Q: What is the correct way to install a two-ply roofing underlayment? I was quoted a price on two-layered underlayment for my roof (Fontana G40 underlayment). I asked how much overlap he uses in his installs and he answered 2-4 inches.
I was very confused as I have read your information and then confirmed with the TRI Installation Guide (page 17) that there should be 18-19 inch overlap.
He clarified that when he installs, he puts two layers directly on top of eachother, and then a 2-4 inch overlap between the double layers. Is this a bad practice? I haven’t seen other guides saying to do it this way and I am afraid if I have warranty issues they may say it was installed incorrectly.
Thanks for all your information, it has been a great help to my research!
You are correct. The standard approach for two-ply roofing underlayment is to lap a 36-inch roll by 19 inches. The lap is half the width of the roll, plus one extra inch for” headlap”. This provides two plies across the entire roof with three plies at the “headlap”. Heavy-weight two-ply underlayment, installed this way, is widely considered to be the industry standard for tile roofs.
By comparison, the standard overlap for single-ply underlayment is only 2 inches at horizontal laps and 4 inches at vertical laps.
The headlap is narrow band of 3-ply coverage at the top of each lap, as shown in the illustration. Most roofing tiles and shingles (as well as sidewalls shingles) are installed with a 3-ply headlap.The headlap provides for extra protection at overlaps, especially by vertical joints in roofing or underlayment.
I’m not sure why this contractor installs underlayment the way he is describing and don’t know how it would perform compared to the industry-standard installation as shown here. I suspect it would would not have the same holding power as true double coverage if exposed to high winds in storm.
Regardless, you are correct that you are risking warranty problems if the installer does not follow industry standards and especially if he fails to follow instructions provided by the manufacturer.
So definitely bring this up, and unless the contractor has a compelling reason to not follow the standard procedure, insist that he do so. Or find another contractor if necessary. — Steve Bliss, BuildingAdvisor.com
See also Best Underlayment for Tile Roofs