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 cost $53,100 to $153,400, with detached ADUs running higher at $106,200 to $259,600 depending on site work, foundation type, and finish level. Mason Remodeling Co Los Angeles is an independent service provider, not affiliated with GAF, but we regularly integrate GAF roofing and weatherproofing systems into additions and ADUs across LA County. Most homeowners come to us after a roofline tie-in fails, a permit review flags improper flashing details, or they’ve discovered their existing GAF underlayment was never extended correctly onto a new addition.

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Here’s what you need to know before making any decisions: who’s coming to your house, what the project will cost in writing, and what’s guaranteed for 365 days after completion. Everything else is just noise.

Key Takeaways

• Full home additions in Los Angeles run $53,100 – $153,400; ADUs typically land between $106,200 and $259,600 depending on site conditions and finishes.
• GAF roofing tie-ins to new additions are where we see the most failures: mismatched underlayment, wrong flashing gauge, or skipped ice-and-water shield at the transition.
• Mason Remodeling Co Los Angeles delivers a written price before any work starts, and every crew member is name-verified and photo-verified before they reach your door.
• Call (213) 632-5711 for a free estimate. A live person answers, every time.

Which GAF Models This Applies To

When we say GAF home additions and ADUs, we’re talking about the roofing and weatherproofing layer that ties your new structure to the existing house. The relevant GAF lines are Timberline HDZ and Timberline UHDZ shingles, Deck-Armor underlayment, StormGuard ice-and-water shield, and GAF’s Cobra ridge vent systems. If your addition or ADU was built with GAF materials, the critical transition is where the new roofline meets the old one, along with any valleys, dormers, or shed-roof tie-ins.

How to identify what you have: look under a shingle tab along a rake edge for the GAF stamped logo on the nailing strip, or check any leftover bundle wrappers in the garage or attic. The shingle thickness tells you the series: Timberline HDZ runs about 3/8″ thick, UHDZ closer to 5/8″. For the underlayment, if you can see any exposed decking during construction or repair, Deck-Armor has a gray, cross-hatched face pattern. StormGuard is a black, self-adhered membrane with a distinct asphalt scent when peeled. OEM GAF materials are always our first recommendation for tie-ins because the warranty stays intact and the color match across roof planes is clean. Quality aftermarket underlayment exists, but for a Los Angeles ADU that will face 40-plus summers of UV and heat cycling, we’d rather you keep the system cohesive.

One compatibility gotcha we see regularly: GAF’s current Timberline HDZ line uses LayerLock technology, which means the nailing strip is reinforced and the shingles seat differently than the previous Timberline HD line. If your existing roof is older HD and the addition gets HDZ, the physical match is fine but the visual texture differs slightly at certain light angles. It’s cosmetic, but if that bothers you, tell us during the estimate and we’ll talk through options before you spend a dollar.

Signs Your GAF Home Additions & ADU Is Failing

• Staining at the roofline transition. Brown or gray streaks on interior ceilings near where the addition meets the original house usually mean the flashing was installed wrong or the underlayment wasn’t lapped correctly. Water travels down the trusses and shows up far from the actual breach.
• Lifted shingles at the tie-in within the first two years. New additions settle, and if the roof decking wasn’t allowed to dry out fully before shingling, trapped moisture causes shingle tabs to cup and lift, especially on south-facing Los Angeles roof planes that take direct sun from 10 a.m. to 6 p.m.
• Granule piles in the gutters near the addition. A new roof shouldn’t shed granules. If you’re seeing dark gray or black grit collecting in gutters under the new roofline, the shingles may have been walked on before they cured, or they were installed in extreme heat, which breaks the granule bond.
• Musty attic smells in the addition after rain. This is almost always an underlayment failure. In LA’s winter rain events, a small gap at the ridge vent or a torn section of Deck-Armor lets water in slowly, and it soaks insulation before you ever see a ceiling stain.
• Visible gaps at the valley where addition meets existing roof. Los Angeles homes with older roof lines often have non-standard valley angles. If the valley flashing isn’t custom-bent on site, you’ll see daylight through the gap from the attic side. This is an active leak waiting for the next good storm.

Replacement Cost & What’s Included

For the full scope of a home addition or ADU in Los Angeles, plan on $53,100 to $153,400 for a ground-up addition, and $106,200 to $259,600 for a detached ADU with kitchen and bath. If the issue is specifically a GAF roofing tie-in repair on an existing addition, that’s a much smaller job: roof repair runs $470 to $2,125, and a full roof replacement on the addition alone lands at $9,440 to $23,600 depending on square footage and tear-off requirements.

What drives the range on additions and ADUs: foundation type (post-tension slab versus raised), sewer lateral distance for the new bathroom, electrical service capacity at the main panel, and the finish level you choose. A simple one-bedroom ADU with laminate counters and a fiberglass shower is a different animal than a two-bedroom with a full kitchen, custom cabinetry, and a GAF Timerbline UHDZ roof. Mason Remodeling Co gives you a written line-item price before any work starts, so you see exactly where the money goes: site prep, foundation, framing, roofing, plumbing, electrical, drywall, paint, and finishes. No change orders cooked up mid-project. No verbal estimate that grows. The written price is standard on every job, and the 365-Day Done Right Promise covers the completed work for 365 days from completion date.

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