Underlayment is the part of your roof you never see and rarely think about, but it is the layer that saves you when the shingles fail. It is the water-resistant barrier laid directly on the roof deck, under the shingles, and there are three main types at three price points. Here is how felt, synthetic, and peel-and-stick underlayment compare, and why it matters more than you would think, especially in Florida.
What underlayment does
Underlayment is your roof's second line of defense. Shingles or tiles are the first barrier against water, but wind can lift them, they can crack, and water can get past them. When that happens, the underlayment is what keeps the water off the wood deck and out of your home. On a tile roof especially, the underlayment is really the waterproofing layer, and it usually wears out before the tile does.
The three types
Almost every roof uses one or a mix of these:
- Felt, the traditional tar paper. It comes in 15-pound and 30-pound weights and is the cheapest option, around 0.15 to 0.75 dollars per square foot. The downside is that it absorbs water, tears easily, and breaks down in the sun, and it lasts roughly 12 to 20 years.
- Synthetic, made from woven polymer. It is lightweight, much stronger and more tear-resistant, water-resistant, and holds up to UV, and it has become the standard for new roofs. It costs about 0.50 to 1.50 dollars per square foot and lasts 20 to 30 years or more.
- Self-adhered, the peel-and-stick membrane also called ice-and-water shield. It is fully waterproof and seals around the nails driven through it. It costs about 0.75 to 2 dollars per square foot and is used in the most vulnerable spots, the valleys, eaves, and around penetrations, or across the whole deck in high-wind zones.
A common, smart setup is synthetic across the main field of the roof with self-adhered membrane in the valleys and around chimneys and vents, where leaks are most likely. Ask what your quote uses where, since it is easy to cut corners on the part nobody sees. See what a complete estimate should list in what a roofing estimate should include.
What Florida requires
In Florida, underlayment is not just a good idea, it is code, as a secondary water barrier under the roof covering. In the high-velocity hurricane zone around Miami-Dade and Broward, a sealed roof deck with self-adhered underlayment is required. And a sealed deck, or secondary water resistance, is one of the biggest credits on a wind mitigation inspection, so it can lower your insurance too. See the Florida wind mitigation inspection.