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®. 

    Also see our blog posts Why is my roof leaking? and What's the average lifespan of a roof?

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  To learn more about roofs and attics, see these other blog posts:

• Why is my roof sheathing sagging between the trusses?

• Why is granule loss a problem for an asphalt shingle roof? 

• What are the mistakes to avoid when doing attic improvements?

• What causes roof shingles to curl up at corners?  

• What causes shingles to buckle along a line on the roof?

• What causes leaks at a fake roof dormer? 

• What causes a sagging roof ridge line?

• What causes bubble-like blisters in a built-up and gravel roof?  

• Why does it cost so much more to replace a steep roof than a low slope roof? 

• What is "ponding" on a flat roof?

• What causes a lump or dip in the roof? 

• If my roof is not leaking, why does it need to be replaced?

• How can I be sure my roofing contractor got a permit?

• How many layers of roofing are allowed on a home? 

• What are the dark lines running parallel to shingles on my roof?

• Can metal roofing be used on a low slope/pitch roof? 

• How can I make my roof last longer?  

• What are the warning signs of a dangerous attic pull-down ladder?

• How can I find out the age of a roof? 

• Should I buy a house that needs a new roof?

• Should I buy a house with an old roof? 

• What are those metal boxes on the roof?

• What does "lack of tab adhesion" in an asphalt shingle roof mean?

• Why do roof edges start leaking?

• Why do my dormer windows leak? 

• Do home inspectors go on the roof? Do they get in the attic?

• Should I put gutters on the house? 

• How can I tell if the house needs a new roof?  

• Why does my homeowner's insurance want a roof inspection?

• What are the hazards to avoid when going into an attic? 

     Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

Photo - W.R. Grace & Co.