What is Roof Underlayment?
Roof underlayment is the continuous water-resistant sheet installed across the entire roof deck, directly beneath the shingles. It is the secondary layer of weather protection: if water gets past the shingles (during wind-driven rain, a small leak, a high-wind event, or normal end-of-life shingle wear), the underlayment shed it down to the gutter before it reaches the wood deck. Modern underlayment is synthetic polypropylene; older roofs use asphalt-saturated felt. Underlayment over the whole deck is required by code on every shingle roof.
The cyan layer between the shingles and the plywood roof deck is the underlayment. It runs across the entire deck, edge to edge and ridge to eave, as a continuous water-resistant sheet.
What is the most common roof underlayment?
Today, synthetic polypropylene underlayment is the most common product on new residential roofs in Virginia and Maryland. It replaced asphalt-saturated felt (the original “tar paper”) starting around 2010 and is now installed on roughly 80 percent of new roofs nationally. Synthetic is lighter, stronger, less slippery, more resistant to UV exposure during the install window, and stays in service longer.
You will still see felt on some budget jobs and on older roofs being repaired in place. There is nothing inherently wrong with quality felt installed correctly, but synthetic is a clear upgrade in every measurable way and the installed price difference is small.
Synthetic vs felt underlayment
Direct comparison.
| Synthetic Underlayment | Asphalt-Saturated Felt | |
|---|---|---|
| Material | Woven polypropylene | Cellulose or fiberglass mat saturated with asphalt |
| Weight per roll | About 25 to 30 lb (10 squares) | About 60 lb (4 squares) |
| Coverage per roll | 10 squares | 4 squares (15 lb felt) or 2 squares (30 lb felt) |
| Walkability | Anti-slip surface, safer for crew | Slippery when wet or hot |
| UV exposure tolerance | Up to 6 months exposed | A few days to a few weeks |
| Tear strength | Very high (won’t tear at fasteners) | Moderate (can tear at fasteners in wind) |
| Service life on the roof | 25 to 50 years | 15 to 25 years |
| Installed cost per square | About 10 to 15 percent more than felt | Baseline |
On a typical roof, synthetic underlayment adds a few hundred dollars over felt and is a strict upgrade. There is almost no scenario where felt is the better choice on a new install.
Is roof underlayment necessary?
Yes, and it is required by code. The International Residential Code section R905.1.1 requires underlayment over the entire roof deck on every asphalt shingle roof, with specific minimum standards for the product type and the way it is fastened. Skipping it (or doing only partial coverage) is a code violation that fails inspection and voids the shingle warranty.
The functional reason is that shingles are not perfectly watertight on their own. They shed water as long as wind is calm and the roof is not damaged. Wind-driven rain pushes water sideways under shingle edges; small punctures or end-of-life wear let water through; ice dams force water uphill underneath the shingle field. Underlayment catches all of that and channels it down to the gutter.
How often does roof underlayment need to be replaced?
Underlayment is replaced every time the shingles are replaced. It is not removed and reused; the existing underlayment comes off with the old shingles and the new shingles go onto fresh underlayment. This is why a proper roof replacement is a full tear-off, not an overlay.
An overlay (new shingles laid on top of old shingles without removing them) leaves the original aged underlayment trapped underneath. Code permits one overlay layer in some jurisdictions, but the underlayment is then 20 plus years old and likely brittle. DreamHome does not do overlays for this reason; the savings vanish when the next replacement is needed.
What DreamHome installs
DreamHome installs synthetic polypropylene underlayment across the entire roof deck on every replacement, fastened per IRC R905.1.1 with capped fasteners at the spec interval. Ice and water shield (a separate product, a fully self-adhering rubberized membrane) goes in valleys, around penetrations, and at the eaves where ice dams form. Together, the two-product stack covers every part of the deck.
Underlayment is included in the base price; it is never an upsell. The product brand and weight (and the ice-and-water-shield coverage zones) are documented on the work order so the homeowner can verify what was installed.
Red flags on someone else’s quote
- Felt only. Acceptable on a budget repair; on a full replacement it is a quiet cost-cut that shortens the roof’s life and lowers your resale story.
- Partial coverage. “Underlayment in valleys and at the eaves only” is a code violation. The whole deck has to be covered.
- No mention of ice and water shield at the eaves. Mandatory for ice dam protection in Virginia and Maryland (IRC R905.1.2 climate zone trigger). If it is missing from the quote, ask.
- Overlay quoted at a steep discount. Skipping the tear-off saves money today and traps the old underlayment underneath; the next replacement is much more expensive.
- Underlayment brand not specified. Industry-standard practice is to name the product (e.g., Owens Corning ProArmor, GAF FeltBuster, etc.). A spec that does not name the product is a spec that can be substituted at install time.