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Hi - I am building a small cabin in the woods of West Virginia with a 12/12 pitch roof. The plans call for a pretty thick sandwich of materials for the roof - I have a 2x deck down on the rafters (looks nice from inside) covered in Protec 160 to get dried in while I get ready to do the next couple layers. These nest layers will be 2 inches of XPS insulation sandwiched on the deck with OSB on top, with another layer of underlayment and then the metal.
Here's my concern: We had some pretty heavy rain last weekend and while inside the building I noticed 5 or 6 spots on my Protec that were leaking pretty steadily. I have the overlaps right and used a Stinger to secure it to the deck in all the recommended locations (laid out on the material). I'm wondering if using the Stinger was my mistake - I am wondering if the Stinger staples ripped small gashes in the underlayment in places when I was slapping them down or if the Stinger caps don't seal the hole the staple makes very well. Maybe I should have used cap nails?
Now I am a bit afraid to trust Protec on the final OSB layer and am considering Grace Ice and Water shield but it's so dang expensive! I will do it for peace of mind though if I have to. Is a few leaks in the Protec normal when installed with a Stinger? I want ZERO leaks of course, should anything work it's way through the metal in the future. Thanks in advance for your advice.
Synthetics like the Protec product are used regularly. However, they should only be fastened using plastic cap nails ... staples, even with plastic caps, act like little knives that create slits in the underlayment. A lot of times, too, leaks through exposed synthetic underlayment occur from the underlayment not being adequately fastened at seams and overlaps and also from it "flopping over" where it is run up against protrusions such as skylights and chimneys. As long as the roof has 3:12 or better pitch, I think you will be fine with synthetic if properly applied.
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