Synthetic Felt Shrinkage

I am using synthetic felt for the first time. When the hot sun hits it, it shrinks. Which is great in removing all the little wrinkles, but in the valley where I had it loose it is now tight and raises in the air.

Should I loosen the nails and make it loose in the valley right before shingle or will it be O.K.

Either that or just slit it. I run I&W in the valley after letting it relax so it doesn’t stretch out like that, then run my felt to about 6" from the center of the valley.

Traditional felt paper will do this also. Although technically speaking it isn’t “shrinking” it is expanding and contracting. A nice, uniform nail pattern will keep it to a minimum. Another trick if you can manage it is to install the underlayment during the hottest part of the day. You shouldn’t be spanning a valley with it in any case.

Remember not to nail it in the valley… ice and water in the valley is the best solution.

We use Titanium UDL synthetic underlay. And I know exactly what you mean. What I do is place (not nailed) a long 2x4 in the valley to hold the underlay in the valley. You can do this when your shingling the valley, just keep sliding it up in front of you.

We use 30# or peel&stick in valleys and along eaves…