GAF Home Additions & ADU Nationwide

GAF Home Additions & ADU: The Los Angeles Owner’s Guide

GAF home additions and ADU construction in Los Angeles typically runs $53,100 to $153,400 fully installed, with detached ADUs starting around $106,200 depending on foundation work, utility runs, and finish level. We install GAF roofing systems, underlayment, and weather barriers as part of every addition we build, and we keep GAF-spec flashing, ventilation, and accessory parts on hand for existing structures. Most owners discover a part has failed only after a storm, when water stains appear on a ceiling, or when an inspector flags a roof-to-wall transition during an addition project. In Los Angeles, where we get long dry stretches followed by sudden heavy rain, a failed GAF flashing or underlayment detail can hide for months before it shows itself.

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Which GAF Models This Applies To

GAF’s product line for additions and ADUs breaks into a few families that behave very differently on a roof. The Timberline series, including Timberline HDZ and Timberline UHDZ, is what most Los Angeles homeowners know: dimensional asphalt shingles with GAF’s LayerLock bonding. The Glenwood and Grand Sequoia lines are premium architectural options used on higher-end additions. For flat or low-slope sections common on modern ADUs, GAF’s TPO and Liberty SBS membrane systems come into play, and they use entirely different flashing and termination details than shingle roofs. Identifying what you have matters. Check the shingle packaging if any remains, look for the GAF logo printed on the underside of a shingle tab, or measure the exposure. Most GAF shingles sit at 5-5/8 inches of exposure on standard spacing. For membranes, look at the seam pattern. TPO is heat-welded with visible seam lines, while Liberty SBS is torch- or adhesive-applied. One gotcha we see constantly in Los Angeles additions: a homeowner matches the shingle color but not the shingle line, and the nail line, exposure, and warranty all end up mismatched. GAF warranties operate as a system, meaning a Timberline shingle installed over non-GAF underlayment may not carry the full GAF warranty. When we install, everything goes in as a matched GAF system.

Signs Your GAF Home Additions & ADU Is Failing

• Water staining on the ceiling or walls of an addition or ADU during the first winter rain. This usually means a roof-to-wall step flashing detail was installed backwards or lapped incorrectly. GAF specifies exact overlap direction, and it’s one of the most common mistakes on LA additions where a lanai or patio cover meets the house.
• Granules piling up in gutters at the base of a new roofline. Shingle granule loss in the first year signals a defect or improper nail placement. GAF’s LayerLock strips need nails at the correct height or the shingle flexes and sheds granules fast.
• Curling or lifting shingle edges near roof penetrations like vents or skylights on the ADU. In Los Angeles heat, a shingle that’s not properly set against a penetration will curl from thermal expansion. The fix is re-flashing and re-sealing, not just nailing it back down.
• A musty smell inside an attached ADU with no visible water. Moisture trapped in a wall cavity often comes from a failed underlayment seam. GAF Deck-Armor and WeatherWatch should overlap at least 2 inches on a slope, and when someone cuts corners, moisture wicks in silently.
• Loose or missing ridge cap shingles after a Santa Ana wind event. Ridge caps take the brunt of LA’s gusty fall winds. GAF Seal-A-Ridge caps should form a continuous sealed line. If they’re flapping, the adhesive strip was never activated or the caps were nailed too high.

Replacement Cost & What’s Included

For additions and ADUs specifically, the $53,100 to $153,400 range covers the full build: foundation, framing, roofing, drywall, flooring, electrical, plumbing, and finishes. A straight garage conversion sits lower, around $35,400 to $94,400, while a ground-up detached ADU with a kitchen and bath runs $106,200 to $259,600. When the scope is limited to the GAF roof system on an existing addition, a roof replacement falls between $9,440 and $23,600, and targeted repairs run $470 to $2,125. Every estimate from us is a written, line-item price before any work starts. You approve the number, and that’s the number on the final invoice. No change orders appear because someone forgot flashing in the first quote. The 365-Day Done Right Promise applies to every project: if something isn’t done right within a year, we make it right. Written in the contract, not promised on a handshake.

Key Takeaways

• GAF additions and ADUs in Los Angeles cost $53,100 to $153,400 for the complete build.
• GAF warranties work as a system. Mixing a Timberline shingle with non-GAF underlayment can void coverage.
• LA’s hot, dry climate followed by sudden rain hides roof failures until they become interior water damage.
• Every GAF system we install carries the 365-Day Done Right Promise in writing.
• Written pricing before work starts means the number you approve is the number you pay.

DIY or Call Us?

Some things on a GAF roof you can check yourself. Look at your gutters after a rain and note whether granules are collecting. Walk the perimeter on the ground and look for lifting shingle edges or missing ridge caps. But climbing onto a roof to fix a flashing detail yourself is where I’d stop you. Roof failures on additions and ADUs are often structural, not cosmetic. The water you see on a ceiling has already passed through multiple layers, and finding the actual entry point takes knowledge of how GAF systems lap and seal. Working on a roof or a tall addition wall is fall-hazard territory, and a misapplied GAF patch can void your warranty entirely. If you see staining, curling, or loose caps, call (254) 272-5876. We’ll inspect it, tell you what’s actually wrong, and price the fix in writing before anything happens.

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