What's the difference between 15-lb felt and 30-lb felt?

Weight per 100 sq ft (a 'square'). 15-lb felt weighs 15 pounds per square; 30-lb is heavier and roughly 2x thick. Both are asphalt-saturated paper. 15-lb is the absolute cheapest underlayment and has been industry standard since the 1930s. 30-lb is slightly more durable. Neither is competitive with modern synthetic — they're 80-year-old technology being installed because they're $10-15 per square cheaper.

What's synthetic underlayment?

Polypropylene or polyethylene-based sheets engineered specifically as a roof underlayment. Sub-categories include GAF Tiger Paw, GAF Deck-Armor, Owens Corning ProArmor, Carlisle WIP-300HT. Synthetic underlayment is 6-10x stronger than felt, lighter to handle, doesn't wrinkle in summer heat, doesn't tear when it gets wet, and lasts 30-50 years vs 20-30 for felt. Costs ~$30-50 per square installed vs ~$15-25 for felt.

Do I need peel-and-stick (ice & water shield) in SoCal?

SoCal doesn't get ice damming, but peel-and-stick underlayment (GAF WeatherWatch, etc.) is still required by California Building Code in valley flashings, around skylights, around chimneys, and along the eaves on steeper pitches and in coastal/wet zones. Premium installs cover the full roof with synthetic + add peel-and-stick at the critical leak zones. A 'budget' install skips the peel-and-stick.

Why do contractors still install felt?

Price. 15-lb felt costs the contractor $8-15 per square; synthetic costs $25-40 per square. On a 25-square house, switching from felt to synthetic adds $400-625 to the contractor's material cost. Most low-bid quotes use felt to keep the bid number low. The homeowner pays back the difference (and more) when the roof underlayment fails 10 years sooner than it should.

Will the homeowner ever see the underlayment?

Practically never. The underlayment goes down on top of the roof decking and gets covered by shingles or tile. You only see it during a tear-off or a serious leak event. That's exactly why cheap contractors save money there — homeowners can't physically inspect it after install. The only way to know is to ask before signing the contract.

Roof Underlayment: The Invisible Layer That Decides How Long Your Roof Lasts

The shingle is the visible roof. The underlayment is the actual waterproof layer. When the shingle fails (storm damage, tree branch, foot traffic), the underlayment is what keeps water out of your house. And it’s the line item that most cheap roofing bids quietly downgrade — saving the contractor $400 and costing the homeowner 5-10 years of roof life.

What underlayment does

Underlayment is a continuous water-resistant layer between the roof decking (the plywood or OSB sheathing) and the visible roofing material (shingles, tile, metal panels). Three jobs:

Secondary waterproofing. Catches water that gets past the shingle or tile from blow-back wind, ice dam, or punctured shingle.
Vapor barrier. Stops moisture rising from inside the attic from condensing on the underside of the shingle.
Slip surface. Lets shingles or tile shift slightly with thermal cycling without dragging the sheathing.

The four underlayment types

1. 15-lb felt (asphalt-saturated)

The original underlayment, since the 1930s. Weight: 15 pounds per square (100 sq ft). Asphalt-impregnated paper. Cheap, easy to install, but:

Tears easily during install (especially in wind)
Wrinkles in summer heat (the wrinkles telegraph through the shingles within a couple of years)
Absorbs water once torn (rots the decking beneath)
20-30 year service life

Felt is the SoCal industry default for builder-grade installs and the cheap-bid go-to. The 80-year-old technology never got better.

2. 30-lb felt

Same idea as 15-lb but heavier — 2x the thickness. Slightly more durable, slightly tougher to install, slightly longer life. Used on tile and steeper pitches where slip is a concern. Still asphalt-paper, still 80-year-old tech, but a step up from 15-lb.

3. Synthetic underlayment (the modern standard)

Polypropylene or polyethylene-based sheets engineered as underlayment. Common products:

GAF Tiger Paw™— TMC’s default. Slip- resistant walking surface, 6 months UV exposure rating, lays flat, doesn’t wrinkle.
GAF Deck-Armor™ — Vapor-permeable version for high-humidity coastal applications.
Owens Corning ProArmor™ — Equivalent synthetic option.

Synthetic underlayment is the right answer for almost every SoCal residential roof:

6-10x stronger than felt — survives install + storm without tearing
Doesn’t absorb water
Doesn’t wrinkle in heat
30-50 year service life
Lighter to handle (lower install labor cost partially offsets the higher material cost)

4. Peel-and-stick (ice & water shield / leak barrier)

Self-adhering rubberized asphalt membrane. Stickier-back, applied in critical zones (valleys, around penetrations, eaves on coastal applications). Common products:

GAF WeatherWatch® — The most common premium leak barrier in SoCal.
GAF StormGuard® — Standard-tier leak barrier.

Peel-and-stick is required by California Building Code in valley flashings, around skylights, around chimney bases, and along eaves on steeper pitches. Premium installs (TMC default) extend peel-and-stick coverage to additional zones — especially on coastal homes where wind-driven rain is a real concern.

The right SoCal underlayment system (what TMC installs)

On every standard residential reroof, TMC’s system is:

Full coverage: GAF Tiger Paw synthetic underlayment, lapped 4-6 inches at horizontal seams.
Valley flashings: GAF WeatherWatch peel-and- stick extending 36 inches into the valley + onto the field on both sides.
Around penetrations: Peel-and-stick collar around every vent stack, chimney, skylight.
Eave starter zone: Peel-and-stick along the first 24-36 inches above the eave on coastal applications and steeper pitches.
Drip edge: GAF Pro-Start metal edge installed ON TOP of the underlayment at the rake and UNDER the underlayment at the eave (per GAF spec).

The line item to watch on roofing quotes

When you compare roofing bids, the underlayment line item is one of the fastest tells about quality:

“15-lb felt” — Cheap. Walk away (or ask them to upgrade and re-quote).
“30-lb felt” — Cheap-mid. Better than 15-lb but still 80-year tech.
“Synthetic underlayment” — Mid-tier. Acceptable, but ask which synthetic.
“GAF Tiger Paw synthetic + WeatherWatch peel-and- stick at valleys and penetrations” — Premium. The baseline for any Golden Pledge eligible install.
Not mentioned in the quote at all— Reddest of red flags. They’re installing the cheapest felt they can find and not telling you.

The decking question

Underlayment sits on top of decking — the structural plywood or OSB sheathing. If the decking is rotted at tear-off, no underlayment will help. TMC always:

Inspects every sheet of decking after tear-off
Publishes a per-sheet decking replacement rate on the original quote (typically $110-$170 per 4×8 OSB sheet)
Photo-documents any replaced sheets
Sends the homeowner the photos for the records

A roof with rotted decking and good underlayment fails. A roof with sound decking and bad underlayment fails too. Both matter.

The 7-year math

On a typical 25-square asphalt roof:

15-lb felt install: $400-$500 in underlayment material. ~20-25 year underlayment life.
GAF Tiger Paw + valley peel-and-stick install: $800-$1,200 in underlayment material. ~35-50 year underlayment life.

The upgrade costs $400-$700 once. It buys you 10-25 extra years on the underlayment, which buys you a full re-felt cycle on a tile roof, or a clean tear-off on an asphalt roof. The math is one of the most obvious wins in roofing.

Next step

If you’re evaluating roofing quotes, look at the underlayment line item first. If you’re replacing, request a written quote from TMC — every TMC quote names the exact underlayment SKU + peel-and-stick coverage zones. Or call (951) 840-9935.