Synthetic vs. Felt Underlayment: What to Put on Every Job

Underlayment is the layer that saves your job when the shingles fail, the wind lifts a tab, or the snow melts backward. It's also the layer crews are most tempted to cheap out on — and the one that quietly causes leaks a year later.

What Underlayment Actually Does

Shingles are the first line of defense, not the last. Underlayment is the secondary water barrier that protects the deck when water gets past the surface — during wind-driven rain, at a slipped shingle, or before the roof is dried in.

It does three jobs:

• Sheds water the shingles let through.
• Protects the deck during installation and in a storm.
• Backs up your details at valleys, penetrations, and eaves where leaks start.

Skip it or spec it thin and you've built a roof that only works as long as every shingle is perfect. They won't be.

Felt: The Old Standard

Asphalt-saturated felt — the classic 15 and 30 pound rolls — has covered roofs for a century. It still works, and it's cheap.

Where felt holds up:

• Lowest material cost per square.
• Familiar to every crew.
• Fine on simple, short-timeline jobs that get covered the same day.

Where felt hurts you:

• It tears easily underfoot and in wind, especially the 15 pound.
• It wrinkles and buckles when it gets wet, telegraphing through the shingles.
• It degrades fast if left exposed — a rain delay can ruin a felt-covered deck.

Felt isn't wrong. It's just a lower floor with a narrower margin for error.

Synthetic: The Modern Default

Synthetic underlayment is a woven or spun polymer. It costs more per roll but changes the math on the roof.

Why crews switch:

1. Lighter rolls cover more square footage, so fewer trips up the ladder.
2. Won't tear underfoot and grips better, which is a real safety and speed gain.
3. Handles exposure far longer if the job gets weathered in.
4. Lays flat and won't wrinkle through the shingles.

The higher material cost is often offset by faster, safer installation and dramatically lower callback risk. For most contractors, synthetic is now the standard field underlayment — and a selling point when you explain it to the customer.

The cheapest underlayment is the one that doesn't send you back to fix a leak.

Ice-and-Water: Non-Negotiable Zones

This is a different product and a different purpose. Ice-and-water shield is a self-adhering membrane that seals around fasteners, and it belongs in the leak-prone zones regardless of what field underlayment you run:

• Eaves in any climate that sees ice dams. IRC R905.1.2 puts the ice barrier from the lowest edge of the roof to a point 24 inches inside the exterior wall line, measured horizontally — so on a steep roof that is more than one course of membrane. Plenty of jurisdictions amend it upward, so check yours.
• Valleys, where water concentrates and moves fast.
• Around penetrations — pipes, chimneys, skylights.
• Low-slope sections and wall flashings.

Check your local code for the eave requirement, then treat valleys and penetrations as mandatory on every job whether code names them or not. This is where roofs leak, and this membrane is what stops it.

Speccing It Right on Every Quote

A clean underlayment spec is three decisions, made the same way every time:

1. Field underlayment — synthetic for most work; note when a customer's budget forces felt so it's a documented choice, not a surprise.
2. Ice-and-water zones — eaves per code (IRC R905.1.2 to 24 inches inside the exterior wall line as the baseline, plus any local amendment), plus all valleys and penetrations, as a standing rule.
3. Line-item both so the customer sees the full system and understands they're not just paying for shingles.

Under-speccing to win a bid on price is a false economy. The leak that comes back doesn't care that you saved forty dollars on the roll — it costs you a return trip, a reputation hit, and sometimes the deck.

Running a Sharper Business

Underlayment is a small share of the job cost and a huge share of whether that job ever comes back to haunt you. The contractors who never argue about a mystery leak are the ones who standardized their underlayment spec instead of deciding it fresh on every roof. RidgeSync makes that automatic — put your synthetic and ice-and-water products in your catalog once, and every estimate and bill of materials builds them in at the right quantities from a single measurement. There's a 30-day free trial if you want to spec your next roof in it. Get the layer under the shingles right, and the shingles get to just be shingles.