Ask a homeowner what protects their roof and most will say “the shingles.” The shingles get all the credit. But there’s a quiet second layer doing a critical job underneath them — the underlayment — and most people never see it until their roof is torn off.
Underlayment is the sheet material laid directly on the wood deck before the shingles go on. When shingles work perfectly, it’s backup. When shingles fail — in a windstorm, an ice dam, a backed-up gutter, or just old age — it’s the only thing between rainwater and your drywall. Choosing the right one matters more than its invisibility suggests.
What Underlayment Actually Does
Underlayment has two jobs. First, it’s a secondary water barrier. Shingles shed most water, but wind-driven rain can push water up under shingle tabs, and ice dams can shove it underneath from below. The underlayment catches that water and channels it down the slope and off the roof instead of letting it soak into the deck.
Second, it’s a temporary weather barrier during installation. A good underlayment lets a roofer deck and dry-in a roof in the morning and shingle it in the afternoon — or days later — without a surprise storm ruining the job. That alone is worth plenty.
Everything else is detail. Let’s look at the three main options.
Felt Underlayment: The Old Standard
Asphalt-saturated felt — roofing felt, tar paper — is the classic underlayment. It’s a mat of fiberglass or cellulose fibers saturated with asphalt, and roofers have been rolling it out for over a century. The common weights are 15-pound and 30-pound, which refers to how heavy a standard roll was, not to any thickness spec worth worrying about. Thirty-pound felt is thicker and tougher than 15-pound.
Felt still works. It’s cheap, it tears cleanly enough, and it gets the job done on budget roofs in mild climates. But it has real weaknesses. It absorbs moisture if left exposed — a 15-pound felt can wrinkle and ripple if it gets rained on before the shingles go down, which telegraphs through the finished roof. It’s also the most tear-prone of the three in high wind before the shingles are on, which is when it matters most.
Some roofers still prefer felt on steep roofs in dry climates because it’s cheap and familiar. But it’s steadily losing ground, and on most jobs today it’s the value choice rather than the best choice.
Synthetic Underlayment: The Modern Default
Synthetic underlayment is made from woven or spun polypropylene or polyethylene — think industrial-strength house wrap engineered for roofs. It has become the default on most professional jobs, and for good reasons.
It doesn’t absorb water, so it stays flat and wrinkle-free even if it rains before shingling. It’s much stronger against tearing — a crew can walk on it in the wind without punching through. It’s lighter per square, so one roll covers more roof with fewer laps and seams, and the printed lap lines make installation faster and more consistent. On steep roofs, the surface grip is noticeably better underfoot than felt, which is a real safety factor for the crew.
The trade-off is cost. Synthetic runs roughly two to three times the price of 15-pound felt per square. In the context of a whole roof replacement, though, the difference is small — a few hundred dollars on a typical home — which is why so many roofers spec it as standard now. If your roofer includes synthetic underlayment without asking extra, that’s normal and it’s a good sign.
One note: not all synthetics are equal. Cheaper ones can be slippery and can degrade under prolonged UV exposure if the job gets delayed. A reputable roofer picks a brand they’ve used before and knows.
Ice-and-Water Shield: The Leak Specialist
Ice-and-water shield is a different animal. It’s a self-adhering rubberized asphalt membrane with a sticky backing that bonds directly to the deck. Where felt and synthetic hang loose and overlap, ice-and-water shield seals itself around every nail that penetrates it. That self-sealing property is what makes it special.
It’s used in the vulnerable zones: along the eaves where ice dams form, in valleys where water concentrates, around chimneys and skylights, and at any low-slope-to-steep transition. In cold climates, building codes often require it from the eave edge up to at least two feet inside the exterior wall line — the area most vulnerable to ice-dam backup.
Why does it matter? Because ice dams and wind-driven rain push water *upward* — against the direction shingles are designed to shed it. Normal underlayment channels water down; ice-and-water shield seals water out. When water backs up under the shingles at the eave, this membrane is the last line of defense, and it’s a very good one.
The downside is price. It’s the most expensive underlayment per square foot by a wide margin, and applying it is slower because every sheet has to be pressed and rolled to bond properly. That’s why it’s used selectively — eaves, valleys, penetrations — rather than covering the entire deck, though full-deck coverage is done on low-slope roofs and in the harshest climates.
How the Three Work Together
On a well-built roof, you don’t pick one — you combine them. The typical professional assembly is ice-and-water shield at the eaves, in valleys, and around penetrations, with synthetic (or felt) underlayment covering the rest of the deck. Each material goes where its strengths matter most.
When you’re comparing roofing bids, this is worth asking about. Some low bids cut the ice-and-water shield at the eaves, or skip valleys, to shave cost. In a warm climate with no ice-dam risk, that’s a smaller deal. In any climate that freezes, missing eave protection is a future leak waiting for a bad winter. Ask where each type of underlayment is going on your roof, and make sure the answer matches the weather you actually get.
What Underlayment Tells You About a Roofer
Underlayment choices are one of the honest signals in a roofing bid. A roofer who specifies synthetic across the deck and ice-and-water shield at eaves and valleys is building the roof to last. A roofer who specs the cheapest 15-pound felt everywhere and skips the shield zones is building to a price — and the price will eventually be paid by someone.
For more on the rest of the roof system hiding under the shingles, the guide to what a roof is made of covers every layer from deck to ridge — and if you’re pricing a full redo, the roof replacement page walks through how the whole job comes together.