GAF Underlayment & Deck Repair Nationwide

GAF Underlayment & Deck Repair: The San Antonio Owner’s Guide

GAF underlayment and deck repair in San Antonio typically runs $400 to $1,800 installed, depending on how much decking has rotted and which underlayment your system calls for. We stock GAF-compatible synthetic underlayment and CDX plywood decking on our trucks, so most repairs are completed same-day once we’re on the roof. Most owners discover the problem two ways: a leak shows up on the ceiling after a hard Hill Country storm, or a roofer doing a shingle replacement peels back the old layer and finds the decking underneath has turned to sponge.

Call (254) 276-4369

Which GAF Models This Applies To

This repair path applies to any GAF shingle system sitting on a damaged deck or failed underlayment. The most common lines we see in San Antonio are Timberline HDZ, Timberline UHDZ, and Royal Sovereign, but the underlayment requirements don’t change much between them. What matters is which underlayment your roof was originally built with. Older GAF systems in San Antonio, especially those installed before 2010 around Alamo Heights and Terrell Hills, often sit on 15-pound felt that’s now brittle and tearing. Newer GAF installations use feltbuster, Tiger Paw, or Deck-Armor synthetic underlayment, and those have different tear strengths and exposure ratings.

Here’s the compatibility gotcha most people miss: GAF’s system warranty requires the underlayment to be a GAF-approved product. If you swap in a generic felt from a big-box store, the shingle warranty stays intact, but the system warranty won’t cover the underlayment layer. For decking, the spec is simpler: most GAF installs in San Antonio call for 15/32-inch or 1/2-inch CDX plywood, minimum 24/0 span rating, nailed at 6 inches on center along edges and 12 inches in the field. Some older homes in Monte Vista and King William have 1×6 plank decking instead, which needs a different repair approach entirely.

We’re an independent roofing contractor, not affiliated with GAF, but we install and repair using manufacturer-recognized GAF products so your warranty story stays clean.

Signs Your GAF Underlayment & Deck Repair Is Failing

• Water stains on ceilings or walls after rain. This is usually the decking itself has rotted through, or the underlayment has torn at a seam where water is getting past. In San Antonio, the worst leaks often show up after those spring storms that dump an inch in an hour.
• Soft spots you can feel from inside the attic. If you can push a screwdriver into the underside of your roof deck, the plywood is compromised. That’s rot from trapped moisture, and it’s not going to dry out on its own in our humidity.
• Shingles lifting, curling, or blowing off in wind. When the deck is soft, nails lose their grip. The shingles may look fine, but if the substrate underneath is spongy, the whole section is only one good norther away from coming apart.
• Visible felt or synthetic underlayment showing through gaps in the shingles. This is a sign the original installation cut corners, or the shingles have shifted enough to expose the layer below. Once UV hits that underlayment, it degrades fast under the Texas sun.
• Granules piling up in gutters every month. GAF shingles shed granules as they age, but heavy granule loss combined with any of the above means the system is working too hard. The underlayment becomes the last line of defense, and older felt was never meant to stand alone.

Replacement Cost & What’s Included

The installed price for underlayment and deck repair in San Antonio breaks down like this:

• Decking repair only (1 to 2 sheets of CDX plywood): $180 to $420
• Full underlayment replacement on a standard 1,800 to 2,500 sq ft roof: $1,200 to $1,800 when paired with shingle replacement
• Spot underlayment repair (5 to 10 squares, no shingle swap): $350 to $650
• Full deck replacement on a 2,000 sq ft home where rot is widespread: $2,500 to $4,800, depending on how many sheets come up

Every Mercer Roofing estimate includes a written line-item price before any work begins. No change orders, no surprise add-ons once the old layer comes off and we find something unexpected. The written price covers tear-off, haul-away, the new decking or underlayment, fasteners, and labor. We also include our written 365-Day Done Right Promise on every repair: if the work isn’t right within a year, we make it right, and a second visit is free if the repair fails. Grant Whitlock reviews each estimate personally because he’s the one who has to stand behind it.

DIY or Call Us?

This is one repair where the honest answer is: leave it to a professional. The reason isn’t the tools, it’s the position. Underlayment and deck repair means being on a pitched roof, handling 4×8 sheets of plywood, often in the heat of a San Antonio afternoon when the surface temperature is 140 degrees or higher. One wrong step on a soft deck and you’re through the roof, literally. We’ve seen it happen to experienced guys, and the hospital bill always costs more than the repair would have.

What you can safely check from inside: go into the attic on a clear day and look for stained decking, light coming through cracks, or moisture around vent penetrations. Take photos. Note where the problems are. Then call (254) 276-4369 and tell us what you found. We’ll schedule a free estimate, and the technician who shows up will have their name and photo sent to you before they arrive and a photo ID badge when they’re at your door.

How much does GAF underlayment replacement cost in San Antonio?

How long does GAF underlayment last?

GAF synthetic underlayments like Tiger Paw and Deck-Armor are rated for 25 to 30 years when properly installed, but the old 15-pound felt many San Antonio homes had installed 15 to 20 years ago was realistically good for a decade in our heat. If your roof is past year 12 and you’re seeing leaks or lifting shingles, the underlayment is likely contributing to the failure. If you’re unsure what you have, we can tell you during a free estimate.

Can GAF underlayment be repaired instead of fully replaced?

Yes, when the damage is confined to a small area. A spot repair on five to ten squares runs $350 to $650 in San Antonio. But if the felt is brittle across whole sections of the roof, patching becomes a game of whack-a-mole, and full replacement is the cheaper option long-term. Call (254) 276-4369 and we’ll give you a straight answer on which path makes sense.

How long does a GAF underlayment and deck repair take?

Most spot repairs are completed in one day. Full underlayment replacement paired with a shingle tear-off takes two to three days on a standard San Antonio home. Deck replacement on a widespread rot job adds a day, sometimes two if we’re replacing 20-plus sheets. We’ll give you a written timeline as part of your estimate.

Do you stock GAF underlayment and decking on the truck?

We do. Our trucks carry GAF-compatible synthetic underlayment, 15/32 and 1/2-inch CDX plywood, and the fasteners needed for both. For homeowners with Owens Corning or CertainTeed systems, we stock those compatible options as well. If we arrive and find something unexpected, there’s a good chance we can still finish the job that day. Call (254) 276-4369 for a free estimate and a written price before the first nail goes in.

Why San Antonio Homeowners Call Mercer Roofing First

You should know exactly who’s coming to your door before they ring the bell. Every Mercer technician is background-checked, identity-verified, and arrives in a clean uniform with a photo ID badge. We send their name and photo to you ahead of the visit, and that’s the Threshold Verified standard, not just a promise we made up. The technician who walks your roof is the same person whose name you already have.

Mercer Roofing Co San Antonio is part of the Threshold Group family, founded in 2000. Each location is independently owned and operated, but we’re all held to the same standard on vetting, pricing, and written guarantees. A roof shouldn’t be something you worry about, that’s our job. Call (254) 276-4369 for a free estimate, or just to talk through what you’re seeing on your roof. A live person answers, any hour.

Written by Grant Whitlock, Owner, Mercer Roofing Co San Antonio at Mercer Roofing Co San Antonio, serving San Antonio and nearby areas.