Roof Underlayment in Arizona: Why It Fails First and How to Replace It

What underlayment is and why it matters so much here

Underlayment is the sheet material fastened to the roof deck under the tile, shingles or metal. On a shingle roof it's a backup layer. On a tile roof it's the main waterproofing.

Tile is not watertight. Water runs between tiles, around their edges and under them in wind-driven rain. The tile's real job is to shield the underlayment from sun, foot traffic and hail, and to shed most of the water. Whatever gets past the tile runs down the underlayment to the eaves. When the underlayment cracks, tears or crumbles, that water reaches the wood deck and eventually your ceiling, even though the roof still looks perfect from the street.

That's why, in the Valley, a tile roof with sound tile and worn-out underlayment is so common, and why the usual fix is replacing the underlayment and reusing the tile.

How Arizona heat wears it out

Phoenix roof surfaces reach 150 to 170 degrees on summer afternoons, and the Valley now sees around three weeks of 110-degree days in a typical year, with a record 70 in 2024. The tile blocks direct sun, but the heat still soaks through to the underlayment and the deck all day, then backs off overnight. Year after year, that cycle does the damage.

Traditional asphalt-saturated felt is the most affected. Heat drives the oils out of the felt, and it turns dry, brittle and eventually crumbly. Brittle felt splits where it's nailed, where tiles shift and wherever someone steps. Older felt systems under Valley tile often reach the end of their life in roughly 20 to 25 years, and sooner on hot, south- and west-facing slopes.

The Tile Roofing Industry Alliance points out that there's no single lifespan for tile underlayment, because organic felts, synthetics and self-adhered membranes all age differently. The useful number is the one in the manufacturer's literature and warranty for the product actually on your roof.

The main types of underlayment

The building code lists the standards underlayment must meet (IRC R905.1.1), and the tile section adds tile-specific requirements. In practice, you'll see four families:

• Organic felt (ASTM D226 or D2626). The traditional 15- and 30-pound felts and heavier tile felts. Inexpensive, but the most sensitive to heat aging.
• Fiberglass-reinforced asphalt underlayment (ASTM D6757). A heavier, inorganic-reinforced sheet built for steep-slope roofs. Fontana's Vulca Seal G40, which we install under tile, meets ASTM D6757 and D4601.
• Synthetic underlayment (ASTM D226 and D4869 properties). Woven or spun polymer sheets that are light, strong and don't absorb water. The TRI-BUILT synthetic we use under shingles lists a service range of -40 to 240 degrees, meets ASTM D226 Types I and II and D4869 Types II and IV, and isn't recommended below a 2:12 pitch. SwiftGuard is a synthetic we install under tile.
• Self-adhered membranes (ASTM D1970). Peel-and-stick sheets that bond to the deck and seal around nails. TRI-BUILT HT SA is a high-temperature self-adhered underlayment rated for use under metal, shingles and tile.

There's no single best product for every roof. The right choice depends on the roof covering, the pitch, the warranty you want and the budget. What matters most is the right product installed to the manufacturer's instructions and the tile installation manual.

Mechanically fastened vs. self-adhered

Most underlayment is mechanically fastened: nailed with cap nails or plain roofing nails through the sheet into the deck. Every fastener is a small hole, and laps between sheets are where water can work in sideways.

Self-adhered membranes bond to the deck and seal around fasteners that go through them. That makes them well suited to the places water concentrates:

• Valleys
• Around pipes, vents, skylights and solar mounts
• Roof-to-wall transitions and headwalls
• Low-slope areas near the 2.5:12 tile minimum

A common approach is a self-adhered membrane in those critical areas and a fastened underlayment across the open field of the roof. Full self-adhered coverage costs more, and it's a judgment call on most Valley roofs. Note that self-adhered membranes stick permanently, so the next underlayment replacement may involve more deck work to remove them.

One more detail: fasteners. Fontana's G40 warranty doesn't allow staples of any kind as underlayment fasteners, requiring corrosion-resistant nails of No. 11 gauge or heavier instead. That's the kind of detail that decides whether a warranty claim is honored.

Double layer on low-slope tile roofs

Pitch changes the rules. The building code allows tile on slopes as low as 2.5:12, but from 2.5:12 up to 4:12 it requires two layers of underlayment. Many Valley homes have pitches in that range.

Manufacturers go further. Fontana's G40 warranty is 10 years for a single layer and 20 years for a double layer, but both apply to slopes of 4:12 or steeper, and the 20-year warranty also requires an extra 18-inch ply in all valleys, eaves, rakes and around penetrations, valley details that keep end laps at least 18 inches away from valleys, and flashing at every penetration, including solar mounts.

If your roof is near the low end of tile's range, ask how the underlayment will be layered and what warranty actually applies at your pitch.

Signs your underlayment is failing

Underlayment failure usually shows up indoors first, because you can't see the underlayment from outside.

• Ceiling stains after rain, even though no tiles look broken.
• Leaks that show up in a different place each storm.
• Stains, dark spots or soft areas on the underside of the roof deck, visible from the attic.
• Daylight through the deck at the eaves or around penetrations.
• Tiles that have slipped, or broken tiles that have exposed the underlayment to sun.
• A tile roof that's 20 years old or more and has never been checked underneath.

The only reliable check is to lift tiles. During an inspection, a roofer lifts sample tiles on different slopes and photographs the underlayment. Brittle felt that cracks when bent, tears at the nails or crumbles at the eaves is done, no matter how good the tile looks. Our standard roof inspection is free, and a comprehensive inspection with a written photo report is $149.

Tile underlayment replacement, step by step

A tile underlayment replacement, often called a lift and relay or a re-felt, keeps your tile and replaces everything under it that wears out. Here's how it generally goes:

• Protect the property. Landscaping, pools, walls and driveways are covered, and the work area is set up for debris.
• Lift and stack the tile. Tiles come off by section and are stacked on the roof or the ground, sorted so they go back where they belong. Cracked tiles are set aside.
• Remove the old underlayment. The old felt, battens where needed and debris are cleared down to the deck.
• Repair the deck. Soft, rotted or delaminated sheathing is replaced, so the new underlayment goes on solid wood.
• Install new underlayment and flashings. New underlayment goes on to the manufacturer's and tile manual's details, with new valley metal, pipe flashings, vent flashings and edge metal as needed.
• Relay the tile. The original tile goes back on and is fastened, with broken pieces replaced. The building code requires reused tile to be sound and undamaged (IRC R908.5).
• Reset ridges and hips, check fastening at the perimeter, clean up and do a final walk.

If you have solar panels, they need to come off first. We detach and reset them during the job for $150 per panel. Permits depend on the city: Phoenix exempts a reroof with the same type of material, while Scottsdale requires a permit when sheathing is replaced, and Queen Creek requires one for any reroof.

What it costs

Our installed price for a tile underlayment replacement is about $4.00 to $7.00 per square foot of roof area for concrete tile, and about $5.00 to $8.50 for clay, because clay is more fragile to lift and reset and usually needs more replacement pieces. For a typical Valley home, that commonly lands somewhere around $10,000 to $22,000.

What moves the price:

• Roof size and pitch. Steeper roofs are slower and need more safety setup.
• Complexity. Valleys, hips, skylights, chimneys and multiple levels add labor.
• Deck repairs. Rotted sheathing found after the tile comes off is replaced as a line item.
• Underlayment choice. A double layer, more self-adhered membrane or a premium synthetic costs more.
• Broken tile and matching. Discontinued profiles or colors can take extra work to match.
• Solar. Panel detach and reset adds $150 per panel.

For comparison, a new concrete tile roof runs about $6.50 to $10.50 per square foot. Our roof cost calculator gives a starting range for your roof, our estimates are free and itemized in Xactimate, and financing is available through Service Finance and GreenSky, including 0% for 24 months for qualified buyers, subject to credit approval.

Shingle and metal roofs

On shingle roofs, the underlayment is replaced every time the shingles are, so it doesn't need its own replacement schedule. We use TRI-BUILT synthetic underlayment under shingles, which carries a 20-year material warranty for the original purchaser. It has to be covered by the shingles within 60 days, and it's not recommended on slopes under 2:12. Under 4:12, the code requires two layers.

Metal roofs typically go over a high-temperature underlayment, because metal panels can get very hot. Self-adhered high-temperature membranes are common there, and the panel manufacturer's instructions control what's allowed.

Whatever the roof, ask every contractor bidding your job to name the underlayment product, the number of layers, how it will be fastened, and which warranty applies at your pitch. If a bid just says "underlayment," you can't compare it to anyone else's.

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• Tile Underlayment Replacement
• Roof Inspection
• Roof Cost Calculator
• Roof Financing
• Tile Roof Cost in Arizona
• Roofing Warranties in Arizona
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• Roofing in Scottsdale
• Roofing in Mesa
• Roofing in Gilbert
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