What is Roof Underlayment?
Roof underlayment is the water-resistant membrane installed directly on the roof deck, beneath the shingles. It is the secondary waterproofing layer that protects the home if wind, ice, or a damaged shingle lets water past the surface. In Maryland and Virginia, code requires underlayment under every asphalt-shingle roof.
Jim Dodson, HAAG Master Inspector · #992109047 Roof underlayment is the water-resistant or waterproof membrane installed directly on the roof deck, beneath the shingles. It is the secondary line of defense: when wind, ice, or a torn shingle allows water past the surface, the underlayment keeps it from reaching the deck. The two common types in Maryland and Virginia today are synthetic underlayment (woven polypropylene or polyester) and 15-pound or 30-pound asphalt-saturated felt. Synthetic is now the JDH standard.
What it does
Underlayment is the secondary waterproofing layer of a roof. Shingles are the primary defense, but they are not waterproof on their own. They shed water by overlapping. When wind drives rain sideways, ice dams force water uphill, or a tree limb tears a shingle off entirely, the underlayment is what stops that water from soaking into the OSB roof deck. It also serves as a temporary weather barrier during installation, before the shingles go on. Without underlayment, a single torn shingle in a Mid-Atlantic storm can drop water directly into your attic.
Where it sits
Underlayment is sandwiched between the shingles (above) and the roof deck (below). It runs in horizontal courses, starting at the eave and working up to the ridge, with each upper course overlapping the lower course by 2 to 6 inches depending on slope. At critical areas (eaves, valleys, around chimneys and skylights), underlayment is supplemented or replaced by ice and water shield, a self-adhered waterproof membrane.
Common problems with underlayment
15-pound felt on a 30-pound job
Some contractors install the thinner, cheaper 15-pound felt when 30-pound or synthetic was specified. Same look from the curb; very different performance under wind-driven rain. Visible only at tear-off.
Insufficient overlap at horizontal seams
Code requires a minimum 2-inch overlap between courses, more on lower slopes. Roofers rushing the install often skim that. Water then channels along the seam onto the deck during a storm.
Felt left exposed too long before shingles
Asphalt-saturated felt degrades in sunlight within days. If a job is interrupted by weather and the felt sits exposed for a week, it can wrinkle, tear, or lose its water-resistance. Re-do, not install over.
Wrong product for the slope
Low-slope roofs (between 2:12 and 4:12 pitch) require double underlayment or ice and water shield over the entire deck. Standard single-layer install on a low slope is a code violation and a leak waiting to happen.
How to inspect underlayment on your home
Once shingles are installed, underlayment is invisible from the outside. The only ways to evaluate it are during install, at tear-off, or by inferring from the symptoms above. Things to verify when getting a new roof:
- Get the underlayment named on the contract: "synthetic underlayment" is not enough. Look for a specific brand and product (e.g., Owens Corning ProArmor, GAF FeltBuster, etc.).
- Confirm the weight or class on the contract: 15-pound vs. 30-pound felt, or the synthetic equivalent. Cheaper products are not equal substitutes.
- Look for it during the install: if you can see the deck after tear-off, take a photo before the new underlayment goes down. Then photograph the underlayment before shingles cover it.
- Watch for water stains on attic decking from underneath: dark streaks on the underside of the OSB deck mean underlayment is failing or was installed wrong.
Getting a new roof? Verify the underlayment.
A HAAG-Certified JDH inspector will document the existing roof condition, confirm the underlayment specified in your contract is what actually gets installed, and photograph each layer before shingles cover it. About 1 in 4 inspections result in no recommended work because we are not paid on commission.