ASK THE EXPERTS: Roofing on SIPs

Intermediate

I’m in the early stages of building a structural insulated panel (SIP) home in the deep south and want to install a PV system on its roof. However, I’ve encountered the issue of potential incompatibility (and voiding the warranty) of shingles on a SIP roof.

I’m thinking that establishing a “cold” roof using 2-by-4s attached  to the SIP’s top, but secured from the under/inside of an unvented roof with lag bolts (16 inches on center) would serve to secure a mounting base for a roof-mounted PV array. The cold roof would keep heat from building up inside the roof SIPs via simple convection through a ridge vent system, and if there is an unexpected cold snap, there would be no ice dams. The 2-by-4 spacers also would serve as a strong base for attaching the roof-mounted PV array. Of course the lag bolts would need to be applied with suitably sized washers to reduce any potential of pull-through. If there is a need to finish the inside of the attic, simple carpentry would allow this with minimum expense. Can you give me some feedback and expert advice?

Rick Sabb • Detroit, Michigan

Let’s start with the shingle warranty. Although some manufacturers do not offer a warranty for a “hot roof,” I do know that ELK brand does allow this type of installation on SIP roofs. The reason that others might not cover this type of installation is because of a perceived increase in shingle temperature. However, research conducted by Building Science Corporation (buildingscience.com) showed that although asphalt shingle temperatures increase slightly (2° to 3°F) in an unvented roof assembly, the color of the shingles and the roof orientation have a much more profound impact on their durability. If you are going to install a PV system over the roof, I would recommend a light-colored roofing material to keep your PV modules’ temperatures lower.

According to the “Builder’s Guide to Structural Insulated Panels (SIPs)” published by Building Science Corp., the typical reduction of shingle life over an unvented SIP roof assembly is between one and two years. As with any construction, there are different options. In all cases, the seams of all panel joints should be taped on top and bottom with a vapor-impervious material to inhibit air and moisture movement through the joints. A good way to go is to apply asphalt shingles on a SIP roof with a synthetic underlayment. A better way might be to install a layer of plywood or OSB over the SIPs. And, in my opinion, the best application would be to add a 3/4 to 1 1/2-inch sleeper and then the layer of plywood or OSB.

These recommendations are also dependent on the climate you are building in. Even in the deep south, it would take more than a cold snap to create a problem with ice dams, since it’s highly unlikely snow and ice would sit on the roof for any length of time.

Lastly, considering the cost of the materials and labor to add a cold roof system, it might be more cost-effective to install a standing-seam metal roof, which doesn’t need a cold roof system, and should last twice as long as asphalt shingle. Plus, a standing-seam roof is easy to mount PV arrays on—no penetrations are necessary.

As to fastening the PVs to your roof, I refer you to an article in HP156 : “Solar on SIPs.” If you do go to a cold-roof system, I would recommend GRK Fasteners (grkfasteners.com) as a better choice than lag screws. Enjoy your new home and PV system!

Patrick Sughrue • ArtisanTinyHouse.com

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