Synthetic Underlayment Vs. Roofing Felt

Synthetic underlay has a clear advantage over roofing paper when installing or renovating a roof.
Synthetic underlay has a clear advantage over roofing paper when installing or renovating a roof. (Image: Jupiterimages/ Images)

Home and property owners are faced with a variety of options when it comes to building a new roof or renovating an existing one. One option is what to place underneath the roofing shingles. Traditionally, this material was nearly always roofing felt. The introduction of several alternatives of roofing underlay, however, provides builders and/or renovators with a choice in materials.

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About Underlay

Synthetic underlay gains its name because unlike felt, which uses partially organic fibers, underlay is 100 percent manufactured from polypropylene fibers interlaced with polymer. These fibers bend and twist extraordinarily well around corners and crevices without ripping or tearing, two common complaints about felt paper.

About Felt

Felt roofing is also known in the contracting industry as roof paper or tar paper and is soaked in a petrol-based chemical that’s been interwoven with asphalt-coated organic fibers. This makes roofing felt extremely sticky to work with but overall slippery to walk on, a hazard when walking across the pitch of a roof. Unlike synthetic underlay, roof felt is also more flammable by comparison.

Physical Differences

The weight differential between synthetic underlay and roof felt is significant. Underlay weighs approximately six times less than standard roofing felt, weighing in at 28 pounds per 10 squares versus 275 pounds covering the same amount of space. This not only reduces the physical strain of hauling rolls of the material up onto the roof but reduces the amount of weight the walls and buttresses must bear. Felt’s weight increases should it be exposed to moisture, whereas underlay is entirely waterproof. Absorbed moisture also contributes to mold growth and insect permeation, damages not found by using underlay which doesn’t rot. Underlay is also highly resistant to ultraviolet rays which tend to warp exposed areas of roofing paper.

Installation Differences

Rolls of roofing paper come in thinner rolls, meaning more rolls are required to cover the same distance. In terms of costs, underlay has the upper hand if you’re paying a contractor for labor hours. Roofing paper, however, is by the roll, somewhat cheaper than underlay when purchasing roofing materials. Most rolls of each come preprinted with grids that show installers the exact points of where to nail. The bottom side of underlay features a self-adhering surface to make installation easier.


  • Black & Decker the Complete Guide Roofing, Siding & Trim; Chris Marshall
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