Good practices are the reason why all products perform well, and perform to their expected lifetime. Shingles are no different. In the 90’s a certain roofing product came out that was sold as a saving grace for problem area’s. It is easy to apply and does have value in certain problem areas, such as in valleys and along the eave’s if the house isn’t properly vented(that’s a whole different story.) It goes by a few names: Ice & water shield, ice & moisture guard, ice & moisture barrier.
We use I&W in the valleys, which replaced our method of using 90# roll roofing in the valley. It still takes proper installation techniques to keep water out, this is only for a backup of extreme weather and snow buildup. On a 3:12 pitch roof, it’s not a bad idea to install it over everything if you must see shingles. Technically, 3:12 is a low slope roof and less than ideal for architectural shingles! I&W over the entire roof will work for a while, but as soon as enough water has touched the nail heads, they will rust out and cause leaks!
Look at the roof above. The low portion is less than a 2:12 pitch. I was called out to look at the three skylights that are leaking. EVERY shingle that I lifted revealed rusty nail heads! If it wasn’t over I&W, this roof would be leaking everywhere! The installer’s warranty is over, so now this homeowner is left with a time bomb. We are going to tear this roof off and install a low slope product such as EPDM, TPO, or PVC. It’s always sad to tear off such a new roof, only six years old.