When most people picture a roof, they picture the tile or shingle on top. But there is a layer beneath that surface that does the real waterproofing, and it is the one that most often decides when a roof needs work. It is called underlayment, and understanding it explains a lot of roofing mysteries, especially why a tile roof can leak while the tile looks perfect. Here is underlayment explained in plain terms.
Underlayment is a waterproof sheet laid over the roof deck, beneath the tile or shingle. Think of a roof as two lines of defense. The tile or shingle on top sheds most of the water and takes the sun, but it is not a perfect seal, so water gets driven under it by wind, backs up in valleys, and works through small gaps. The underlayment is what catches that water and carries it back out. Without it, every little gap in the surface would be a leak, which is covered on our page on what roof underlayment is.
There are a few kinds, and the choice affects both cost and lifespan. Traditional asphalt-saturated felt is the older, lower-cost option with a shorter life. Synthetic underlayment is lighter, stronger, and lasts longer. Self-adhering membranes seal around the nails that pass through them and are used in the most vulnerable areas, like valleys and eaves. A quality roof often uses a combination.
| Type | Notes |
|---|---|
| Asphalt-saturated felt | Traditional, lower cost, shorter life |
| Synthetic underlayment | Lighter, stronger, longer lasting |
| Self-adhering membrane | Seals around nails, for key areas |
Here is the crucial point. On a tile or slate roof, the tile lasts far longer than the underlayment beneath it. A tile roof can last fifty years, but the felt underneath usually wears out in twenty to forty. So a tile roof does not fail from the sun cracking the tile, it fails from underneath as the underlayment ages, which is why a tile roof leaks while the tile looks fine, covered on our pages on tile roof maintenance and what to know about tile roofs.
Because the underlayment is the part that ages out, fixing a tile roof often means lifting the tile, replacing the failed underlayment, and resetting the same tile, which is a reroof rather than a full replacement, and it is far more affordable than new tile, covered on our tile roof repair page. On shingle, the shingle and underlayment tend to age together, so it is less of a separate concern.
When you get a roofing estimate, the underlayment is one of the most important lines, even though you will never see it, since a cheaper roof often means cheaper, shorter-lived felt, covered on our page on reading a roofing estimate. And when you buy a home with a tile roof, the age of the underlayment matters more than the look of the tile. A roof inspection can gauge where the underlayment stands, which is the real question behind a tile roof's condition.
We check the layer that matters. Call (310) 806-5959 or request an estimate below.
Tell us about your roof and we will assess the underlayment and give you a written price for any work, with no obligation.