Most steep-slope roofs have what is called an underlayment – an additional layer of material between the shingles and the roof deck. It is not waterproof, but adds a measure of water protection to the installed roof, as well as protection from resins and other chemicals in the wood decking that can leech into and damage the asphalt shingles. An underlayment can also provide a temporary shield in the event of storm damage and can help with heat and sound insulation.
The two most common types of underlayments are felt and synthetic. Traditional felt underlayment usually contains cellulose or fiberglass felt that’s been treated with asphalt and other chemicals. It is heavier than synthetic underlayment, more labor intensive to install, and can degrade more easily.
Synthetic underlayment was developed as an alternative to felt, and it is made of polyester, fiberglass, or polyethylene. Synthetics typically provide longer-lasting leak protection than conventional felt materials (which can rot or become brittle before the shingles reach the end of their lifespan). Synthetics are also lightweight, safer for roofers to walk on while they’re installing roofing shingles, and better able to stand up to high winds in case shingles are blown off during a storm.
As you might expect, synthetic underlayment is more expensive than traditional felt – although the cost of synthetic is getting more competitive. Are the performance advantages of synthetic worth the additional expense? After all, felt underlayment has been in use for a long time and has a solid performance history. The answer depends on several factors – climate, roof slope, preference of the contractor and, perhaps most important, the project budget.