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Q&A Spotlight

There’s Rot in the Roof

An unvented low-slope roof is showing signs of water damage. What’s the best fix?

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A small section of this roof shows signs of rot and needs repair.. But how can the job be done without disturbing the steep-slope roofs around it? That's the homeowner's dilemma.
Image Credit: Chuck Kramer
A small section of this roof shows signs of rot and needs repair.. But how can the job be done without disturbing the steep-slope roofs around it? That's the homeowner's dilemma.
Image Credit: Chuck Kramer
With more material removed, the homeowner realizes that the problems are not limited to the area immediately around a poorly installed skylight. The eventual solution on the lower section of the roof included a total of 7 1/2 inches of rigid foam and 1 inch of Styrofoam. This photo was taken just before the 1/2-inch plywood sheathing was added. The uninsulated areas are the roof overhangs.

Chuck Kramer’s home in Enumclaw, Washington, was built in the 1980s with unvented cathedral ceilings, insulated with cut-and-cobble rigid foam insulation and roofed with cedar shakes. A small section of the roof is showing signs of water damage, and now Kramer is trying to find a way of repairing the problem area without tearing into the rest of the roof.

As Kramer explains in a Q&A post at Green Building Advisor, the trouble seems to be in an area measuring about 150 square feet. This part of the roof, with a slope of 4-in-12, is surrounded by two other roof areas with a 12-in-12 pitch.

From inside to outside, the roof assembly consists of 2×6 tongue-and-groove boards, 15-pound asphalt felt, 2×6 rafters, 1×4 skip sheathing, and cedar shakes interwoven with strips of 15-pound asphalt felt. The rafter cavities are insulated with two 2-inch-thick layers of foil-faced polyisocyanurate foam insulation and 1 inch of Styrofoam beadboard.

“Most of the roof is 12/12 pitch,” he writes. “A small section above two bathrooms is 4/12, with two 12/12 pitches joining two sides with valleys. In that section, a skylight was cut in after initial construction, maybe in the late ’80s… The 1x4s [the skip sheathing] and a small part of the tops of the rafters are damaged by rot, particularly badly around the skylight.”

Kramer doesn’t see how he can increase the thickness of the low-slope portion of the roof without affecting the steeper parts of the roof. His proposed fix is to pull off the 1×4 skip sheathing, add a 1/2-inch layer of OSB with a peel-and-stick membrane, then a layer of 30-pound felt, and metal roofing. The 5,000 square feet of 12-in-12 roofing will not be affected.

Inside, he’s improved the bathroom fan capacity and covered the tongue-and-groove ceiling with drywall.

“In the long run, it is possible that I am going to have to have the entire roof replaced, which will allow adding the R-10 or greater rigid foam and completing the other details, at a very large cost,” he says. “For an interim solution on this small section of roof, what improvement to my plan does anyone have?”

That’s the background for this Q&A Spotlight.

It’s worse than I thought

As is often the case when repairing an older house, the damage Kramer found in his initial assessment was just the beginning. What at first looked like a localized problem in the area of the skylight goes much deeper than that.

When workers installed the skylight, “they literally cut a hole in the entire roof structure, added no framing, screwed the deck-mounted skylight on, and covered it with trim,” Kramer writes. “No insulation at the cut, and this was directly above the bathtub/shower…”

A few hours later, after he has removed more roofing, Kramer adds this: “The damage is throughout the roof, as you can see in the photo [see Image #2, below]. Not sure what the fix is, but for now I’m just going to remove all the sheathing remnants and ponder if there is a way to move the transitions and valleys up enough to add the rigid foam at R-10. Any suggestions are appreciated.”

His plan now seems to be to remove the Styrofoam and replace it with 1 inch of closed-cell foam, sealing the edges with spray foam, then seal all joints with tape. That would be followed by a 5/8-inch OSB roof deck, 30-pound felt, and standing-seam metal roofing.

Cut-and-cobble is ‘risky’

The photos tell GBA senior editor Martin Holladay that moisture has probably been accumulating in the roof for years, a result of various air leaks between the framing and the rigid foam insulation that has been cut to fit in the cavities. “The exfiltrating air,” he says, “carries moisture, and the moisture probably condensed on the underside of the cold asphalt felt strips that were interwoven with the cedar shingles.”

The bottom line is this: a cut-and-cobble approach to insulation is risky.

“Fixing the open section with more cut-and-cobble is only a temporary solution, since you’ll probably end up with more air leaks as the rafters expand and contract due to changes in humidity and temperature,” Holladay writes. “Eventually, you’ll probably need to fix this problem on your entire roof. To really fix this issue, you’ll need to strip all of the roofing off, install new OSB or plywood roof sheathing, some type of air barrier, and an adequate thickness of continuous rigid foam above the roof sheathing.”

A better approach now, he adds, would be to strip off all of the rigid foam from above to expose the ceiling boards, and fill the cavities with closed-cell spray foam.

Ordinarily, skip sheathing would be fine

Most houses have solid roof decks — which these days are made of plywood or OSB, but in the past consisted of sawn boards. The person who roofed Kramer’s house used skip sheathing, 1×4 boards attached to rafters so there’s a gap in between each board. That was a common technique when roofs were finished with wood shingles, because the skip sheathing allowed the shingles to dry out better than solid sheathing did.

The technique typically allows any accumulating moisture to escape, Dana Dorsett says.

“Cedar shingle roofs on skip sheathing are inherently ventilated to the exterior,” he says, “and in most climates air leaks from the interior would just leak without leaving enough moisture in the skip sheathing to be a problem, but on the foggy-dew western foothills of the Cascades it’s still not always adequate.”

It appears from the photos Kramer has provided that the only issues with the skip sheathing appear to be around the roof vent (probably because it wasn’t sealed adequately) and at the bottom edge of the roof over the eaves, “which implies that the inherent venting was ‘working mostly,’ ” he adds.

“The damage at the overhang was likely due to a combination of wind-driven leakage of the shingles combined with slower drying due to the colder temperature of the roof at the eaves where it isn’t being heated from below, and scant winter/spring sun to warm it from above,” Dorsett says.

Kramer will have to do better than R-10 in insulation above a new roof deck if he wants to meet the 2015 International Residential Code minimum, Dorsett says. “But R-15 above the roof deck would meet code on a U-factor basis (not on an R-value basis) and would give you reasonable dew point margin,” he says. “Using 3 inches of reclaimed roofing polyiso would be the cheapest (and greenest) way to get there.”

Not enough room for all that insulation

Dorsett may have a point, Kramer replies, but there doesn’t seem to be a way of adding 3 inches of rigid insulation to this small roof area without diving into the adjoining roof sections. And that is not something he’s prepared to do.

“I can see that the right way would be to add the rigid foam over the top, but I’m trying to find a reasonably efficient compromise which will stop the moisture problems in the future, even if not meeting full R-value,” Kramer says. “The rest of the home has about 5,000 to 6,000 square feet of this roof construction. That’s going to be my next challenge.”

His latest, and best plan so far, is to remove the rigid foam from between the rafters and to spray 1 inch of closed-cell foam directly on top of the 15-pound felt separating the roof cavity from his tongue-and-groove ceiling boards.

“Then I will re-install the foil-faced Thermax and tape the top surface across the rafters,” Kramer adds. “Essentially, I’m going to be treating the tongue-and-groove as the roof deck, and installing all of my insulation above that. Then I’ll install the metal roof over 30-pound felt. That will give me R-27, (in the cavity, anyways…). I’ll also have to be sure to seal every penetration (drain vent, bathroom fan, skylight).

“This seems like it will address the issues for this small space,” he adds. “I’m proceeding with the belief that this small section will be sub-optimal in energy efficiency, but fully durable from a moisture standpoint. “

More evidence comes to light

As Kramer dives further into the project, he posts a series of photos that suggest asphalt felt woven into the courses of shingles had something to do with the rot that developed later.

“Your latest photos provide another puzzle piece,” Holladay says “Now we know the condensing surface for the escaping moisture. The original roofer installed strips of #15 asphalt felt, interwoven with the cedar shingles. It looks like a strip of asphalt felt was applied to the upper half of each course of shingles, as the shingles were installed. In winter, this asphalt felt was cold. This upper layer of asphalt felt was the condensing surface for the escaping moisture.

“The moisture was piggybacking on the exfiltrating interior air that was escaping through cracks in the cut-and-cobble foam, and that moisture condensed on the underside of the roofing felt,” Holladay continues. “The moisture was trapped between the asphalt felt and the skip sheathing. The result: sheathing rot.”

The problem would be more pronounced on boards higher on the roof because the warm, moisture-laden air would have a tendency to rise.

Kramer does some exploratory digging into the steeper sections of roof and concludes it will have to be replaced at some point. So he plans a complete repair for the lower section of roof so it will not have to be reworked later. Once the rafter bays have been insulated, he plans to cover the entire section with 3-inch-thick rigid insulation, taped at the seams, before adding 1/2-inch plywood sheathing and his roof covering. (See image #3 below.)

“This means I’ll cut into the two adjoining 12/12 pitches to raise the valley flashing,” he says. “I experimented with this and it wasn’t as bad as I thought.”

Our expert’s opinion

GBA technical director Peter Yost added these thoughts:

Chuck Kramer has gotten much more than he bargained for with his roof and with us! He and I and the GBA online professional volunteer crew have been going full tilt on this one.

I have worked on a number of projects with 2×6 tongue-and-groove ceilings and/or roof decks (here’s one example).They work great as finish and structure but they leak air like a sieve. And because the t&g deck extends through and beyond the gable and eaves to make great overhangs, they leak everywhere: through the field of the ceiling, at penetrations and to the outdoors directly at eaves and gables. The pattern of moisture damage often follows these pathways, as well as up into peaks due to stack effect and moist air being more buoyant than dry air (given the same temperature).

Here is some interesting additional information about this home:

1. The current occupant moisture load is apparently quite modest: just two occupants, few plants, dryer properly vented to outdoors, no wood stored inside conditioned space, no open combustion (e.g. propane cook stove or oven) etc.

2. The previous occupancy moisture load is likely to have been quite high. Evidence of moisture upon move-in by current occupants includes: staining streaks (watermarks) on window frames, ceilings, and walls; and a wood stove with a big, open pot of water on top; and poor or no exhaust venting in bathrooms.

3. Although a blower door test has been conducted on the home, Kramer never got the results. He does, however, remember quite a bit of the “flow” coming from upstairs.

4. Much of the moisture damage — rot and insect tunneling — appears to be historical rather than current or recent.

So, Chuck Kramer is and needs to continue to address two connected issues: household moisture loads and roof air leakage. He needs to manage air leakage at all of the pathways enumerated above, and assess the continuity of his air barrier with a blower door aided by smoke stick and/or IR imaging. He needs to monitor indoor relative humidity, especially during the winter, with a reasonable monitoring device (see the link to “Measuring (and Understanding) Humidity” in the sidebar above) and identify and manage sources of household moisture should levels exceed 35% to 40%.

By the way, it would be a good idea to check the original Q&A post because Kramer has built a great photo series there.


  1. Antonio Oliver | | #1

    Oddball question?
    With no specific response to the photos, is there ever a case for continuing insulation all the way out to end of overhangs, beyond the conditioned envelope? As an extension to the initial question, should one ever consider insulating a porch roof--especially when that roof extends from over the conditioned space?

  2. User avater
    Jon R | | #2

    One can certainly stop roof
    One can certainly stop roof air ex-filtration by reducing building pressure. Cost effectiveness is up for debate.

  3. User avater GBA Editor
    Martin Holladay | | #3

    Response to Antonio Oliver (Comment #1)
    Q. "With no specific response to the photos, is there ever a case for continuing insulation all the way out to end of overhangs, beyond the conditioned envelope?"

    A. Yes, of course there is (assuming we are talking about rigid foam installed above the roof sheathing). Even though there is no need for insulation over roof overhangs, you'll still need some type of shim or material to make the surface co-planar. If you need 2 inches or 4 inches of material over the roof overhangs for this purpose -- to match the thickness of rigid foam on the main area of the roof -- the cheapest way to proceed (considering both labor and materials) is usually to just extend the same type of rigid foam you're using for the rest of the roof.

    Q. "Should one ever consider insulating a porch roof--especially when that roof extends from over the conditioned space?"

    A. The same analysis applies. For a small porch, extending the rigid foam may be the easiest way to proceed. If it's a large porch, it may make sense to raise the height of the porch rafters, or to add 2x4 sleepers of some kind, over the porch instead of shimming with rigid foam.

  4. Mark Heizer | | #4

    SIP panel roof
    GBA has covered rot in SIP roofs and lessons-learned: moisture will get into the assembly. This may not have the top sheathing of a SIP, but close enough to consider the side-effects. Might want to consider treating similarly with vapor relief at the upper end. Otherwise, the 2x's might see similar fate. The work also appears to be above a bathroom (more moisture than normal); upsized bath fan might help a little.

  5. User avater
    Jon R | | #5

    overhang insulation
    Some say that the sun warms the walls and this heat rises to the overhang area and causes ice dams.

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

    Response to Jon R
    Good point. In case GBA readers missed Jon's point -- which he didn't make explicitly -- here it is: insulating a roof overhang on the south side of a house can limit potential ice dams caused by solar heat gain on the south-facing siding.

  7. Skip Harris | | #7

    I thought COLD eves create ice dams... (5, 6)
    My understanding is that the snow melts on the warm roof and runs down to the cold eves. Warm eves should be a way to stop this, as evidenced by folks installing heating cable on their eves...

  8. Skip Harris | | #8

    NOT a SIP! (response to 4)
    Isn't the issue with SIP roofs the fact that they leak at the joints if those are not well-sealed? The owner, here, is attempting to create a jointless SIP (well, IP) and that seems like the perfect solution.

    My only question here is using tape for sealing to the insulation. This SHOULD work, at least with foil-faced isocyanurate, but I'd prefer something like Grace Ice and Water Shield, especially for making the air-tight tie to the other sections of roof when those are eventually modified.

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

    Response to Skip Harris (Comment #7)
    If you want to prevent ice dams, the best approach is to design a roof system that results in uniformly cold roofing -- from the eaves to the ridge. The colder the roofing, the better the roof system will resist ice dams.

    Installing heating cables at the eaves to melt accumulating ice is a band-aid approach that wastes energy. GBA has never advocated the use of heating cables at eaves to address ice dams.

    If solar heat gain (on the siding) is warming the eaves, the heat can melt snow sitting on the roof. In some cases, the liquid water resulting from this snow melting process can freeze in the evening, setting the scene for ice damming problems. One way to address this type of solar heat gain is to insulate the roof overhang, to separate the hot soffit from the roofing above.

  10. User avater GBA Editor
    Martin Holladay | | #10

    Response to Skip Harris (Comment #8)
    You are right that this house does not have a SIP roof, and that the problems encountered here differ in important ways from problems with leaky SIP roofs.

    That said, there is a basic lesson here: Any time a builder attempts to seal a crack with canned spray foam -- whether the crack is between two adjacent SIPs, or whether the crack is between rigid foam and a rafter in a cut-and-cobble installation -- it's worth remembering that air leakage is still possible. Canned spray foam in this type of seam is not a long-term solution in a roof system.

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