Underlayment is the waterproof layer that sits between the roof deck and the tiles or shingles on top. The tiles shed most of the water, but the underlayment is the real barrier that catches whatever gets past them. It matters because on tile and slate roofs, the underlayment usually wears out long before the tile does, which is why a fifty-year tile roof still needs work partway through. When people say a tile roof is leaking but the tile looks fine, the underlayment is almost always the reason.
Think of a roof as two lines of defense. The tiles or shingles on top take the sun and shed the bulk of the rain, but they are not a perfect seal. Water gets driven under them by wind, backs up in valleys, and works through small gaps. That is where the underlayment earns its keep, as a continuous waterproof sheet laid over the wood deck that catches this water and carries it back out. Without it, every little gap in the surface would be a leak.
This is why the underlayment matters so much on long-life roofs. The tile can outlast the felt beneath it by decades, so a tile roof does not fail all at once, it fails from underneath as the underlayment ages. Replacing it, with the same tiles lifted and reset, is exactly what a reroof is, and it is central to tile roof repair.
| Type | Notes |
|---|---|
| Asphalt-saturated felt | Traditional, lower cost, shorter life |
| Synthetic underlayment | Lighter, stronger, longer lasting |
| Self-adhering membrane | Seals around nails, used in key areas |
The grade of underlayment is a real factor in a roof's lifespan and cost, as our page on how long a roof lasts covers.
When you get a roof quote, the underlayment is one of the most important lines on it, even though you will never see it once the tile is on. A cheaper roof often means cheaper, shorter-lived underlayment, which is a false saving on a long-life roof. If you have a tile roof that is leaking while the tile looks fine, the underlayment has likely aged out, and a roof inspection confirms it. Our pages on roof replacement and tile roof cost cover how the underlayment factors into the bigger picture, and roof repair handles a failing section.
We check the layer under your tile. Call (310) 806-5959 or request an estimate below.
Tell us about your roof and we will inspect the underlayment and give you a written price for any work, with no obligation.