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10 Helpful?

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 9, 2011 10:53 AM ET

All roof and little else
by Doug McEvers


I really like your list, I see so many homes with a super complicated roof design and wonder why? Simplify the roof and spend the money saved on insulation and comfort features.

Dec 9, 2011 11:24 AM ET

Ices dams
by Lucas Durand - 7A

I have a through fastener type steel roof on 1x4 strapping, half-lapped 15# felt and 1/2" plywood over an unconditioned attic.

I have been struggling with whether or not to install "snow guards" around the perimeter of the roof.
This seems a bit like installing an "ice dam" - incresing the risk of a leak.

I take your point about locating gutters lower on the facia to avoid having them torn off.

But what about the possibility of injury from avalanche?
I wouldn't want to be standing under the eave if 12 inches of snow suddenly broke loose and came careening down.

Because the steel panels are back-vented I think it helps create avalanche scenarios...
Also because the steel is back-vented, I think the felt has excellent drying potential so maybe snow guards are low-risk...

What do you think?

Dec 9, 2011 11:27 AM ET

Response to Lucas Durand
by Martin Holladay

Snow guards are sometimes the best solution to the issue you raise.

My own preference: if possible, entry doors should be located on the gable side of the house. Let the snow slide when it wants to.

Dec 9, 2011 12:54 PM ET

I wish I knew then...
by Andrew Henry


Good post! I have a regrettably complex roof that I backed myself into because of the desire to add to the size of our house while keeping the very small original structure. It would have been far easier and cheaper to tear down (deconstruct) the original structure and build a house that would be easy to maintain, and also more resilient to weather and future energy constraints.

Looks cool though, if you don't know any better! Unfortunately I do know better, but most people don't. We all want our fairy tale castles.


Dec 9, 2011 1:16 PM ET

Edited Dec 9, 2011 2:01 PM ET.

by Keith Gustafson

I've got one, how about a minimum 4 pitch roof. Mine is 3/12 and 2/12 and leaked like a sieve. I ended up with asphalt and EPDM respectively, all the interesting/green/cool alternatives would not warranty down that low, for good reason. With a lot of neurotic attention to detail, it is watertight, but steeper roofs shed water better. My neighbor has an A frame that is losing quite a few shingles, and does not leak[except for where they made a shallow pitch addition on the back]

RE: glaciers
My first winter with the rubber, giant 4 foot glaciers would slowly slide off the roof and fall with huge impact on the deck. I bonded strips of walking pad to the bottom edge and it works perfectly, no slides. The textured surface allows water to pass easily, it actually melts the dam from the bottom. May not be relevant on a steeper pitch, but it does prevent holes in the roof from mounting snow guards

Dec 9, 2011 1:46 PM ET

Response to Keith
by Martin Holladay

You're right, of course. I've always been in favor of 12/12 roofs -- they look best, and you can use your speed square to cut the rafters angles -- all cuts are 45°. The ridge is a 90° angle, and the rake trim angles are easy too.

Dec 9, 2011 2:13 PM ET

Edited Dec 9, 2011 2:18 PM ET.

12/12 looks best?
by John Brooks

Now that sounds arbitrary to me.....
Is 12/12 really optimum for shedding snow?
I am just asking cause we don't get that much snow in Texas
If 8/12 or 9/12 will do thejob...why waste all that extra material and make the house that much taller.
The speed square can be used for pitches other than 12
does it really save that much time to cut rafters at 45 degrees?

around here there is a markup on roofing when the pitch goes above 8/12
12/12 is not-so-safe for the roofers

Dec 9, 2011 2:22 PM ET

Just an opinion, John
by Martin Holladay

As I stated in my blog, "I have strong opinions about roofs. Without apology, I hereby present my opinions."

In Vermont, 12/12 roofs are traditional. They shed water well, even when using primitive roofing like hand-made wooden shingles. I have seen light through old wooden shingles (standing in the attic) in a house with a 12/12 roof, and the roof didn't leak when it rained.

Steep roofs are forgiving; shallow pitched roofs are not.

Dec 9, 2011 2:28 PM ET

but your not building with handmade shingles
by John Brooks

what is the optimum roof pitch in Vermont for shedding snow with a metal roof?
how about asphalt?
do shingle installers really prefer to work on a 12/12 pitch roof???

Dec 9, 2011 2:34 PM ET

Edited Dec 9, 2011 2:41 PM ET.

Response to John
by Martin Holladay

I'm a roofer. Steep roofs shed water better than shallow-pitched roofs, even when the roofing is wearing out and the flashing has a few minor defects. I like the way steep roofs look, so that's the way I design and build the houses I have designed and built.

If you don't like the looks of a 12/12 roof, and you don't like to work on a steep roof, you are free to build a roof with any pitch you want.

I don't know what you mean by "the optimum roof pitch in Vermont for shedding snow with a metal roof." My roof has a 12/12 pitch and metal roofing, and it holds snow all winter, until sometime in late March. I guess if you wanted the snow to slide off faster than 5 or 6 months, you would have to make it steeper.

Dec 9, 2011 2:51 PM ET

Holding snow on the roof...
by Lucas Durand - 7A

My roof has a 12/12 pitch and metal roofing, and it holds snow all winter, until sometime in late March.

Is there strapping (ventilation) under your metal?
What kind of fastening schedule did you follow?

Dec 9, 2011 2:55 PM ET

Response to John
by Lucas Durand - 7A

I think 6/12 is about maximum before the rules change a bit.
At 6/12 my hammer will just stay put on plywood.

Dec 9, 2011 2:56 PM ET

Response to Lucas
by Martin Holladay

The metal panels are screwed to 1x3s, 24 inches on center, installed parallel to the ridge. So there is air between the roofing and the asphalt felt.

My memory is that each 36-inch wide panel has four screws into each 1x3 -- but I can't verify that because my roof holds snow. I'd have to get up there with a broom, and I don't feel like that right now.

Dec 9, 2011 4:10 PM ET

Reducing Algae & Moss - Not allowed!
by Jim Hassi

Martin -

You wrote:
"Otherwise, install a galvanized steel or copper ridge cap; leachate from the ridge cap will keep your shingles algae-free."

While I live in mossy roof country and disagree with the policy because of the life cycle of the asphalt roof (water quality vs. useful life of the product) I am compelled to point out that the practice of using copper or galvanized strips to reduce moss are not allowed in most of Washington's Green Building programs, including our local HBA's Built Green program. This is due to the metal's leachates entering the storm water/surface water.

Dec 9, 2011 6:44 PM ET

Great list, can't argue with
by Aaron Vander Meulen

Great list, can't argue with any of it. If one were to make a list of rules for building, the KISS principle should be #1. I wish I had a picture of the house I just looked at for a friends daughter. Pitched roof hacked onto a sloped flat roof, OSB soffit (guess how that looks) and the kickers, no drip edge or gable overhang. Rule #11, If you don't know what you're doing, don't!

One question, I understand the pitch change as you described it, feel different about a Gambrel roof? I realize it's a form of cathedral ceiling, but it would be better, right?

Dec 9, 2011 6:51 PM ET

Response to Aaron
by Martin Holladay

I can live with a gambrel, although it's not my favorite. It's certainly better than the type of roof in the photo accompanying Rule #5.

Dec 9, 2011 6:55 PM ET

Response to Jim Hassi
by Martin Holladay

Does this green building program allow builders to install a galvanized steel roof or a copper roof? If so, why not allow the use of a little flashing? in any case, how can they regulate the installation of flashing? It's virtually impossible to build a roof without flashing.

Dec 9, 2011 7:45 PM ET

by Keith Gustafson


the amount of copper or zinc that gets offsite is minuscule. Sounds like a theoretical answer to a theoretical problem. What and there is no problem with aluminum?

perhaps relevant if you were rebuilding a house on the shore of a reservoir, but only perhaps

Dec 9, 2011 8:25 PM ET

Edited Dec 9, 2011 8:26 PM ET.

Built Green - No Moss control allowed
by Jim Hassi

Metal roofs are allowed, but copper flashing is not. We did a project a couple of years ago that was an all copper roof, including copper shingles - but it was not a Built Green certified project.

Our local Built Green checklist line item states: "No zinc galvanized ridge caps, copper flashing, copper wires or copper/zinc impregnated shingles for moss prevention". We have just reviewed the statewide (proposed) consolidated checklist, and a variation is included there as well.

As I said - I don't necessarily agree because it is a miniscule problem and it is not uncommon for the North side of a shaded roof to need replacing after 15 years because of moss damage. One local roofing supplier flat out told me that spending money on a 40 or 50 year shingle would not be worth it because wind or moss would damage the shingle long before the 30 year mark anyway. So I look at the cost both to the homeowner and our resources to do that when zinc strips could prolong the useable life.

Our current project should have a long lasting roof - at least on the South side where it is almost entirely covered with PV panels ;p

Dec 9, 2011 10:54 PM ET

Edited Dec 9, 2011 11:00 PM ET.

snow sliding off metal roof
by Jack Woolfe

My roof has a 12/12 pitch and metal roofing, and it holds snow all winter, until sometime in late March. I guess if you wanted the snow to slide off faster than 5 or 6 months, you would have to make it steeper.

I have a 3:12 through-fastened metal roof. Snow slowly slides off it, especially on the south-facing side. How is it that snow stays on your 12:12 metal roof for months at a time?

Dec 9, 2011 11:07 PM ET

by Lucas Durand - 7A

That is what I was wondering too...
I've heard that sometimes the heads of the screws can help hold snow up.
I thought maybe he had used a LOT of screws...

Dec 10, 2011 5:16 AM ET

Edited Dec 10, 2011 5:18 AM ET.

Response to Jack Wolfe
by Martin Holladay

I'm not sure why my roof stays snow covered. My best guess is that the snow stays on because our weather is so cold. We really don't get any temperatures above freezing during the winter. Occasionally we'll have a thaw for a few hours, but not enough to get the slow to slide.

My solar thermal collectors are covered with snow, too, until late March, and it's too complicated to try to clear them. (There isn't enough sunlight to make it worth the effort, and I get hot water from a coil in my wood stove, so I don't really need the solar-heated water in winter anyway.) I have a system for removing the snow from my PV array -- I do it with a broom.

Dec 10, 2011 5:20 AM ET

Edited Dec 12, 2011 4:24 PM ET.

Another response to Jim Hassi
by Martin Holladay

If a green building program wants to avoid ridicule, it needs a minimum of consistency and logic. "Metal roofs are allowed, but copper flashing and galvanized flashing are not?" Whoever wrote those guidelines wasn't thinking too clearly.

Dec 10, 2011 10:23 AM ET

Question re penetrations
by David McNeely

I am planning >= 3' eaves with the soffit at the plate line:
Is there any reason a vent stack cannot turn 180 deg. to vent down from the soffit?
Same question for bathroom fans?
Same question for range hood vents?
Same question for fireplace (just kidding...).
Seems like avoiding all these penetrations would be a good thing for everyone.

Dec 10, 2011 10:58 AM ET

Response to David
by Martin Holladay

DWV (plumbing) vents have to go through the roof.
Bath fan venting from the soffit is possible, but it's a bad idea -- moisture gets pulled back into the soffit vents, and these vents create icicles, rust stains, etc. Go through the gable or the roof.
Range hood vents - NO! Grease and mess .

Dec 10, 2011 3:10 PM ET

Roof Rules
by Ron Smaron

Waxing to my poetic days in architecture school, I recall a philosophical discussion a group of us had one time on what defines a house. After many brain enhanced opinions, we all boiled it down to the roof as the defining element of the essence of shelter. Nothing else matters if you don't have a roof.
That aside, I have four things to add to what you said:
1. Roof ridge vents (and soffit vents) should be used generously. Especially ridge vents, which eliminate the need for other types of roof venting penetrations. And that includes ridge vents flashed in at vertical walls.
2. Clerestories are a good way to get light into the center of a building while minimizing roof breaks. It also allows for ventilation, and sun control via overhangs. True, it's a dormer, but at least it's a dormer on the ridge, higher up.
3. Another roof type might be the combination of a gable and hip, as demonstrated by what I call the traditional Japanese farm house. It's basically a gable roof that partway down changes to a hip, leaving small triangles of wall at the end of the higher gables. Nice form, minimal vertical wall meeting a roof.
4. Because of global warming issues, roofs should be as light in color as possible. Pure white may not be aesthetic, but we need to bow to a higher directive and start replacing all the lost snow surfaces of our poles with a greater reflectance elsewhere, as in our roads, sidewalks and roofs.
Now I'd like your opinion on the 10 rules for flat roofs, when you absolutely have to have one.
Ron Smaron

Dec 11, 2011 12:16 AM ET

roof rules for snow zones
by kevin o'meara

I live in snow country, my personal record was 8 feet in 3 days! When you live in snow country you absolutely have to consider where all that snow is going to go, especially if you have a metal roof. Several years ago a child was killed when playing about their house and the snow from the metal roof suddenly slide at all once ("roof-alanch") and buried and killed him! Metal roof tend to shed their snow in sudden sloughs. I have seen this take out underlying decks and deck rails. My favorite is when the roof sheds right in front of the garage door and leaves a wall of heavy concrete like snow barricading entombing the cars inside!

Dec 11, 2011 1:36 AM ET

Edited Dec 11, 2011 1:45 AM ET.

New Plumbing Vent Option for Heavy Snow
by Kevin Dickson, MSME


If the snow doesn't slide off your roof until March, your attic is obviously overinsulated :)

But seriously, do you think your ridge vent or vents are really working when they are sealed shut with snow? Or do you have gable vents only?

I couldn't find a link to the new code, but I'm told that DWV stacks are now allowed to go out the gable. This was helpful for a cabin I built in Montezuma, CO at 10,600'. You can also eliminate most of the exterior plumbing vents by using AAVs (Studor vents).

Also just an opinion, but I believe cost effective insulated roofs can be accomplished with 12" SIPs. In fact, if you just add a 3' kneewall to the attic, then SIPs gain you a bonus room that is worth far more than the extra cost of the SIPs. Just watch those air sealing details between panels and at the ridge beam.

Here again, a simple gable is preferred, and ice damns never occur on a 12" SIP. Ever.

Here's a roof problem with no solution that I'm aware of:
Gutters are needed in snow country in the summer. Unfortunately, snow will tear them off if the ice doesn't tear them off first. Heat tape is still the only non-solution that I see in CO.

Another idea tried and failed is to drop a sturdy gutter below the point that sliding snow would touch it. The problem is that the water from heavy summer showers will also overshoot the gutter.

In Europe, their answer is super heavy duty roofs and gutters with massive tile and 3/12 pitch. The idea is to let the entire winter's worth of snow pile up and melt in the spring. I hate that solution because of cost.

Has anyone seen a gutter design that works for heavy snow areas?

Dec 11, 2011 7:52 AM ET

Response to Kevin Dickson
by Martin Holladay

Q. "Do you think your ridge vent or vents are really working when they are sealed shut with snow?"

A. Snow is not an air barrier, so there is some ventilation that occurs through ridge vents during the winter, although not as much as later, when the snow melts. My plumbing vent pipes are high enough to poke through the snow. As Bill Rose as taught us, however, worrying about attic ventilation is misplaced worry. My usual advice is, if you have a ceiling air barrier and install deep insulation, your ventilation details become less important.

Concerning roof-alanches: snow guards can limit the problem, and asphalt shingles are always an option.

Concerning gutters in snow country: It's possible to have a roof without any gutters. That's what I did on my own house.

Dec 11, 2011 11:58 AM ET

No gutters are OK, but they are desirable.
by Kevin Dickson, MSME

Here's one more thing for your "Yes, I know list":

"Yes, I know that the insulating value of the snow on the roof (R-1 per inch) is wasted with a ventilated attic."

It's sort of like having a roof that has free extra insulation during the coldest part of the year.

Dec 11, 2011 12:36 PM ET

Question for Kevin
by Garth Sproule 7B

You say that ice dams never occur on a 12" sip roof...Seems to me that as snow accumulates on a sip roof that the thermal gradient will keep moving outwards, raising the surface temperature, until such a point that the snow will start to melt at the surface of the sip, causing an ice dam at the eaves. How much snow would it take?? Can someone do the math?

Dec 11, 2011 1:00 PM ET

R-value of snow
by Martin Holladay

The R-value of snow is often overestimated. The most reliable sources report an R-value of about R-0.5 per inch for fresh, dry snow. As the snow gradually consolidates and becomes denser, its R-value drops.

Dec 11, 2011 1:06 PM ET

by Jesse Lizer

You mention not using gutters. My question is how are people getting away with not using them. I would love to not use them, however with a basement, you have water issues if you allow the water to run off the roof. I typically do my best in designs to put a gable over the garage and door ways to eliminate the need for them there (and now falling infront of the doors) but gutter the rest of the house typically.

Dec 11, 2011 1:34 PM ET

Response to Jesse
by Martin Holladay

In many locations, sloping the grade away from the foundation is enough to keep your basement dry, especially if the foundation has good footing drains that drain downhill to daylight.

If you need to, you can install "underground gutters" at the dripline of your roofs -- basically trenches lined with EPDM rubber, filled with crushed stone and perforated drainage pipe. Again, slope the pipes downhill to daylight. Cap with landscape fabric and more crushed stone.

Dec 11, 2011 2:15 PM ET

by Doug McEvers

It appears to me some of the "leaf guard" gutters may have some snow and ice shedding ability. They do cover most of the gutter, I will keep an eye out for homes that have them and observe this winter.

Dec 11, 2011 2:21 PM ET

by Keith Gustafson

Large overhangs are good for several things, eliminating gutters is one of them

I end up with 4 feet from drip line to foundation, and I have found evidence of only one water problem[where it had smaller overhang]

Dec 11, 2011 2:34 PM ET

Edited Dec 11, 2011 8:39 PM ET.

Response to Garth
by Kevin Dickson, MSME

Yes, because of the insulation value of snow, it is theoretically possible for ice to occur under the snow on a 12" SIP. But in the real world it's a rare combination of phenomena.

From the BSC website for Aspen, CO:
"Ice dam protection – The combination of adequate insulation just above the exterior wall, and air sealing at the wall-roof assemblies transition are essential to prevent ice dams. But ice dams can occur even in properly detailed roof assemblies from differential solar snow melt. A water protection membrane over the entire roof surface is recommended on all roof assemblies in this climate."

Higher roof slope reduces the thickness of the snow, and the likelihood of buildup
Martin's recommendation of corrugated roofing does solve the problem, since it's self-vented. The external fasteners do hold the snow a bit. Fasteners hold the primary room for improvement for self-vented metal roofing.

Dec 11, 2011 9:03 PM ET

Lot of good sense in list, but...
by James Morgan

roofs that change pitch as in #5 are traditional and ubiquitous down our way. They work just fine. Boy, am I glad I don't live in snow country. What's an ice dam? ;-)

12/12 has a certain appeal although it tends to looks more like a 60° or tighter ridge angle from close up, 10/12 is more likely to look like an actual right angle at the ridge. Regional variations are important: FWIW 10/12 is the most common roof slope for older farmhouses in my area, and we have a lot of quite handsome 1920's/1930's bungalows with 9/12 roofs. My roofer buddy tells me his favorite slope is no less than 7/12 and no steeper than 9/12. Easier on the back than a flatter pitch, and safe to walk.

Dec 11, 2011 9:15 PM ET

Chimneys not required on a new home
by Kevin Dickson, MSME

Talk about your vanity feature - a brick chimney on a new house in 2011? Really?

Dec 12, 2011 5:27 AM ET

Response to Kevin Dickson
by Martin Holladay

For more on brick chimneys, see Farewell to the Chimney?

Dec 12, 2011 6:51 AM ET

step flashing
by Michael Fetterman

The last photo shows a kickout flashing and a step flashing. Your comment is that only one thing is wrong...the flashing should only be nailed through the roof and not to the wall. I was taught and have always thought that the opposite was true. I place one nail high in the corner on the downside of the slope to secure the flashing to the wall. I have always felt it is best to avoid a nail through the roof at that location. i also install a membrane flashing up the wall and on the roof surface, if possible. On a re-roof where the siding is left intact I lay a strip of membrane on the roof along the wall. I try not to use any roof cement on the flashing either. I think this traps more water than it keeps out. I would like to hear your thoughts.

Dec 12, 2011 9:05 AM ET

Response to Michael Fetterman
by Martin Holladay

It's true that you will hear conflicting advice when it comes to the question of nailing step flashing. Some roofers advise only nailing the roof; some advise only nailing to the wall sheathing; and some (these are the ones you should certainly ignore) nail both.

I don't think that nails in the roof are a problem. If the roofing is asphalt shingles, you have plenty of nails in the roof already, and these nail holes don't leak, because the nails are lapped by the next course of shingles. So it's not as if one nail in the step flashing (as long as you nail high enough) is going to cause any problems. If you ever need to readjust or remove that nail, it's a simple matter to lift the nail with a flat bar.

However, if you nail the step flashing to the wall, removing the step flashing in the future is a major operation that usually involves removing the bottom courses of siding. If you are lucky, however, and the step flashing has no nails into the wall, you may be able to remove the step flashing with a pair of pliers. And you may be able to slip in new flashing without removing the siding -- at most, you may have to loosen the siding nails to create a little wiggle room.

Dec 12, 2011 11:09 AM ET

Mansard Roof insulation
by Eric Johnson

I have a mansard roof that needs additional insulation. This design does not have any soffit ventilation. (The almost vertical 2nd story mansard walls have no space at the top to carry air into attic.) If I blow additional insulation into attic, it will touch roof deck near eaves. Is this OK? How to best remedy a bad roof design?

Dec 12, 2011 11:15 AM ET

Response to Eric Johnson
by Martin Holladay

Most codes allow for the construction of unvented insulated roof assemblies, and these can work well. If your insulation is touching your roof sheathing, the insulation can't be air-permeable. You either need to use spray foam insulation against the underside of the roof sheathing, or you need to install rigid foam insulation above your roof sheathing. Either method will keep your sheathing warm enough to prevent problems from condensation or moisture accumulation.

For more information on building unvented insulated roof assemblies, see How to Build an Insulated Cathedral Ceiling. Although the article discusses unvented cathedral ceilings, the same principles apply to other types of unvented roofs.

Dec 12, 2011 12:40 PM ET

roof in Finland
by donald mallow

How are boards run vertically up the roof slope fastened without through nailing...and do those roofs leak with all the through nailing and all the through joints??? Looks nice, but very risky...

Dec 12, 2011 12:50 PM ET

Edited Dec 12, 2011 12:56 PM ET.

Response to Donald Mallow
by Martin Holladay

First of all, you have a good eye. You're right that the roof on the house in Bomba Village, Nurmes, Finland, appears to have board-and-batten roofing, with the boards installed parallel to the rakes.

I'm not familiar with this style of traditional Finnish roofing, but here are some observations:

1. As I said before, the steeper the roof, the more forgiving it is of minor imperfections. Presumably the nail holes don't leak because of the roof's pitch.

2. I assume that the boards and battens are nailed to purlins installed across the rafters.

A close-up photo is attached below.

Roof in Finland.jpg

Dec 12, 2011 3:41 PM ET

Walking pad
by William Stilwell

Re: Post #5

What is a "walking pad"?

Dec 12, 2011 3:45 PM ET

roof in Finland
by donald mallow

Martin... Apparently roofs like that were also built in Russia...Either the occupants accepted leaks or they had a detail to prevent them...(Certainly there was no Ice and Water Shield)... If they have any drainage plane at all ( felt?) nails have punctured it. It is counter to the roofs we build.... How does one find out about it? Nothing on Google....

Dec 12, 2011 3:55 PM ET

Response to William Stilwell
by Martin Holladay

I assume that Keith Gustafson was talking about a type of membrane installed on low-slope roofs to provide a durable walking surface for maintenance personnel. One brand is Roof Trak. The illustration below comes from this document:

Walking pad.jpg

Dec 12, 2011 4:05 PM ET

Edited Dec 12, 2011 4:17 PM ET.

More on Finnish board roofs
by Martin Holladay

I found another reference to Finnish board roofs here:

"The design and construction of the historical building of Pärnu Old Town Basic School started in 1901 and the building was completed in 1902. ... Increase in price of the building didn’t allow building an expensive stone roof, therefore a board roof characteristic to Pärnu was built, which was painted so it would look like a stone roof from a distance. With a correct maintenance the board roof last nearly half a century."

Here's a web page with links to several photos of Finnish buildings with board roofs:

Here's a photo posted on Flickr, showing a board roof in Finland. Notice that the boards and battens on this roof are trough-shaped -- a feature that would certainly help make the roof more watertight:

Board roof - Finland - Mikatus.jpg

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