Roof Underlayment: Synthetic vs Felt for Southern California

Underlayment is the layer nobody sees and everybody blames. It sits between the deck and the shingles, it keeps water out while the roof is open, and it is the last defense when wind drives rain under a tile or a shingle lifts. Southern California asks something unusual of it: months of blistering UV exposure and dry heat, then a handful of intense winter storms. This guide covers how synthetic and felt underlayment differ, and which one belongs on your deck before the rains arrive.

Which underlayment should you use?

For most steep-slope roofs in Southern California, use synthetic. It handles long UV exposure while the job is open, it does not wrinkle or tear the way felt does in heat, it covers far more area per roll, and it is safer to walk on. Felt still makes sense on small repairs, on jobs where a specification calls for it, and where budget drives the decision. Low-slope roofs are a different system entirely — those need modified bitumen membranes, not underlayment.

What synthetic underlayment actually is

Synthetic underlayment is a woven or spunbond polypropylene or polyethylene sheet, usually coated on both faces. It is a plastic fabric, not a saturated paper, which is why it behaves so differently on a hot deck.

The ProArmor synthetic roof underlayment roll is the reference product in the Roof Underlayment collection, and its spec sheet reads like a list of answers to Southern California problems. It carries a fusion back coating for slip resistance on both surfaces, resists stretching and tearing around fasteners, withstands UV exposure for up to 90 days, and works under both asphalt and metal roofing. It comes in 42-inch rolls covering roughly ten squares at about 25 lb per roll, with printed fastening guides, and carries a ten-year limited warranty.

Three of those numbers matter more than the rest in LA. Ninety days of UV tolerance means a roof can sit dried-in through a permit delay, an inspection backlog, or a materials wait without the underlayment cooking. Ten squares per roll means dramatically fewer roll changes and seams than felt across a whole roof. And 25 lb per roll means one person can carry it up a ladder, which changes how a small crew staffs a tear-off.

Where felt still belongs

Asphalt-saturated felt is organic or fiberglass mat soaked in asphalt. It has roofed North America for a century and it still works. The 216 sq. ft. felt underlayment roll covers roughly two squares, which is a sensible unit for a patch, a small addition, a shed, or a section of roof being repaired rather than replaced.

Felt is also what many older specifications and some tile assemblies call for by name. If the plans say felt, use felt — substituting a synthetic without approval is the kind of thing that surfaces during a warranty claim.

The limitations are real, though, and they are worse in this climate than almost anywhere. Felt absorbs moisture and wrinkles, then dries and shrinks; on a hot inland deck it can start cupping within days of being laid. It tears at fasteners in wind. It becomes slick and brittle. And left exposed to Southern California sun for more than a short window, it dries out and loses the asphalt that made it waterproof in the first place.

Low-slope roofs need a different product

Plenty of LA housing stock — mid-century flat-roofed homes, additions, garages, and commercial buildings across the basin — has slopes too shallow for shingles. On those roofs, underlayment is not the answer; a built-up or modified bitumen membrane system is.

The collection carries that system as a set. A base sheet such as the Tri-Ply #75 in 3 ft. x 100 ft. rolls goes down first, a smooth modified bitumen APP membrane builds the waterproofing layer, and a granular cap sheet finishes it. The white granular cap sheet options matter in Southern California specifically: a reflective surface on a flat roof cuts cooling load in a climate where that is a year-round cost, and reflective roofing intersects with Title 24 requirements on many projects.

Self-adhering starter rolls handle the transition details — eaves, rakes, and the first course — where wind-driven water is most likely to get underneath.

Fastening and installation for SoCal conditions

Cap nails, not staples. Plastic or metal cap fasteners spread the load and stop the underlayment from tearing free in Santa Ana wind events. Follow the printed fastening pattern on synthetic rolls rather than eyeballing it; the spacing is what the tear-strength rating assumes.

Lap correctly and lap generously. Follow the manufacturer's printed overlap lines, run laps so water sheds over rather than into the seam, and pay extra attention at valleys, penetrations, and the eave line where most leaks actually start.

Start on a clean, dry, smooth deck. Both product types require a properly prepared deck free of protruding nails and depressions — a nail head left proud will wear a hole through underlayment from below over a few seasons of thermal cycling. If the deck itself is being replaced, our guide to sheathing vs. structural OSB covers panel selection, and radiant barrier OSB is worth considering on a re-deck in a hot valley.

The wildfire angle

In high fire hazard severity zones and the wildland-urban interface, the roof is the assembly that matters most. Chapter 7A drives the whole roof assembly — covering class, deck, edge details, and the vents that penetrate it — and underlayment is one component within a listed assembly rather than a free choice. Our guide to fire-hardening your LA property with WUI and Chapter 7A compliant materials covers how those assemblies are put together. Confirm the specific assembly listing before you substitute any layer on a roof in a mapped fire zone.

Estimating and ordering before the rains

Roofers count in squares: one square is 100 square feet of roof area. Take the roof area, add for waste at hips, valleys, and starter courses, and add the overlap you lose at every lap — a common working figure is ten to fifteen percent over the flat area, more on a cut-up roof with dormers and multiple planes.

Then do the roll math. A synthetic roll covering ten squares replaces roughly five felt rolls, which is fewer seams, fewer roll changes, and less lifting. On a 30-square house that is three synthetic rolls against fifteen felt rolls.

Timing is the LA-specific part. The dry season is when roofs get opened, and the window closes in late fall. Underlayment demand spikes right before the first storms every year, so order the whole roof at once rather than in stages. Browse the full Roof Underlayment collection and the broader Roofing & Facade collection for shingles, membranes, and accessories, with delivery across Los Angeles County. If shingle selection is still open, our guide to choosing roof shingles and colors for LA homes covers the layer above.

Frequently asked questions

How long can underlayment stay exposed before shingles go on?

The ProArmor synthetic is rated for up to 90 days of UV exposure. Felt should be covered within days, not weeks — check the specific product's rating and treat it as a maximum, not a target.

Can I install synthetic underlayment over old felt?

Roofing over existing underlayment is generally a bad idea and often not permitted. Both products require a clean, dry, smooth deck; old felt hides deck damage and prevents proper fastening.

Do I need underlayment under tile roofs?

Yes, and on tile the underlayment does more of the waterproofing work than it does under shingles, since tile sheds water rather than sealing it out. Tile assemblies often specify a particular underlayment type — follow the assembly.

Is synthetic worth it on a small repair?

Usually not. A two-square felt roll is the right unit for patching a section. Synthetic earns its keep across a whole roof, where the coverage, walkability, and UV tolerance compound.