A. Roofer Joseph Bublick responds: Although I prefer a base of polyisocyanurate, OSB or plywood will work fine under fully adhered EPDM. Install the OSB smooth side up. To ensure an even surface, I make sure the fasteners are flush, and install duct tape over the sheathing seams to soften any transitions that may occur if the sheathing begins to curl. There is no need to prime the OSB before installing the EPDM adhesive.
EPDM sticks more tenaciously to OSB or plywood than it does to polyisocyanurate. If you are used to being able to adjust the EPDM slightly as it's installed, this can be a disadvantage. Once the EPDM is cemented to OSB, you won’t be able to pull it up for readjustment.
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