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Q&A Spotlight

Solving a Roof Dilemma

A mix of pitched and flat roofs creates a risk for leaks

Image 1 of 2
This Passivhaus-in-the-making has several potential problems with the roof design, including the inability of the sheathing to dry should it become wet, the proposed use of slate roofing on a relatively low slope, and possible ponding of water on a flat roof section next to a chimney.
Image Credit: Illustrations courtesy of Joe B

Joe B is building what he hopes will be a Passivhaus-certified home in Port Washington, New York, a town on the north shore of Long Island in Climate Zone 4. The house is well underway, but Joe worries about the potential for trouble in a very complex roof design.

In a post at GBA’s Q&A forum, Joe runs down the basics of his architect’s plans. Roughly 70% of the roof is pitched at 5-in-12, with the remaining roof area all but flat — with just enough of a slope to drain water. Three pitched sections form a U-shaped perimeter to the low-slope section, with with water draining toward the eave where it will run past a chimney (see Image #2, below).

Pitched roof sections are being framed with 11 7/8-inch laminated veneer lumber (LVL) rafters and covered with Zip sheathing.

“We are planning to spray 3 inches of closed-cell foam between the rafters and under the Zip sheathing and the balance (about 9 inches) of the rafter cavity will be filled with open-cell foam (to save on cost),” Joe writes. As required by the manufacturer, he adds, the sheathing will get a band of peel-and-stick waterproofing membrane at the perimeter of the roof, in the valleys, and at the overhangs. That will be followed with a vapor-permeable roof underlayment called , and, finally, Vermont slate shingles.

The 1,000-square-foot low-slope area consists of 3/4-inch AdvanTech sheathing over 9 1/2-inch LVLs. Beneath the roof deck will be 3 inches of closed-cell spray foam and another 6 inches of open-cell foam. The entire surface will be covered with Grace Ice & Water Shield. That will be followed by a 4-inch layer of rigid foam insulation, and another layer of tapered foam insulation that will maintain the roof pitch at 1/4 inch per foot for drainage.

The low-slope roof will be treated with made by Kemper.

Joe’s greatest worry? A risk that condensation will lead to sheathing rot in this unvented roof assembly. That’s the topic for this Q&A Spotlight.

There’s a better way

Rather than sandwiching the roof sheathing between an impermeable barrier above and below, GBA senior editor Martin Holladay would design the roof so it could dry in at least one direction.

“A better approach to this low-slope roof (in my opinion) would be rigid insulation above the roof sheathing, combined with fluffy insulation (cellulose, mineral wool, or fiberglass) below the roof sheathing,” Holladay writes. “In your climate zone (Zone 4), you would need a minimum of R-15 insulation above the roof sheathing to make this approach work.”

With vapor-permeable insulation instead of spray foam below the sheathing, the roof could dry to the inside. (For more information on this approach, see the first two entries in the “Related Articles” sidebar, above left).

When it comes to the pitched roof sections and Joe’s choice of slate, Holladay (who describes his approach to slate roofing specs as “old school”) says that the pitch is probably too shallow, but in any event the slate should be applied over skip sheathing, not a solid layer of Zip sheathing.

“Slate is expensive roofing, so why cut costs where it counts?” Holladay says. “Install skip sheathing (1x4s or 2x4s, spaced according to the courses of slate) above the Zip — you won’t regret it. (The skip sheathing allows the slate to dry quickly, which will extend the lifespan of the slate.)”

Closed-cell foam isn’t necessary

Joe plans a layer of closed-cell foam throughout the house to ensure airtightness, but Dana Dorsett says this won’t be necessary. Open-cell foam will do as good a job, he says, at a much lower environmental cost.

“Open-cell foam at 3 inches or more on plywood or OSB sheathing is every bit as airtight as closed-cell foam,” Dorsett says. (For more information on this topic, see Air Leakage Through Spray Polyurethane Foam.)

“When the thermal bridging of the framing is factored in, the performance gain of the higher R per inch is almost vanishingly small,” Dorsett continues. “That 3 inches of closed-cell foam buys you at best another R-1.5 of ‘whole-wall R-value’ in an advanced-framing/low-thermal-bridging wall. You could get the same thermal benefit by adding 1/2 inch of continuous EPS over the sheathing. Closed-cell foam uses four times the polymer of half-pound open-cell foam, and the global warming footprint of the HFC245fa blowing agent is about 1,000 times that of CO2, compared to the effectively zero impact of the water used to blow half-pound foam.”

Three inches of polyiso insulation, or 4 inches of expanded polystyrene (EPS), over the roof deck and 10 inches of open-cell spray foam between the rafters would get Joe to code minimums for his climate zone. That 10-inch layer of open-cell foam would be a Class III vapor retarder, he says, and no additional treatment would be needed on the interior to lower the vapor permeance and protect the OSB sheathing.

Potential leaks are a big concern

Long Island will get plenty of wet and snowy weather, Dorsett points out, and the roof design lends itself to problems. “The sloped roof pitches and valleys dumping onto a flat roof and blocked on the drain end by an exterior chimney are mistakes,” Dorsett says.

There have been six snowstorms in the last 20 years that dumped 18 inches of snow in New York City, he says, adding, “As configured, that flat roof section is guaranteed to collect deep snow drifts no matter which direction the wind blows during those storms.”

His concerns are echoed by Kevin Zorski. “I know that you are in the middle of this project and basic things cannot be changed,” Zorski says, “but the choice of this architect was not the best, I’m afraid. The first thing I saw were the things Dana pointed out: three roofs draining into a wide, flat roof that slopes toward a chimney. I hope this architect knows more about energy efficiency than water management.”

If there is any way to increase the pitch of the flat roof section, Joe should take advantage of it, says Zorski. As it is, the risk of leaks is a much bigger threat than the condensation that Joe originally asked about.

“You have a three-sided swimming pool that is slowly draining water,” he says. “You may want to run the design by an experienced architect or engineer as a precaution before going too much further ahead. I apologize for my negativity, but this design looks to me to be a problem waiting to happen.”

Make a waterproof pan

Holladay, a one-time roofer, suggests Joe could solve the potential leak problem on the low-slope portion of the roof with a soldered copper roof.

“When I worked as a roofer, we were occasionally called in to solve problems like this — steep roofs that dumped snow, water, and ice onto a low-slope roof,” he says. “The only dependable long-term solution is to install a soldered copper roof. The soldered copper roof should extend up the slopes of all the adjacent slate roofs, creating a 4-foot-wide band of copper at the eaves of the sloped roofs that dump onto the flat roof, and every single seam should be soldered.”

In addition, flashing for the chimney should be soldered copper, ideally installed as “through-chimney” flashing, a more reliable solution than inserting flashing into kerfs in the mortar.

A soldered copper roof won’t leak and is tailor made for a situation such as that faced by Joe B. [Photo credit: Dennis Crookshanks,]

Joe notes that the color of copper isn’t really what he had in mind for the roof, but Holladay tells him this will be a fleeting problem. The metal will quickly turn a dull brown in color. And even if it didn’t, that shouldn’t be a cause for worry. “The only people who will ever see this low-slope roof are people in helicopters and roofers trying to address the leaks,” he says, “so you shouldn’t worry about the color.”

And as to Dorsett’s suggestion for adding insulation on top of the roof deck, that’s not going to happen.

“Due to design of the house, we want to keep size of fascia about +/- 10 inches total, and if we add some rigid EPS insulation at this stage on top of pitched roof, my roof fascia will grow in height,” Joe says. “Rafters are cut already and the 9 1/2-inch LVL fascia is installed… By adding height, we will have to extend the eave.”

Our expert’s opinion

Here’s what GBA technical director Peter Yost has to say:

The saying goes that when given lemons, make lemonade. But the followup to that should be that some solutions are just not potable. (Sorry, I just could not resist the double-entendre on solutions and lemonade).

I am going completely with Martin on this one: Redesign the low-slope roof with topside rigid insulation and air-permeable cavity insulation, because if there was ever a roof design that begged for drying potential, it’s this one. Get the architect who was clever enough to give you this roof design to modify or detail your fascia and overhang on the low-slope roof to accommodate this roof redesign.

Can you spec and construct a no-drying-potential roof assembly to not leak? Yes, but it will have to be done pretty near perfectly. If you stick with this roof assembly I would strongly consider building in leak detection (Detec, for example) along with an inspection-maintenance schedule. A roof is such an out-of-sight, out-of-mind feature that these are more than just good ideas.

I am also more than just a little curious how such a chimney made it into and survived Passive House muster. And if this chimney will be completely outside the air and thermal barrier — which it appears to be in the image — the draw on this flue will be sorry at best.

I hope this does not feel like piling on at this point, but anyone who is aspiring to meet one of the toughest performance programs on the planet can’t start with a “lemony” design.


  1. Christopher Welles | | #1

    Is this really a problem?
    I hate to ask, and I know it's counter to the prevailing wisdom, but is this assembly really that dangerous? I recently read: "BSI-092: Doubling Down—How Come Double Vapor Barriers Work?", and it would seem that this assembly, or some similar, could work.

    I'm well aware that I'm not an expert on this, so I'm not trying to disagree here so much as understand. How is this different from the described "assemblies that work"?

    I do see the risk of thermal bridging showing up through the melt pattern, so I certainly see the benefits of continuous insulation, but unfortunately, after hearing all the horror stories saturated roof insulation, vapor permeable insulation under the roof deck makes me a little nervous.

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

    Response to Christopher Welles
    Like you, I don't think that the insulation system originally proposed by Joe B was "dangerous," and I never said it was.

    Joe B posted a question on the Q&A page, asking for advice. I noted that his roof assembly didn't address thermal bridging through the rafters, and prevented moisture in the roof sheathing from drying out, so I suggested a better assembly. I still think that the assembly I proposed is better than the one that Joe B was planning to use.

    When GBA readers tell me that they plan to install closed-cell foam on the underside of the roof sheathing, and a vapor-impermeable type of roofing above the roof sheathing, here's my usual advice: "Installing rigid foam above the roof sheathing is a better approach than the one you are proposing. But many people insulate as you plan to do, and these installation generally work fine -- although they do have thermal bridging through the rafters. Just make sure that the roof sheathing is bone dry on the day that the spray foam is installed, because there won't be much drying through the spray foam."

  3. Keith Gustafson | | #3

    boy is that a lot of water
    boy is that a lot of water dumping in that roof

    don't forget a significant cricket behind the chimney

    I vote EPDM[I always vote EPDM] with a minimum of a foot exposed climbing up the roof pitch and a foot behind the shingles. But I am paranoid. . Preferably all in one sheet, maybe with one short vertical seam below the chimney[you can use it to take up the slack created by the cricket and thus avoid a vertical seam above the chimney

    Foam right under the EPDM is a standard install. They make tapered for the edges, to keep your fascia slim

    actually looking at it again, 2 pieces one from roof edge to the uphill side of the chimney, the other overlapping that one heading upward, no vertical seams. You could also hip roof the flat roof foam below the chimney to maximize foam above the roof and still have a consistent slim fascia

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