Synthetic underlayment is a woven or spun polymer sheet that replaces traditional asphalt felt under shingles, tile, or metal. It is lighter, much stronger against tearing, more slip-resistant to walk on, and handles a few days of weather exposure better. For most Bay Area reroofs it is the better choice, paired with self-adhered membrane at valleys and penetrations.
Key takeaways
- Synthetic underlayment has largely replaced felt on new residential roofs.
- It resists tearing in wind and holds up better if rain arrives mid-job.
- It does not replace self-adhered membrane in valleys and around penetrations.
- On tile and metal roofs, underlayment choice matters even more because it does most of the waterproofing.
What underlayment does
Shingles, tile, and metal shed water, but wind-driven rain and water that backs up at flashings can still get underneath. Underlayment is the second line of defense. It also keeps the deck dry during installation, between tear-off and the covering going on.
Felt vs synthetic
| Property | Asphalt felt | Synthetic |
|---|---|---|
| Weight per roll | Heavy | Light, more coverage per roll |
| Tear resistance | Tears in wind and under foot | High |
| Wrinkling when wet | Common, can telegraph through shingles | Lies flat |
| Short-term exposure | Limited | Rated for a set exposure period by the manufacturer |
| Walking surface | Slippery when dusty or wet | Many have a textured walking surface |
| Cost | Lower material cost | Higher material cost, often offset by labor savings |
Why synthetic fits Bay Area conditions
Wind. Ridge-top homes in the Oakland and Berkeley hills, along Skyline, and on the Peninsula take strong gusts. Felt can tear loose before shingles cover it. Synthetic stays put.
Surprise rain. Bay Area weather is mostly dry from late spring to early fall, but showers arrive early some years. If a section is dried-in with synthetic underlayment and a storm hits, the deck stays protected for the short time it takes to finish.
Fog and moisture. On coastal roofs where mornings are damp, felt can absorb moisture and wrinkle. Synthetic lies flat, which gives a flatter finished roof.
Heat. Under metal roofing in hot inland areas, a high-temperature-rated underlayment is the right choice. We match the underlayment to the covering and the manufacturer’s requirements.
Installation details that decide performance
Good underlayment installed poorly still leaks. The details we watch on every job:
- Lap direction. Every sheet laps over the one below it, like shingles, so water always runs over a seam rather than into it.
- Lap widths. Side and end laps follow the manufacturer’s minimums, which are printed on many synthetic products as guide lines.
- Fasteners. Most synthetics call for cap nails or cap staples, which have a wide plastic or metal head that resists tearing. Plain staples can pull through in wind.
- Flat installation. Wrinkles and bubbles show through thin shingles and create channels for water. The sheet should lie tight to the deck.
- Edge order. At the eaves, drip edge goes under the underlayment; at the rakes, it goes over.
- Turn-ups. At walls and chimneys, underlayment turns up the vertical surface a few inches before flashing goes on.
Questions to ask your roofer about underlayment
Underlayment is a line item that is easy to cut without the homeowner noticing, because it disappears under the shingles within hours. Ask which brand and product they plan to use, whether it is approved by the shingle or tile manufacturer, where self-adhered membrane will go, and how long the product can stay exposed. A contractor who answers with specifics has thought about the system underneath, not just the shingles you will see from the street.
Where synthetic is not enough on its own
Mechanically fastened synthetic underlayment has fasteners through it. At the places where water concentrates or moves slowly, we use self-adhered membrane instead, which seals around each nail:
- Valleys
- Around chimneys, skylights, and pipes
- Low-slope transitions and dormers
- Roof-to-wall intersections
Our article on what’s inside a quality roof system shows where each layer goes.
Underlayment on tile roofs
With concrete or clay tile, water regularly runs under the tiles, and the underlayment carries it to the eaves. When tile roofs leak, worn-out underlayment is usually the cause, not the tile. That is why a tile reroof often means lifting the tiles, installing new heavy-duty underlayment, and relaying the tile. See tile roof underlayment replacement.
Fire-rated assemblies
In very high fire hazard severity zones, California requires a Class A roof assembly. Some coverings earn that rating only when installed over a specific underlayment or cap sheet listed by the manufacturer. In wildfire-exposed areas of Marin, the East Bay hills, and the Santa Cruz Mountains edge, we install the assembly exactly as listed. The Class A roofing answer page explains the rating.
Exposure limits are real
Synthetic underlayment is not a finished roof. Each product lists how long it can sit exposed to sunlight before UV starts degrading it. We plan work so the covering goes on well inside that window. If a job pauses, the underlayment should not be left baking in the sun for weeks.
What it means for warranties
Shingle manufacturers often have stronger warranty options when their own accessories, including underlayment, are used as a full system. Read the specific terms, and see our article on registering a manufacturer warranty. We list the underlayment brand and type in every estimate so you know exactly what you are getting.
Underlayment is part of every new roof installation and roof replacement we do, and it is photographed before shingles go over it. If you are in Daly City or Redwood City and want to see it on your own roof, ask.
Get a free inspection and written estimate that names the underlayment we would use on your roof: request a quote or call (415) 410-7917.
Frequently asked questions
Is synthetic underlayment better than felt?
For most residential reroofs, yes. It is stronger, lighter, lies flatter, and tolerates short-term exposure better. Felt still works, but synthetic has become the standard on quality installs.
How long can synthetic underlayment be left exposed?
It depends on the product. Each manufacturer lists a maximum UV exposure period. A professional crew covers it well inside that window.
Does underlayment need to be replaced when I replace shingles?
Yes. During a tear-off, the old underlayment comes off with the shingles and new underlayment is installed over the repaired deck.
Do I need self-adhered membrane if I use synthetic underlayment?
Yes, in valleys, around penetrations, and at roof-to-wall transitions. Synthetic covers the field; self-adhered membrane seals the spots where water concentrates.
Can synthetic underlayment be used under tile and metal?
Yes, but the product has to suit the covering. Tile roofs need a durable underlayment because water runs under the tiles regularly, and metal roofs in hot areas often call for a high-temperature-rated product. We follow the covering manufacturer’s list of approved underlayments.
