Roof Underlayment

You know when something looks so easy to do when you watch the instructional video? How the people demonstrating it just crank it out like it's nothing? How the instructions sound so simple, almost foolproof? And then when you actually try to do it, it all goes wrong.

Grace Ice & Water Shield, you are so guilty.

Or at least, at first you were.

Every roof needs an underlayment to protect the sheathing from any water that gets past the roofing material (e.g. standing-seam steel). Like the house wrap, the underlayment is applied from the bottom up, so that every upper course overlaps the one below it.

The cheapest, most common type of underlayment is roofing felt, which is a thick, asphalt-saturated paper. It comes in two thicknesses, 15# and 30#; 30# is what's required under the metal roof that I will be installing. The disadvantage of using roofing felt is that on a slope as low as mine, you're supposed to overlap the courses by 19 inches! The roll is only 36" wide, so that ends up basically doubling the amount of product you need. Plus, the waterproof membrane that roofing felt creates is open; you don't tape the seams or anything like that to create a really water-tight surface, but rather rely entirely on the overlapping.

What I mostly used, instead, is Grace Ice & Water Shield. This is a self-adhesive rubberized asphalt underlayment, super-sticky on one side and with a fabric facing bonded to the other. You only need to overlap the courses by 3 1/2" and the stickiness means that it seals itself against all water (and ice) intrusion. It comes in a big roll with a paper backing, which you're supposed to be able to pull off as you unroll it onto the roof--application in a single motion!

I divided the big 75' roll into three 22'-6"rolls, to be easier to handle. My aim was to run the underlayment down over the fascia and verge rafters by about 2" to protect them, too, from any water that might get in under the roof edge flashing.

The first course was not so good. We tried to tack the end down with staples to keep it in place while we unrolled it, but the staples pulled out unbeknownst to us and we didn't notice until it was too late. Once that stuff is down, it's down--it'll sooner delaminate your plywood sheathing than pull free from it! It went down crooked and off the line we'd snapped, so not enough of it hangs over the fascia/verge rafter on the front of the house and too much on the back. Boo.

And I realized that we'd be a few inches short of covering the whole roof with the stuff. This necessitated another trip to the store, to buy the (thankfully inexpensive) regular 30# roofing felt. (That's the un-logo'ed strip in the picture.)

One course of the roofing felt, properly overlapped, was enough to finish the rest of the roof with Grace, and since the felt was much easier to install, we managed to create a nice, straight line to guide the rest of the application. We also came up with a better method than stapling the Grace to hold it down while we worked. After that, it went on pretty easily!

Lesson 52: Tack the end of the roll of Grace Ice & Water Shield down with something stronger than staples; we used three screws in a board to hold it down and in place while we rolled it out, then I went back with scraps and patched the screw holes. Roll it out halfway with the backing still on, check and double-check for positioning, then cut the liner and roll the rest of the roll onto the roof deck while pulling away the liner. Untack the end of the roll, roll it up, and roll it back out while removing the liner. Press it down firmly, all over, to make sure it adheres!

I also ordered my roofing material ahead of the Independence Day weekend, but it won't be for another few days. I've settled on clear-coated galvanized Snap-Loc standing seam panels from Champion Metals. It comes pre-cut to length and with matching flashing pieces. I can't wait to start installing it!