Tools & Materials
Synthetic Vs. Felt Roofing Underlayment
Prospectors during the California gold rush covered the roofs of their temporary shacks with roofing felt also known as felt paper or tar paper. Fast forward one hundred seventy years and roofing felt is still the most popular roofing material (now used as an underlayment) being installed today. But it looks like we will reach the tipping point in the next couple years where more homes in the United States will be covered with synthetic underlayment than with roofing felt.
What is roofing felt?
Roofing felt is essentially a polyester or fiberglass fleece that is soaked in some waterproofing agent. It’s available in two thicknesses, 15-lb.and 30-lb., 30lb being the thicker of the two. Those numbers refer to how much 100-sq.-ft. of the paper used to weigh. The actual weight-per-sq. these days is actually less than the label says, but the 15-lbs. and 30-lbs. moniker stuck. Roofing felt comes in 36” wide rolls. A 15-lb. roll covers about 4-sq. (400-sq.-ft.) and a 30-lb. roll covers about 2-sq.
What is synthetic underlayment?
Different types of synthetic underlayment have been around since the early 2000s. Most synthetics are woven or spun from either polyethylene or polypropylene. While the ingredients are similar, the manufacturing process and thickness of each type of underlayment can be fine-tuned to produce different properties, such as exposure time and walkability. One way of comparing synthetic underlayment is by weight, similar to how felt is compared. Instead of pounds-per-100-sq.-ft., synthetics are rated by grams-per-sq.-meter (GSM).
Price: synthetics vs. felt
The price of synthetics is based on weight and performance. Heavier products typically have more high-performance qualities baked in like improved walkability, durability, and longer exposure times. The cost of the thinner synthetics right now is comparable to a high quality 15-lb. felt but still more expensive than the cheaper felt. Thicker synthetics run about 30%-40% more. Some thinner products are actually cheaper than felt, but Dale Walton, a Product Manager at CertainTeed, warns against going too thin. CertainTeed sells two synthetic products, RoofRunner which has a GSM of 110 and DiamondDeck which weighs 185 GSM. Dale said that their testing has shown that products with a GSM less than 110 are prone to tear outs and puncture holes from imperfections in the decking surface.
Benefits: Larger rolls
Roofing felt rolls are 3-ft. long and cover about 400-sq.-ft. Synthetic underlayment rolls are 4-ft. long and cover closer to 1000-sq.-ft. It doesn’t take a roofing scientist to figure out that a roof can be coverd faster with larger rolls. And more coverage per roll translates to fewer trips fetching additional product.
Benefits: Stays down in the wind
When fastened properly, synthetic products will stay put better than felt. No more chasing airborne underlayment around the neighborhood or having to pull it out of the neighbor’s trees.
Benefits: Less heat
Here is a benefit that not many folks on the synthetic fence don’t consider. Roofing felt is black. Many synthetics are gray. Black gets creates a much hotter surface in the sun than gray.
When synthetics first came out, one of the main complaints was that they got slippery when wet. Just the opposite is true these days. Some products actually get a little tacky when wet. Improved walkability is the sole reason many roofing contractors have made the switch from felt.
Prolonged exposure is another terrific benefit of synthetic underlayments. Even exposed for 24 hours, roofing felt starts to bubble, buckle and wrinkle. And if not remedied, those humps and bumps can telegraph right through roofing materials, especially thinner asphalt shingles. Synthetics, on the other hand, can be left exposed for many weeks without a problem.
Be Aware: Fastening is critical
An installer needs to follow the directions when installing synthetic underlayment, just like any other building material. Most synthetic manufacturers require the product to be fastened with capped nails, not staples. This is especially true if the product will be left exposed for any length of time.
Be aware: Low perm rating
Most synthetic products have a perm rating much less than felt. This means that any moisture in the attic will NOT be escaping through the roof itself, making proper attic ventilation very important. And it’s best to check with the manufacturer to confirm their product is recommended on a “hot roofs.”
Be aware: Types of roofing material matter
Not all types of roofing can be installed over all types of synthetic underlayments. When installing any kind of roofing other than asphalt (steel, wood, tile, etc.), make sure the synthetic you install is compatible.
Be aware: Don’t skip peel and stick
Synthetic underlayments still need to be used in conjunction with peel and stick (aka. self-adhering or ice and water) underlayments, which are required to cover overhangs, valleys and wall intersections. There are synthetic peel-and-stick products available, but they are not as widely used because they are seen as most contractors as cost prohibitive. Roofing codes vary from region to region; make sure to check with your local building officials on the peel and stick requirements in your area.
Ron Heather is Vice president of Roof Depot, that employs up to 150 roofing crews. When asked about the future of synthetic underlayments he said, “Ten years ago we wouldn’t have touched the stuff. Ten years from now, when the prices are the same or less than felt, we will use nothing but synthetic. It’s safer to work with. The installers love working with it. It’s just a superior product all the way around. ”
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