Image Credit: All photos: Allan Poole Terry Lindenberg is opening up the roof to discover the extent of the OSB rot caused by moisture accumulation. Rot was extensive. Workers scraped the rotten OSB off of the foam panels with garden hoes. At each SIP seam, some of the EPS spline was removed. The channel formed by this operation was filled with canned spray foam. The cured foam was later trimmed flush with a Japanese saw. On the north side of the roof, most, but not all, of the exterior OSB facing on the SIPs was so rotten that it was removed and replaced with new plywood. The 18-foot-long 2x6s in the photo were temporarily installed to enable pressure to be applied to the new plywood sheets as they were being glued in place. Once the glue had set, the 2x6s could be removed. There was less OSB rot on the south side of the roof. Once the SIP seams were re-foamed, new plywood sheathing was fastened to the existing OSB. Jim McCarthy: a hulk, a sage, and the business owner of UP Top Roofing. He likes to go windsurfing after work.
A year ago, I looked on the web for information about repairing a poorly designed SIP (structural insulated panel) roof system. I found Green Building Advisor and posted questions here. It was suggested that I share my roof repair experience for the benefit of others who may be grappling with the same misfortune as mine. Here is my story.
In 1997, my two sisters remodeled a summer cottage built by my great-grandfather, Victor Poole, in Bailey’s Harbor, Wisconsin. They replaced most of the house with a SIP system. At the time, this was a new method. The roof is made of 16 foot long panels, either 3 feet, 4 feet, or 6 feet wide, spanning from the ridge to the eaves. Each panel has a groove in each edge which receives a 3 3/8-inch spline made of extruded polystyrene (XPS) faced with OSB on the inside and outside surfaces. There is no structural lumber incorporated in the panel joints.
The core of each panel is made of 7 1/4-inch-thick expanded polystyrene (EPS); the EPS is faced with 1/2-inch OSB on both the inner and outer surfaces. The total panel thickness matches that of a 2×8.
No help from the SIP manufacturer
Three or four years ago, the mounts for the rooftop TV antenna gave out and we began to notice swelling under the asphalt shingles where the seams between the roof panels were splined together (see Image #1, above).
The design of the SIPs and caulking method prescribed for sealing the seams between the panels did not prevent moist inside air in wintertime from traveling upward within the seams and eventually condensing on the asphalt felt underlayment. Over the years, water accumulated on the OSB, especially at the seams. A spot inspection over a seam revealed that the glue that held the OSB chips together was decomposing from the excess moisture. Galvanized roofing nails were rusting through and some shingles were coming loose. Though the outside OSB on the panels was structurally compromised, and the panel’s unsupported span was 16 feet in many places, structural failure was not a concern.
I sought help from the company that sold us the panels. I knew that the SIP design and the prescribed caulking material were at least part of the problem. Other SIP roofs with similar construction details must also be failing or have failed. The company should have already encountered these problems and I hoped that they would be eager to help their customers with proven remedies. However, they were quick to refuse any guidance at all. They advised me to look to the building contractor for solutions and seemed unwilling to acknowledge that their design was flawed.
On the north side, most of the OSB was rotten
The most important thing, I felt, was to find a way to stop air movement in the panel seams. If that could not be accomplished, the problem would eventually return. I had to wait till the roofing was torn off before I could decide what to do.
My sister located a coalition of two locally well-known contractors: Up Top Roofing, operated by Jim McCarthy from Sturgeon Bay, Wisconsin; and Appleport Builders, operated by Terry Lindenberg of Sister Bay, Wisconsin.
Terry had experience with SIP construction. He and his crew were involved in the job for as long as we needed them to adequately address the unique problems of repairing a rotting SIP roof. The combined manpower of the two crews was enough to get the roof done quickly.
On August 1st, we set up scaffolding for the north-facing roof, which has a 12/12 pitch. Terry adapted two long ladders so that they could be hooked over the peak. The asphalt shingles were loosely attached and came off in large gobs (see Image #2 at the bottom of the article). We quickly learned that that the OSB was rotten — not just at the seams, but across almost the entire roof (see Image #3). Our plan for repair and cost estimates were discarded.
I found two garden hoes in the shed, and they became our most useful tool for removing the OSB chips from the EPS panels (see Image #4, below).
Adding canned spray foam to the SIP seams
I began to hope that all the OSB on the north roof was soft and rotten — that would have made removal of the OSB easier — but that was not the case. After the panels were scraped clean of the rotted OSB, they still looked like OSB, because the texture and color was embedded in the foam.
All the rotted OSB on the roof was scraped off the foam panel surface. We pulled the partially rotted 3 3/8-inch-wide OSB strip from the block spline of each joint.
When these splines were manufactured, the foam in the block splines was kerfed with a saw to a depth of about 2 inches, presumably to give the splines some “give” (see the illustration above). We broke out 2 inches of the EPS spline, down to this kerf, to create a channel to lay down a layer of spray foam in the panel joint where there was once space that allowed interior air into the seam. I then drilled 3/8-inch holes on 8 inch centers, to blow Great Stuff canned spray foam into the space between the block spline and panel. (Canned spray foam produces more volume and cures faster if it is sprayed with a mist of water.)
The illustration above shows, top to bottom: (1) the block spline with the 3 3/8-inch-wide rotted OSB strip removed, (2) the EPS foam broken out to the depth of the kerf, (3) the resulting space filled with Great Stuff canned spray foam.
Image #5 (at the bottom of the article) shows how we re-foamed the seam splines. A Japanese pull saw was great for sawing the foam flush with the roof once the foam had cured.
Gluing new plywood to the EPS panel cores
Once this work was done, the roof was swept and blown clean with a leaf blower.
We needed a system to apply pressure to the new layer of 9/16 inch plywood that we were gluing to the EPS panels. The roof was too steep for weights to be a reliable solution.
We talked about using 8-inch-long screws to go through the 7 1/4 inched of foam to the inner OSB (which was also the ceiling). We settled on an idea that used the 2x8s that were located in the ridge and at the eave for fastening a series of 18-foot-long 2x6s, on edge, using 8-inch hex-head wood screws (see Image #7, below).
We would lay down a grid of PL300 foam adhesive from a large caulk gun, match the surfaces of the rigid foam and the new plywood, hold them apart till tacky, and then reset the piece of plywood and quickly bolt down a few of the 18-foot 2x6s on edge over the plywood.
To focus the pressure on the glued plywood, we used 2 or 3 opposing stacked cedar shingles, and drove them together with a hammer until we had the plywood pressed against the roof. Glue setting time was 20 minutes. We ran the 4’x8′ sheets of plywood horizontally and avoided having a plywood seam break over the EPS panel seams.
The south roof wasn’t as rotten
The south-facing roof was in much better condition because the sun baked out more of the moisture from the OSB. Though the panel edges were swollen and crumbling, the OSB elsewhere was solid enough to be nailed into.
We decided to glue and air-nail a layer of 9/16-inch plywood over the original OSB (see Image #8, below). As I did on the north roof, I drilled holes on 8-inch centers along the panel edges to blow Great Stuff canned spray foam into the spaces between the splines and the panels.
Once we had new plywood on the roof surface, the rest of the roofing project proceeded predictably and quickly. The whole job took about two weeks.
We ended up with an unvented “hot” roof
One other approach that we considered, but did not use, would have been to strip the OSB, then glue 1×3 nailing strips on 16 inch centers, vertically, then nail or screw the plywood to them. This would have created ventilation channels for the roof. The ridge and eave would have had to be vented.
SIP roofs can get really hot. Installing asphalt shingles over foam insulation without ventilation channels can reduce the lifespan of the shingles. Some shingle manufacturers void their regular warranties on such installations. Providing ventilation channels would also have allowed any moisture that migrated through the panels to drain off or evaporate.
The ventilation channels would have kept the roofing shingles cooler, and would also have kept the outside layer of OSB (the nailing surface) drier.
This project was a can of worms. Moist air movement in SIP seams can be catastrophic. I was so afraid that my roof was in terrible shape that I considered tearing off the roof and replacing it with an energy-inefficient roof.
I took a vacation from my regular life to be on site for this project. The two crews I hired were great to work with. Their philosophy was that they would find a way to solve the problems that came up, and they did.
The roofing crews tried to give us a quote based on what we thought we would find, but what we found was different. They adapted quickly. They were good at thinking and hashing out ideas in a genuine and diplomatic manner. Since I was right there, we made decisions in five minutes that could have taken hours or days otherwise.
This project took 14 work days to complete, but that time frame included 154 square feet of conventional re-roofing.
Advise to owners of SIP roofs
SIP construction requires extremely tight joints. Joints should be filled with foam continuously or at close intervals so that there is no chance for air movement. If there is any way to incorporate an air barrier on the inside surface of the roof, then that should be also be done — with great care and attention to detail.
Wiring and plumbing passageways in foam panels should be foamed-in where possible to block air movement. Indoor humidity levels in winter (especially in cold climates) should be monitored and controlled.
If you are not good at details or obsessing over a tight fit, then you should not attempt to install a SIP roof. But if your SIP roof has crumbled, my experience is that it can be successfully repaired.
For a SIP roof repair project, these questions must be answered:
- Is the roof structurally sound enough to be worked on?
- Can the upward/outward movement of moist house air be stopped?
- Can I find a contractor who understands this kind of work, who is willing to take on a project of uncertain scope?
For my project, we discovered a roof with no viable outside nailing surface. We were able to scrape off the rotten OSB and apply a new nailing surface on a 12/12 pitch roof. A small amount of the OSB was sound, so we had to fit the new plywood around it. We encountered the most difficult roof conditions and managed to prevail.
I feel that we have adequately addressed the causes of the failure and restored the roof to its original structural and functional condition. Let’s see if the test of time agrees. I will keep you posted.
Allan Poole is a Connecticut Licensed Arborist who lives in Middlefield, Connecticut. He has owned and operated his own tree service business for the past 36 years. He and his wife Nancy designed and built their own energy-efficient oak timber frame home in 1981.