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 each other, 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!
— Bob
A: You are correct. The standard approach for two-ply roofing underlayment is to lap a 36-inch roll by 19 to 20 inches. The lap is half the width of the roll, plus one extra 1 to 2 inches 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.
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 One- vs. Two-Layer Underlayment
M..Gramm says
Does Felt Underlayment Leak?
After putting felt paper on the roof should rain leak through?
buildingadvisor says
To meet the building code, on roofs with a pitch of 4:12 or greater, the horizontal lap should be 2 inches minimum, and end laps 4 inches minimum. End laps should be offset by at least 6 feet from the course below.
For low slopes of 2:12 to 4:12, code requires double coverage as described in the post above.
If felt paper is carefully installed with proper laps, no tears, and sufficient nailing with plastic or metal caps, and carefully woven into flashings at chimneys, plumbing stacks, and other penetrations, the underlayment should prevent leaks in a light rain without a lot of wind. However, some leakage may occur with heavier winds or a less-than-perfect installation.
If the felt has been exposed to the weather for more than a week or so, it will start to degrade and will be more prone to leakage. Also. felt underlayment tends to absorb water and wrinkle, and is prone to tears.
Double-coverage underlayment will provide much better protection, but still should not be left exposed for extended periods.
Many synthetic underlayments can be left exposed to the weather for up to six months and are more water-resistant than felt. Also, they have fewer horizontal and vertical seams, so they are a better choice where they will be left exposed for extended periods.
James C. says
Two-Ply Modified Asphalt Vs. One Layer Synthetic Underlayment?
Deciding between 2 bids to lift and lay cement tile roof in San Diego county, near ocean.
One- suggests 2 layers of Malarkey UDL 40.
Two- suggests 1 layer of UDL 50.
Bid 2 is about $10 k less.
Do these sound prudent, and would you favor either?
Thanks
buildingadvisor says
All things being equal, a two-ply roofing underlayment is better than a single layer. In your case, however, the two systems use completely different materials. So it is difficult to compare the two.
Malarkey’s Right Start UDL is a polymer-modified asphalt underlayment, which is a big improvement over traditional asphalt felt. The rubber additives give it better strength, durability, and water resistance. This is a heavy-duty fiberglass-mat underlayment that weighs about 40 pounds per “square” (100 sq. ft.). A one-ply installation carries a standard 10-year warranty, which can be extended when combined with certain other Malarkey products and installed by a certified contractor. A two-ply installation can be expected to last longer and may carry a longer warranty (check with Malarky’s technical services for more details).
I’m guessing that the other product is Titanium UDL 50, now owned by Owens Corning. This is a 30-mil synthetic underlayment that weighs around 24 pounds per square. It is the heaviest product in the Titanium line and comes with a 50-year warranty. Synthetic underlayments are non-woven fabrics – something like Tyvek, only much heavier and more waterproof.
I have not been a big fan of synthetic underlayments for tile roofs, but they are gaining wider acceptance and do have a number of positive attributes. Roofers like them because they are lightweight, easy to install, and often provide a good walking surface like this one. They also tend to be highly waterproof, tear resistant, and UV resistant. That is, they can be exposed to UV for longer than most asphalt-based underlayments.
Modified asphalt products tend to seal better around nail penetrations, but not as well as the seal provided by a self-adhesive underlayments. Modified asphalt underlayments also have a longer track record than synthetic underlayments. Both products are tested for high temperature use, Right Start to 230 °F and Titanium to 240 °F.
Both products meet the relevant ASTM standards, but Titanium UDL also has approvals from the state of Florida and Miami Dade Country, one of the toughest roofing codes in the US.
Since you are in a pretty mild climate, I think either system would work well. I can’t say which will last longer, but would expect either to last well beyond 20 years. Warranties provide some idea of a manufacturer’s confidence in their product. But most roofing product warranties are pro-rated and for materials only, so their actual dollar value is limited.
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