Roof Underlayment Guide: Felt vs Synthetic vs Ice-and-Water

By the Orlando Roof Inspection team · Updated November 2026 · 9 min read

Roof underlayment is the layer that goes under your shingles, between the roof deck (the plywood or OSB) and the final roof covering. Its job is simple but vital: it acts as a secondary water barrier that keeps rain out if wind lifts a shingle or water gets past the surface. There are three main kinds in use today, and they are not interchangeable: asphalt-saturated felt, modern synthetic, and self-adhered ice-and-water shield. This guide covers what goes under shingles, how the three types compare, where each one belongs, and roughly what they cost.

1. What underlayment is and why it matters

Underlayment is a water-resistant sheet that sits directly on the roof deck, under the shingles or tiles. Think of it as the roof's insurance policy: shingles are the first line of defense, and the underlayment is the backup that stops water once a shingle fails, lifts in the wind, or is removed during a repair.

It matters for three reasons. First, no shingle roof is perfectly watertight forever, and wind-driven rain can push moisture past the surface. Second, the underlayment protects the deck while the roof is being installed, before the shingles go on. Third, most asphalt shingle warranties and building codes require an approved underlayment, so skipping or shortcutting it can void coverage. If you are planning a new roof, our roof replacement process guide shows where this layer fits into the job.

2. The three main types

There are three underlayment families, and a single roof often uses two of them together. The felt and synthetic go across the field of the roof, while ice-and-water shield is used in the high-risk spots.

• Asphalt-saturated felt (15 lb and 30 lb). The traditional "tar paper." It is a paper or fiberglass mat soaked in asphalt for water resistance. The 30 lb version is thicker and tougher than the 15 lb, so it holds up better during installation, per Hunker.
• Synthetic underlayment. Woven polypropylene or polyethylene sheets. Now the most common type on new residential roofs because it is lighter, tears less easily, and resists water and mold.
• Self-adhered ice-and-water shield. A rubberized-asphalt, peel-and-stick membrane that bonds to the deck and seals around the nails driven through it. It is fully waterproof but is used only in specific areas, not across the whole roof.

Water-resistant is not the same as waterproof. Felt and most synthetics shed water but are not sealed at every fastener, while ice-and-water membrane is described as completely waterproof because it self-seals around nails, according to Hunker.

3. Felt vs synthetic

Synthetic beats felt on weight, tear strength, and UV exposure, while felt wins mainly on upfront price. That is the short version, and for most new roofs synthetic is now the default choice.

On weight, 15 lb felt runs about 7.5 to 12.5 pounds per roofing square (100 square feet) and 30 lb felt about 16 to 27 pounds, per Western States Metal Roofing. Synthetic is far lighter. Owens Corning notes it can be up to four times lighter, so a few rolls of synthetic cover what once took more than a dozen rolls of felt. That lighter weight means fewer rolls to carry up a ladder and faster installation.

On durability, synthetic has very high tear strength compared with felt, which is prone to tearing in high winds and during installation, again per Owens Corning. UV and weather exposure is the biggest gap. Felt should go under shingles the same day, because sunlight dries it out and it can fail if left exposed, while many synthetics can stay exposed for roughly 2 months up to 30 months and still stay watertight, per Western States Metal Roofing. Felt also absorbs water and can wrinkle, which makes shingles harder to lay flat. Its one edge: felt may seal a little better around individual nail holes. For how underlayment choice plays into material decisions, see our metal roof vs. shingles comparison.

4. Where ice-and-water membrane is used

Ice-and-water shield is installed in the leak-prone spots, not across the entire roof. The standard locations are the eaves, the valleys, and anywhere water or ice can collect, such as around chimneys, skylights, vents, and other penetrations, per Hunker. It is also used on low-slope sections where water drains slowly.

In cold climates there is a code requirement behind it. The International Residential Code (IRC) ice-barrier rule, Section R905.1.2, requires the membrane to run from the lowest roof edge to a point not less than 24 inches inside the exterior wall line of the building, as explained by Fine Homebuilding. Because that 24 inches is measured horizontally, the actual distance up the slope is longer once the eave overhang and wall thickness are added. The same source shows that on a 12:12 pitch with a 12-inch overhang the membrane can reach roughly 59 inches up the slope.

Here in Central Florida we do not get ice dams, so the eave ice-barrier rule does not apply the way it does up north. But the membrane is still valuable locally in valleys, around penetrations, and on low-slope areas where our heavy rain and wind-driven water put the most stress on a roof. It ties in closely with metal flashing details, which our roof flashing guide covers. Where the membrane and flashing meet, poor detailing is a common source of leaks, as explained in our roof leak causes guide.

5. Can you reuse old underlayment?

No. When you replace a shingle roof, the old underlayment comes off with the old shingles and new underlayment is installed fresh. It is a single-use layer, and reusing it is not an option on a proper reroof.

The reasons are practical. Tearing off shingles damages the underlayment beneath, self-adhered ice-and-water membrane is bonded to the deck and cannot be peeled up cleanly, and felt that is years old has usually dried out or absorbed moisture. A tear-off also lets the roofer inspect and repair the decking, which you cannot do if you leave the old layers in place. This is one reason a layer-over (adding new shingles on top of old ones) is a false economy: it hides deck and underlayment problems and costs more at the next replacement. Our roof replacement cost guide breaks down how tear-off affects the total.

6. Typical cost

Underlayment is a small share of a full roof cost, but the type you choose does move the number. Material prices run roughly from a nickel per square foot for basic felt up to the high end for premium synthetic and peel-and-stick membrane.

By published material ranges, 15 lb felt is about 5 cents per square foot and 30 lb about 10 cents, per Western States Metal Roofing, which puts synthetic at roughly 17 to 25 cents. Hunker lists vapor-impermeable synthetic at about 11 to 15 cents per square foot and a sample ice-and-water product at about 70 cents. These are material-only numbers, and installed costs are higher once labor is added, so treat them as planning ranges and compare itemized quotes.

Material-only ranges from the sources linked above; installed prices are higher and vary by region, roof complexity, and labor.

7. How it fits the roof system and warranties

Underlayment is one part of a layered roof system, and it works with the deck, the flashing, and the shingles rather than on its own. A manufacturer warranty usually assumes an approved underlayment is installed correctly, so the choice is about code compliance and coverage as much as cost.

Many shingle makers offer stronger, longer warranties when you use their full system, including their underlayment, starter strip, and ridge products, installed by a certified contractor. Using a cheaper off-brand underlayment can leave you with only a basic warranty. During installation, the sequence matters too: ice-and-water goes down first in the vulnerable spots, then felt or synthetic across the field, then flashing and shingles, each lapped so water always drains over the layer below. When any of those laps or the flashing is wrong, water finds its way in, which is why an independent inspection checks the whole system and not just the shingles.