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What is the difference between roofing felt and synthetic underlayment?
Friday, July 20, 2018
Because most roof coverings are not really waterproof, and would be more accurately described as a surface that sheds water due to a downward slope and the down-lapping of overlapping panels, a secondary roof membrane for backup protection is required under a primary roof to catch any minor leakage at the laps. It is called an underlayment and, for many years, the only choice for roofers was an asphalt-saturated sheet “roofing felt,” which comes in 15-lb. and 30-lb. weights. The poundage once designated the total weight of the felt necessary to cover one roofing “square,” which is a tradesman’s term for 100 square feet, but now the rating is only nominal and actual weights are are somewhat less. The 15-lb. is standard and the 30-lb. is a heavier, premium underlayment.
Because asphalt is a petroleum byproduct, the cost of roofing felts has increased over time, along with the rising price of a barrel of crude oil. Also, more advanced refining processes have meant that less asphalt byproduct is made. These changes provided an opportunity for synthetic underlayment to enter the marketplace.
It is manufactured from polypropylene and polyethylene, which are used make a wide variety of other consumer products. Synthetic underlayments offer a number of advantages over roofing felts, such as tear resistance, UV resistance, and they don’t wrinkle when exposed to moisture like roofing felt. When it first began appearing on roofs in the mid-1980s, the multiple advantages made it a premium-price product. But, as synthetic underlayment’s popularity and production has increased, the price has come down to become comparable to 30-lb. felt, and it will likely be the predominant roof underlayment in the years to come. Popular brands include RhinoRoof®, Grace Tri-Flex®, and Titanium UDL30®.
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To learn more about roofs and attics, see these other blog posts:
Photo - W.R. Grace & Co.
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