GAF Underlayment & Deck Repair Nationwide

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

GAF underlayment and deck repair in San Francisco typically runs $1,200 – $4,800 installed, depending on how much decking needs replacement and which GAF underlayment line your roof carries. We stock GAF-compatible underlayment and decking on our trucks and can usually complete a standard repair in one day once we get eyes on the roof. Most San Francisco homeowners discover the problem the hard way: a ceiling stain after a heavy winter storm, or shingles lifting at the eaves where the deck has gone soft from years of fog-season moisture.

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Here’s the thing about underlayment and decking in this city: you can’t see either one until the shingles come off. The deck is the plywood or OSB layer nailed to the rafters, and the underlayment is the water-shedding barrier between that wood and your shingles. When either fails, the shingles on top don’t matter much. Water gets through, wood rots, and you’re suddenly looking at a repair that should have been caught two seasons earlier. That’s why we inspect the deck with a moisture meter and a hammer tap test on every repair we do, not just the ones that look bad from the street.

Which GAF Models This Applies To

GAF makes several underlayment lines, and the right replacement depends on what’s already on your roof and what your shingle system was designed to work with. The compatibility piece matters more than most homeowners realize, so here’s the plain version:

• Deck-Armor: GAF’s breathable synthetic underlayment, rated for use under GAF asphalt shingles including Timberline HDZ, Timberline UHDZ, and Royal Sovereign. It’s a 25-mil equivalent product that handles footfall better than felt and doesn’t absorb water the way #15 or #30 felt does.
• FeltBuster: A high-traction synthetic used on steeper pitches where installers need grip. Commonly paired with Timberline and Grand Sequoia systems. The traction backing is the identifying feature, and it’s why you don’t substitute standard synthetic on a steep San Francisco Victorian without checking pitch first.
• StormGuard and WeatherWatch: GAF’s ice-and-water membrane lines, required by code in specific locations like eaves, valleys, and around penetrations. StormGuard is the peel-and-stick product used on low-slope sections under 2:12 pitch, common on flat-low portions of Sunset and Richmond district homes.
• Tiger Paw and Shingle-Mate: Older GAF lines still found on roofs from the 2000s and early 2010s. If your roof has these, they’re near or past their effective lifespan in San Francisco’s moisture-heavy climate.

One compatibility gotcha we see often: GAF’s upgraded wind warranty coverage requires their full system, with specific underlayment products listed in the warranty documentation. If a prior contractor mixed brands, the shingle warranty may already be void. We flag this on inspection reports because it changes the repair math. When we replace GAF underlayment, we verify the exact product line against the existing shingle system, measure the deck exposure, and match fastener patterns to manufacturer spec. We’re an independent service provider, not affiliated with GAF directly, but we install their materials to their published specifications because warranty claims depend on it.

Signs Your GAF Underlayment & Deck Repair Is Failing

• Water stains on interior ceilings or walls after rain. This is the most common first symptom in San Francisco, and it means water has already passed the underlayment and saturated the deck. By the time you see the stain, the deck below it is almost always compromised. We trace the stain to the roof leak source, not just the drywall patch.
• Shingles lifting, curling, or missing at the eaves and rakes. The deck edge rots first because fog moisture collects there and drip edges channel water along the perimeter. When the wood swells and softens, nails lose their bite, and shingles start to lift. In neighborhoods like the Outer Sunset and Richmond, we see this before we see field-of-roof failures.
• Sagging or spongy spots when the roof is walked. A healthy deck resists a deliberate step. If a section flexes or feels soft underfoot, the deck has lost structural integrity. This is not a patch situation, that section comes out and new plywood or OSB goes in.
• Granule buildup in gutters with exposed felt or synthetic fibers on the roof surface. This means the shingles are past their effective life and the underlayment is now the only thing keeping water out. In San Francisco’s damp-air environment, underlayment exposed to UV and moisture degrades within a single season.
• Moss growth at the roof edges or in valleys. Moss traps moisture against the deck and underlayment year-round, especially on shaded north-facing slopes common in Glen Park and Noe Valley. The deck underneath moss is almost always wet to the touch, even in summer.

Replacement Cost & What’s Included

Here’s the straight answer on pricing for San Francisco. Full deck replacement over repair sections typically runs $3,500 – $4,800 for a standard 400 – 800 square foot repair area, including new GAF-compatible decking, underlayment, and shingle re-installation. Isolated deck patches of 100 – 200 square feet, with new underlayment and shingles, usually run $1,200 – $2,200. The range depends on three things: how much deck is rotten, what pitch and accessibility your roof has, and which underlayment line the existing system requires.

Every estimate includes the same line items in writing: tear-off of existing materials, dump fees, deck replacement to match existing thickness (usually 1/2″ or 5/8″ CDX plywood), underlayment matched to the GAF system, shingle re-installation or replacement, and flashing work at any penetrated areas. The price we put on paper before work starts is the price. No change orders at the door. The work is covered by the 365-Day Done Right Promise: in writing, for one full year, including a free return visit if the repair fails within that period. Call (831) 318-8819 for a written estimate, and we’ll read your roof honestly before we quote it.

DIY or Call Us?

You can safely check a few things yourself: look for missing shingles from the ground with binoculars, note any interior stains directly under the roof slope, and check your gutters for shingle granules. If you see evidence of a problem, get a professional on the roof. Walking on a compromised deck is dangerous because the surface you’re standing on may not hold your weight, and there’s no way to know where the soft spots are until you’re on them.

Deck replacement and underlayment work are not DIY jobs in any practical sense. You need the right fasteners, the right underlayment overlap and nail pattern for the product you’re installing, and you need to know what the framing below the deck looks like. If the rafters or trusses have water damage, the repair grows fast. Travis Oakley and our crew have been reading San Francisco roofs long enough to know when a deck repair is a two-hour patch and when it’s a two-day structural job. If you’ve got signs of trouble, call (831) 318-8819 and we’ll take a proper look before you spend a dollar.

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