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Q&A Spotlight

Designing a Low-Slope Roof That Works

Metal roofing or EPDM? Insulation above or below the sheathing? Vented or unvented? An owner weighs all the options.

Image 1 of 3
Starting point for a low-slope roof: Kevin Hoene's original plan for the low-slope roof on his new home in Illinois. He asks for guidance as he wrestles with details for insulation and roofing materials.
Starting point for a low-slope roof: Kevin Hoene's original plan for the low-slope roof on his new home in Illinois. He asks for guidance as he wrestles with details for insulation and roofing materials. The author wants an unvented roof assembly but his builder and roofer both like the idea of a vented roof better. This RoofsPlus detail represents a more robust fascia detail for an EPDM membrane roof cladding. The 1/2-inch polyiso rigid insulation is the preferred underlayment for the EPDM membrane; the red “wrap” on the inside 1x3 trim board is the copper screening.
Image Credit: RoofsPlus

If only Kevin Hoene’s choices for a new roof boiled down to a choice between an EPDM membrane and metal, his life would probably seem a whole lot simpler.

But Hoene, building a new home in Illinois and on the boundary between Climate Zones 4 and 5, will soon be weighing the pros and cons not only of different roof coverings, but also of what type of insulation to use, whether it should go above or below the roof sheathing, and whether the roof should be vented or unvented. In other words, nothing seems off the table.

“Our house is being built near the boundary of Zone 4 and 5 in Illinois with a 1:12 pitch metal roof,” Hoene writes in a Q&A post at GreenBuildingAdvisor. “I’ve done a lot of research on low-slope roofs because our builder does not have a lot of experience with flat roofs.”

After reading a couple of articles on the topic by GBA Senior Editor Martin Holladay, Hoene is leaning toward an unvented roof assembly (see the sketch above), which would include cellulose insulation in the rafter bays and 4 inches of extruded polystyrene (XPS) rigid insulation above a layer of airtight sheathing. That would be followed by a second layer of OSB or plywood sheathing and, finally, the metal roofing.

“Does the diagram look like a good approach?” he asks. “Any details that I should pass along to our builder about the roof trusses?”

Those questions are the start of this Q&A Spotlight.

Choosing the type of roofing

A first step, Holladay suggests, would be finding out whether the type of metal roofing Hoene is considering can be installed on the roof he’s planning. The pitch is very low — rising only 1 inch for every 12 inches of run — and some types of metal roofing need more than that.

Hoene has discussed the possibility of using 60-mil EPDM instead, which apparently won’t present any aesthetic issues, but he wonders whether the membrane will perform as well as a standing-seam metal roof.

“On low-slope roofs in the snow zone, EPDM will work better than standing-seam metal, since it can’t leak even under high winds, whereas metal roofs will,” writes Dana Dorsett. “With limited slope to drain well, it can take forever to be rid of any leakage moisture. EPDM won’t last as long as metal, but there’s a reason it’s the most common roofing for very low slope roofs.”

Kevin Dickson also likes EPDM. It’s “great,” he says, and it’s available in white, which would reduce cooling loads in the house.

An alternative, Dickson adds, is spray polyurethane, which will last “indefinitely” if it’s well maintained, meaning that it should be recoated every 10 years or so, and inspected carefully every year.

In further conversations with a roofer familiar with low-slope assemblies, Hoene is told that a fully adhered 90-mil EPDM roof applied over 1/2-inch DensDeck Roof Boards is the best option.

“Is the 90-mil thickness overkill?” he asks. “Most of the applications I have read about use 60-mil or 75-mil, but we do want a roof built to last.”

What kind of insulation will work best?

Hoene’s original plan was to use 4 inches of XPS, which has a nominal R-rating of 5 per inch. Better take that with a grain of salt, Dorsett says.

“Four inches of XPS won’t perform at R-20 for the full 50+ year lifecycle of most metal roofing,” he writes. “As it loses its blowing agent over a handful of decades its performance starts out higher than R-20, but eventually drops to about R-17. The R-5/inch labeling is based on something like a 20-year average performance.

“R-17 might be sufficient for dew-point control, but it might not be,” he continues. “If instead you went with 5 inches of EPS [expanded polystyrene], its R-value will be pretty much the same on day 25,000 as on day 1, since its much lighter blowing agent dissipates quickly, and its labeled-R is its fully depleted R.”

There are two other choices — polyisocyanurate and spray polyurethane foam.

“Another roofer has proposed using 4.4 inches of polyiso insulation above the sheathing for an unvented assembly,” Hoene writes. “This builder said that XPS and EPS aren’t compatible with the EPDM adhesive. Quote from builder: ‘Like pouring gas in a styrofoam cup, it melts it away. They are typically used in ballasted or mechanically attached systems.’

“The issue I have with polyiso is that I have read that the effective R-value in cold weather is lower, and polyiso should only be used in warmer climates,” he adds.

One solution to polyiso’s less-than-optimum performance in cold weather, Holladay says, comes from building scientist John Straube, who once told him, “One option is to stick with polyiso and just make it thicker. If we do that, let’s call polyiso R-5 per inch.”

Either open- or closed-cell polyurethane foam could be sprayed on the underside of the roof sheathing, another possible route to take.

Where should the insulation go?

Hoene has sketched an unvented roof assembly, but his builder and roofer both suggest he consider a vented roof assembly (see Image #2 at the bottom of the page).

“They have not done an unvented roof with the layered foam above the sheathing and seem more comfortable doing the following approach and say it would save a lot of money in time and materials,” Hoene says.

The builder suggests blown-in cellulose 16 inches deep, leaving the area above the insulation open for ventilation. Hoene adds: “He recommended no venting in the center of the roof due to more chances for leaks.”

“It might be time to find a different roofer,” Holladay replies. “If you can locate a roofer familiar with commercial construction, you’ll be in better hands — because installing rigid foam above the roof sheathing is a standard method of insulating low-slope commercial roofs.

“The approach shown in your sketch is associated with failures, because there isn’t enough of a difference in elevation between the ‘inlet’ vents and the ‘outlet’ vents. I call this type of ventilation ‘faith-based venting.’ Air rarely follows the ‘smart arrows’ that some designers draw to indicate where the air is supposed to enter, and where it is supposed to leave.”

The same roofer who has suggested 90-mil EPDM also would prefer keeping all of the insulation below the sheathing. Hoene says he would insulate from below using blown-fiberglass with some sort of adhesive that allows it to bond with the decking, or to use spray foam.

Holladay is unconvinced. “I stand by my advice,” he says. “For an unvented flat roof, you want at least some — or ideally, all — of the insulation to be above the roof sheathing. That way, the roof sheathing stays warm and dry.”

The recessed can light conundrum

There is yet another wrinkle to this roofing saga: the use of recessed light fixtures in the ceiling. They would be installed in the drywall that serves as the air barrier. Hoene wonders if that’s going to be a problem.

Holladay says, “Absolutely, that’s a problem.”

He continues, “You want to minimize all penetrations and electrical boxes in an airtight ceiling, and all penetrations need to be very carefully air sealed. If you care about energy performance, you won’t have any recessed can lights in your ceiling.”

Dorsett adds that certain types of recessed lights come with gaskets to make them airtight, and they are rated for contact with insulation. But, he says, they should be inspected and installed carefully.

“There are also surface-mount LED fixtures that can be mounted on standard electric boxes that present a far smaller and far shallower penetration into the insulation layer, which may be a better alternative,” Dorsett says. “Any penetration of the ceiling gypsum needs to be detailed for air tightness.”

Our expert’s opinion

Here’s how GBA Technical Director Peter Yost sees Hoene’s situation:

Per usual on roofing and attic questions, I like to check in with Brian Knowles at RoofsPlus, a local high-performance roofing company. Here is a summary of my discussion with Brian:

Is a 1:12 standing seam roof OK? Yes, but use a full-coverage membrane such as Ice and Water Shield as the roofing underlayment. Also, use a hidden-clip system rather than exposed fasteners, and back up the standing seam lock seam with a high-performance sealant. Brian’s company uses Geocel Tripolymer.

Using an EPDM membrane would be OK as well. But, exposed roofing membranes typically don’t fail in the field of the installation, but at joints and laps. These need to be inspected annually. Brian’s company never uses 45-mil membranes but finds the 60-mil performs well. They have never used 90-mil EPDM.

For both types of cladding, the devil is in the details at the eaves: With a slope this low, making a watertight seal between either the membrane or the standing seam and the fascia is tricky, particularly if the site has wind exposure. Brian has found that even manufacturers’ recommended eave details are not robust and his company has developed some of their own details.

Here is Brian’s cut: “Fully adhered EPDM systems (where there is no curb or parapet termination) need to be stripped in to metal drip edge with cover tape. The published details will show the EPDM fastened under the drip edge metal. They show cover tape over the drip edge and onto the EPDM. This detail leaves the strip of cover tape vulnerable to damage from sliding ice and snow. Our manufacturer trainers have approved a slightly different detail that has a more successful field application in areas where ice and snow are a factor (see Image #3, below). Bear in mind that these edge details will require regular inspection over the years to maintain the bond at the edge. A parapet or curb detail is preferred when using EPDM.”

Color matters. In cold climates, white membranes and white metal roofs can be associated with significant night-sky radiation and much cooler attic spaces.

On the question of whether to vent the roof assembly: A dedicated space of 1 to 2 inches just underneath or above the structural roof sheathing will vent the assembly. The space created between the structural roof deck and the finished ceiling is an attic space. This space can be vented along any of the margins of the attic but typically with vents at the eaves or sometimes rooftop turbine or “whirlybird” vents. Brian and I agree that the attic (or in this case almost a plenum) space needs to be either completely inside or outside the conditioned space and it needs air flow in either case. But note: If the attic is outside, then it means venting it with outside air; if the attic is inside, then it means introducing inside air flow. (Read on for more details…)

In Brian’s experience, even though a 1:12 pitch gives you negligible stack effect air movement or venting, when the roof cladding — particularly a darker one — is exposed to solar gain, that attic space heats up quite a bit and air flow develops even without stack effect from eave to eave or eave to ridge. So, the “high-low” eave vents in the second hand-drawn image will work fine for venting this “outside” attic space. And of course in this case, the finished ceiling plane is where the continuous air and thermal boundaries will be.

On the other hand, you could insulate and air seal this roof at the roof line, in which case the attic space created between the roof and the ceiling plane becomes an inside space. It’s important to create air flow for this space, even if it is just passive vents that relieve this space.

But note, if you air seal and insulate the finished ceiling plane, then the roof assembly above can dry to the interior. On the other hand, if you insulate and air seal the roof line, and you don’t vent the underside or topside of the structural roof deck, then you’ll have little to no drying potential in either direction.

15 Comments

  1. Kevin Hoene | | #1

    Follow up
    As the homeowner of the project in question, I'd like to follow up about how we ultimately proceeded. After much discussion, we chose to do an unvented assembly. Our roofer used 90 mil EPDM on top of 1/2" densdeck on top of 2" polyiso on top of 7.5" EPS on top of asphalt felt.

    With the insulation at the roofline (and the vertical walls insulated all the way to the roofline), the attic is completely inside the conditioned space.Your article cites the need for venting with inside air. This takes me back to the use of recessed can lights. Since the barrier is now at the roofline and not at the ceiling, is there no longer a need for an airtight ceiling? is it acceptable to use can lights now? Will can lights actually be beneficial because they will help ventilate the attic space with inside air?

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Kevin Hoene
    Kevin,
    Q. "Your article cites the need for venting with inside air. This takes me back to the use of recessed can lights. Since the barrier is now at the roofline and not at the ceiling, is there no longer a need for an airtight ceiling?"

    A. First of all, I am less convinced that Peter Yost or Brian Knowles that the type of attic you are describing -- an unvented conditioned attic that is entirely within the home's thermal envelope -- needs any kind of venting, especially when all of the insulation is on the exterior side of the roof sheathing. I'm not sure what purpose this venting with interior air is intended to serve. While it's true that some attics insulated with open-cell spray foam on the interior need to be deliberately conditioned with a home's HVAC system to lower the chance of moisture accumulation in the sheathing, your roof assembly has no open-cell spray foam.

    So, here's my advice: don't worry about "venting" this unvented conditioned attic.

    You can install all the can lights you want in your ceiling. Both sides of the ceiling are indoors, so you aren't penetrating an air barrier.

  3. Charlie Sullivan | | #3

    Night-sky radiation
    This comment could be misleading:

    "In cold climates, white membranes and white metal roofs can be associated with significant night-sky radiation and much cooler attic spaces."

    Almost any roofing material except shiny metal will have high thermal emissivity. For example, http://coolroofs.org/documents/ICCtoCRRC-HeatWavePowerPoint_3of3_.pdf lists black, gray, and white EPDM as having 0.86 or 0.87 emissivity. Absent some special treatment, any shingles or asphalt coating or EPDM will experience about equal night-sky radiation and cooling as a result.

    The difference is just how much solar heating it gets during the day. If a roof starts out very hot from the solar gain, it won't end up as cold at the end of the night. And the black roof certainly absorbs more solar radiation: 94% vs. 31% for the white EPDM.

  4. User avater
    Dana Dorsett | | #4

    That's right- it's really DAY-sky reflectivity & radiation
    The more reflective a high emmissivity roof covering is to the solar spectrum, the lower the average roof temp, resulting in a colder average temp at the roof deck and in the attic below.

    But the nightly low temp is barely affected by the daily peak temp, usually limited from going colder than the dew point of the proximate air. A black roof get just as cold as white roof on a clear cold night, but the black roof gets much warmer during the day.

  5. Antonio Oliver | | #5

    similar question with more slope
    Martin,

    With regard to venting and insulating an unvented roof, how would your recommendations change for a roof with a 4:12 pitch in a similar climate zone? Assume standing seam metal roofing.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Antonio Oliver
    Antonio,
    Here is a link to an article that explains everything you need to know: How to Build an Insulated Cathedral Ceiling.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Charlie Sullivan and Dana Dorsett
    When I interviewed Bill Rose in 2006 about the phenomenon of sky radiation during daytime hours -- a problem in Arizona with fiberglass-insulated low slope roofs and white roofing -- this is what he told me: "There is nothing standing in the way of the roof radiating out to space. You have a whole lot of heat loss from the roof surface, day and night. With this white roofing, 80 percent of the heat that hits the roof is reflected. The sun can’t keep up with the heat losses to the sky. What you’ve created is a sky-powered cooling coil, and the fiberglass insulation is like a dirty condensate pan."

  8. Keith Gustafson | | #8

    90 mil EPDM, wow that is a
    90 mil EPDM, wow that is a nice roof, I thought I upgraded at 60. The roof field will outlast the tape and flashing, so when you are old and gray and get a leak, make sure they don't sell you a new roof as I am willing to bet that material will outlive you.

    Can lights: I am willing to bet even with R50 above, the area between the sheet rock and the roof will get 'cold' and thus you will get 'drafts' . I would gasket them like they are going in a real attic and not worry about it.

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Alan Benoit
    Alan,
    If the upper edge of a low-slope shed roof intersects a vertical wall, the most logical approach (in my opinion) is to detail the roof as an unvented roof.

    Another approach is to build a cupola or ventilated doghouse in the middle of the low-slope roof -- an approach championed by Joe Lstiburek.

    For more information on either of these approaches, see Insulating Low-Slope Residential Roofs.

  10. Alan Benoit | | #10

    What if it is a lean-to roof and runs into a vertical wall?
    I am working on 3 projects currently where we have a low sloping roof like the one discussed above that terminates against the gable end of the house; think typical Vermont vernacular. We are creating a well ventilated space above the well-insulated, low-slope roof and have vertical 3/4 strapping creating a drainage plane for the gable end wall. Keeping water, rain and snow out of the vent space prevents us from connecting the two air spaces and thus the low-slope roof needs its own vent at its higher termination against the wall. Are there systems/details/techniques others have used that work for this application? I've seen some ridge-vent like systems available, but wonder how they work under wet snow, etc. Here's a preliminary section detail...

  11. Alan Benoit | | #11

    Follow up to Martin Holladay
    Thanks Martin,
    Actually it was my attendance at Joe Lstiburek's building science seminars this year that have driven me to VENT these roofs, when I would have normally designed them as warm roofs. His guidelines for roofs in areas of more than (I think) 50 inches of average annual snowfall, he suggest venting the roof to avoid ice dams. He has been retrofitting many roofs in ski resorts out west that are dealing with huge ice dams. This happens when the insulation of the snow on the roof is so thick that is traps the heat and overpowers the roof's insulation values causing melt at the roof surface; and so on. Maybe what I'm referring to isn't truly a low-slope roof; as they are all 3/12 or 4/12 pitch, but the challenge to venting them still remains...
    Thank you again,
    Alan

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Alan Benoit
    Alan,
    A standing-seam metal roof is at a very low risk for ice dams -- especially if the roof has no valleys.

  13. Derek Roff | | #13

    Day/night clear sky radiation
    I'm not measuring roof temperatures that are in line with Bill Rose's comment, quoted by Martin, on white roofs and sky radiation. Bill says that the sun can't keep up with the radiant heat loss from a white roof in the desert. After reading that comment, I went out to check my roof. The air temperature is 42 degrees outside, right now, at my desert New Mexico home, at 6,300 ft elevation, and the sky is a cloudless, clear blue. The wind is blowing 16 mph, the humidity is 35%, and the dew point is 20 degrees, according to the online weather station nearest me. Sunset is 45 minutes away, so the sun angle is low.

    I just measured the temperature of a section of white metal roof, and got a reading of 73 degrees. I'm not sure I can trust the reading, but the metal feels warm to the touch. On the shaded side of the roof, the metal feels cold. It seems to me that the sun is keeping up pretty well with sky radiation and conductive heat loss. I don't think I will be getting any condensation under this roof, while the sun is out, today. In full summer sun, this white roof is uncomfortably hot to touch, although I don't have a numerical measurement. Bill's comment doesn't match my experience. While I don't get up on my roof that often, I've touched the hood, door, and roof of my white cars hundreds of times in all seasons. They always feel warmer than the air temperature, if the sun is shining.

  14. Malcolm Taylor | | #14

    Alan
    There are number of ways to vent a lean-to roof - some more practical than others. I'd suggest one of these two for venting at the end wall flashing:
    1.
    - Fur out the end wall flashing that covers the top of your roofing panels with strips of 1x3" material so that there is a cavity created behind the vertical portion which is open to the roof air space below.
    - Fasten a Cor-A-Vent strip to the outside of the end wall flashing near the top.
    - Counter-flash above this with a Z flashing bent to cover the Cor-A-Vent.

    2.
    - Treat the end wall flashing as though it were one half of a metal ridge cap.
    - Leave a gap at the top of your roofing panels open to the airspace below.
    - Apply vented closure strips to the top of the roof panels, and cover with the end wall flashing.

  15. Charlie Sullivan | | #15

    Day/night clear sky radiation
    Martin,

    What you quote Bill saying about sky radiation makes more sense than the quote in the article... Perhaps "night" was a mistake in the article. The visible color only matters during the day.

    Derek, some whites reflect near IR in solar radiation, but some absorb it. It sounds like yours absorbs it whereas the ones Bill refers to are probably the "cool roof" materials that reflect the full solar spectrum.

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