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Boral Home Additions & ADU: The San Jose Owner’s Guide
If your existing Boral-finished addition or ADU has a failing component, or you’re planning a Boral-standard addition, you’re looking at an installed cost between $53,100 and $153,400 in San Jose, depending on square footage, foundation work, and finish level. Most owners call us after noticing water intrusion at the transition point between the original structure and the addition, hairline cracks along the exterior cladding, or a roofline that no longer sheds rain the way it did five years ago. We stock Boral-compatible trim, siding profiles, and structural connectors on the truck for common San Jose models, and we can typically complete an inspection and written quote within one visit.
A note before we go further: Penhallow Remodeling San Jose is an independent general contractor. We are not affiliated with Boral, but we have worked with their expansion joint systems, trim profiles, and cladding on enough San Jose additions and ADUs to know exactly which parts fail first and why. Trevor Nash has spent the better part of two decades running remodeling projects, including a stretch of kitchen and bath gut-jobs in San Jose’s Willow Glen neighborhood that taught him how older California homes move, settle, and pull apart at the seams.
Which Boral Models This Applies To
Boral’s product line for home additions and ADUs splits into two broad groups: their exterior cladding and trim systems (think Boral TruExterior siding and trim) and their structural and fire-rated sheathing. The parts you’ll need depend on which system was used when your addition went up. Older San Jose additions, particularly those permitted before 2015, commonly used Boral’s original trim profiles with a 5/4-inch nominal thickness. Newer ADU construction often uses the reduced-profile 1-inch trim board, which is lighter and faster to fasten but more sensitive to expansion gap errors. If you can still read the ink stamp on the back of a removed trim board or the edge of a cladding panel, that’s your fastest route to a correct match.
The OEM-versus-aftermarket decision for Boral matters more than most homeowners expect. Boral’s TruExterior trim carries specific UV-stable polymers that mimic wood grain without the rot. Cheaper aftermarket PVC boards look right from ten feet away but expand and contract at a different rate than the original Boral material. In San Jose’s dry summers followed by rainy winters, that mismatch shows up as corner joints that open up every September. We quote OEM Boral parts where they’re still manufactured and clearly labeled, and we discuss aftermarket options only when a profile has been discontinued.
Styles & brands we work with
Signs Your Boral Home Additions & ADU Is Failing
- Joint separation at the addition’s roofline or corners. This usually means the original installer used the wrong expansion gap for the San Jose temperature swing, or the trim fasteners weren’t long enough to reach structural framing. Hairline gaps under 1/8 inch are cosmetic. Larger gaps let water behind the cladding.
- Soft spots or bubbling under painted trim boards. Boral’s composite trim resists rot but isn’t magic. If a sprinkler head has been hitting the same bottom board for five years, capillary action pulls moisture up behind the paint film. The board swells, the paint lifts, and the substrate behind it starts to rot.
- Stucco or caulk cracking where the addition ties into the original house. California homes move on clay soils. The point where a heavier addition meets the original footprint is where that movement concentrates. The failure is rarely the Boral material itself; it’s the transition detail that wasn’t given room to flex.
- Water staining on interior ceilings near the addition’s exterior walls. By the time you see a brown ring inside, water has been traveling for a while. Common culprit: missing or poorly installed flashing at the step between the addition’s lower roof edge and the original wall.
- Doors and windows in the addition suddenly sticking or binding. This can indicate foundation movement under the newer structure. San Jose’s Willow Glen and Rose Garden neighborhoods, with their older clay soils, are known for this. It’s not always a Boral part failure, but it’s a signal to check the structural connectors between the addition and the main house.
See the difference
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After
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AfterReplacement Cost & What’s Included
A full Boral addition or ADU project in San Jose typically lands between $53,100 and $153,400, with the final number driven by foundation type (raised versus slab), ceiling height, kitchen or bath finish level if the ADU includes them, and how much of the existing structure needs to be tied into and repaired. A modest single-room addition with basic finishes starts near the lower end. A full ADU with a kitchen, bath, and separate entrance climbs toward the upper end, and that’s without factoring in San Jose permit fees or utility trenching.
We give you a written price before any work starts. That number includes the parts, the labor, the fasteners, the flashing, the disposal, and the warranty. You won’t get a change order at the door because the dump fees went up or because we found the sheathing was worse than expected. If we find a condition during the job that changes scope, we stop, we show you exactly what we found, and we hand you a written change order with a new number before we pick the tool back up. Every Boral addition and ADU project carries our 365-Day Done Right Promise: if something fails within a year because of our workmanship, we come back and fix it at no charge. That’s written on every contract.
DIY or Call Us?
Some of this work is safe to inspect yourself. You can walk the addition’s perimeter and note where joints have opened, where paint has bubbled, and whether any trim boards sound hollow when tapped with a screwdriver handle. You can check that sprinkler heads aren’t spraying directly on exterior walls. You can photograph the gaps and email them to us before we arrive. That information makes our first visit faster and more accurate.
What you should not do is remove exterior trim boards or cladding panels on your own. Boral and similar composite systems are installed as part of a water-management assembly with flashing, house wrap, and gap tolerances that all work together. Pull one board without understanding the weather-resistant barrier behind it and you can create a leak where there wasn’t one. The same goes for any work involving the roof-to-wall transition on an addition. Falls from ladder height account for more homeowner injuries than any other project type we see. If you’re not sure, don’t climb up there. Call (669) 274-2362 and we’ll come look. The estimate is free.
FAQs
Boral’s composite trim and siding systems carry a manufacturer’s stated lifespan of 25 years or more, but the real-world life of your San Jose addition’s exterior depends heavily on sun exposure, sprinkler placement, and the quality of the original flashing work. South-facing walls with no shade and chronic overspray will show joint and paint issues within 8 to 12 years. Properly installed and maintained Boral cladding should last two decades or more before boards need replacement. Call (669) 274-2362 if you want an honest assessment of what’s failing and what still has life left.
Yes, often it can. Caulked joints can be cleaned and re-sealed with a high-quality elastomeric sealant. Individual trim boards that have swollen or separated can be replaced without touching the rest of the assembly. The question is whether the repair will hold, and that depends on whether we’re treating a symptom or the cause. If a sprinkler head keeps blasting the same board, no caulk job will last. We’ll tell you that on the first visit. Call (669) 274-2362 to schedule an inspection.
Most trim board replacements, flashing repairs, and joint re-sealing jobs on San Jose homes take one to two working days, provided the structural framing behind the Boral material is sound. Full cladding replacement on a large addition can run a week or more. You’ll get a written timeline with your written price before any work begins. Call (669) 274-2362 and we’ll give you a window you can plan around.
We keep common Boral trim profiles, transition flashing, and fasteners on the truck for San Jose service calls. If your addition requires a specific board width or a discontinued profile, we source it from local supply houses before the job starts and confirm the match in person. We also work with KraftMaid, Cambria, and Silestone products when the addition includes kitchen or bath finishes. Call (669) 274-2362 and tell us what you need; if it’s a part we can confirm over the phone, we’ll bring it on the first visit.
Written by Trevor Nash, Owner, Penhallow Remodeling San Jose at Penhallow Remodeling San Jose, serving San Jose and nearby areas.
