Should I put underlayment on the entire valley and then put the ice/water shield over it? Should I put the ice/water shield directly onto the wood of the osb valley then staple the underlayment within 6 inches of the center of the valley?

I've seen photos by GAF where it seems the ice/water shield is stuck on top of underlayment?

Which is best?

Many thanks


2 Answers 2


You must install the shingles according to the manufacturer’s recommendations or you’ll void the warranty.

GAF, just like all roofing manufacturers, recommend the “peel and stick” membrane to be applied first. There’s many reasons for this, but ice buildup and wind blown rain are the main reasons.

By the way, do not stop the underlayment 6” from the valley, as you describe. Extend the underlayment THROUGH the valley and weave it into the other side of the roof.


The membrane goes directly onto the roof sheathing (plywood). I would run the roofing paper across one pitch of the roof, continuing it across the membrane and then up the pitch on the otherside of the valley. How ever, I would not staple the roofing paper anywhere it overlaps the membrane, especially with staples that may rust away, leaving holes in the membrane. The galvanized nails that hold down the shingles won't rust away.

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