topic: Creating a Conditioned Attic.

Feb 1, 2015 10:42 PM ET

Edited Feb 1, 2015 10:44 PM ET.

Bizarre Japanese design
by Roger Smith

I saw a TV program last night which featured renovations to a Japanese home. When they showed what they did to the roof, I was surprised. Instead of a normal slope, they reversed it with the sides higher than the center.
As it is in snow country they showed a wide drain where the roofs meet covered in electric cables and a drain pipe to presumably take away the meltwater.
That seems to violate just about every one of the tips here for good roof design. The plus-sides are no snow falling off the house to shovel or hit people. I can just imagine ice building up in the gutter, the electric bills, or what happens in a blackout, but apparently it is common.

For pictures of this, search for murakusetsuyane or 無落雪屋根.
I'll see if I can find out if this roof system works in practice.

Feb 2, 2015 6:15 AM ET

Response to Roger Smith
by Martin Holladay

It seems that you are describing a type of low-slope roof, often with a parapet, that depends on a center drain. This Japanese variation is sometimes called a "butterfly" roof -- two low-slope shed roofs that drain toward the center of the house, creating a valley. I agree that this is a stupid design. It's particularly bad in snow country.


Butterfly roof 1.jpg Butterfly roof 3.gif Butterfly roof 2.jpg

Jul 23, 2015 2:47 PM ET

What about solar panels?
by Clay Whitenack


Would your guidelines for no "deliberate" holes in the roof extend to solar panels? For those that have the land, is ground placement by far the best choice?

Jul 23, 2015 3:10 PM ET

Response to Clay Whitenack
by Martin Holladay

Unlike a dormer or a skylight, the racks used to mount PV modules on a roof don't penetrate the thermal envelope. You can have an intact air barrier, insulation layer, and waterproof roofing, and still have solar panels above the roofing. So solar panels don't really create a hole in your roof. (Needless to say, the brackets used to support PV module racks need proper flashing.)

That said, if you have the option of installing a ground-mounted array or a roof-mounted array, a ground-mounted array is always preferable -- as long as the ground-mounted array gets good solar exposure.

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