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James Hardie Home Additions & ADU: The Houston Owner’s Guide
We install James Hardie siding and trim on home additions and ADUs across Houston, with most projects landing between $45,000 and $130,000 depending on square footage, foundation work, and finish level. That’s the installed range for a full addition wearing HardiePlank lap siding, HardieTrim boards, and HardieSoffit panels, permitted and inspected. Most homeowners don’t realize their addition’s exterior is failing until they spot swelling at the bottom edge of a plank or a trim board that’s gone soft behind the paint. In Houston’s humidity, water wicks into Hardie products when they’re installed too close to grade or flashed wrong, and that’s the most common failure point we see on additions built by other crews.
Which James Hardie Models This Applies To
Home additions and ADUs in Houston typically use one of three James Hardie product lines, and each has its own compatibility rules. HardiePlank HZ5 lap siding is the standard for walls, and it requires a 1/4-inch gap between plank ends and a 6-inch minimum clearance above grade. HardiePanel HZ5 vertical siding is common on ADUs with a more modern look, but it’s not interchangeable with HardiePlank at corners, and improvised transitions are where we see the first failures. HardieTrim HZ5 boards come in 4/4 and 5/4 thicknesses, and the 5/4 is required for outside corners on most elevations with deeper shadow lines.
If you have an existing James Hardie addition and need a section matched, pull a plank or check an exposed edge for the mill stamp. HZ5 products are stamped, and the color code is usually printed on the back of the plank. Aftermarket equivalents exist, but Hardie’s ColorPlus finish has a 15-year limited warranty that an aftermarket repaint won’t match. We stock HZ5 lap siding in 8-1/4-inch and 6-1/4-inch exposures, 4×8 HardieSoffit panels, and both 4/4 and 5/4 HardieTrim in the common Houston profiles.
Styles & brands we work with
Signs Your James Hardie Home Additions & ADU Is Failing
- Swollen plank edges at the bottom course. In Houston, this is almost always ground splash or a grade clearance below 6 inches. The plank soaks water from the bottom, swells, and starts delaminating at the factory edge.
- Caulk pulling away from trim joints. HardieTrim expands and contracts with Houston’s heat. If the original installer used a cheap acrylic caulk instead of a high-quality elastomeric, the joint opens and lets water behind the trim.
- Paint blistering on the face of a plank. Hardie’s substrate holds paint well, but if moisture is trapped behind the plank from a missing housewrap layer, the paint lifts from hydrostatic pressure. Repainting won’t fix it.
- Soffit panels sagging or showing rust stains at fasteners. HardieSoffit needs stainless or hot-dipped galvanized fasteners. Plain steel nails rust in Houston’s humidity, and the rust bleeds through the paint.
- Cracked outside-corner trim boards. If the installer used 4/4 HardieTrim where 5/4 was specified, the thinner board flexes more and eventually cracks at corner joints during seasonal movement.
See the difference
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After
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AfterReplacement Cost & What’s Included
Section repairs on a James Hardie addition in Houston run $1,800 to $5,500 when we’re replacing failed planks, trim, or flashing. A full re-side of an existing ADU or addition lands between $18,000 and $45,000. New construction with James Hardie as the specified exterior is the $45,000 to $130,000 range above, because you’re paying for framing, foundation, electrical rough-in, and all interior finishes at the same time.
Every number we give Houston homeowners is a written price before work starts. That includes tear-out of failed product, replacement planks or trim, housewrap repair if needed, flashing replacement, caulk and paint, and haul-off. If the original underlayment or sheathing is rotten, that’s a separate line item, and we tell you before we touch it, not after. All our work carries the 365-Day Done Right Promise: if a repair fails inside a year, Curtis Lane sends a crew back at no charge.
DIY or Call Us?
You can safely check your addition’s grade clearance, look for swollen plank edges, and run a hose along the wall to see where water pools. What you shouldn’t do is pull planks off a wall without knowing what’s behind them. James Hardie products are heavy, and a 12-foot HardiePlank lap siding piece weighs over 20 pounds. Cutting Hardie products generates silica dust that requires proper respiratory protection, and the manufacturer’s installation specs include specific fastening patterns and joint treatments that most first-timers get wrong. If you’ve got a failing section on your Houston addition, call (940) 289-7658 and we’ll give you a written scope and a straight number.
FAQs
James Hardie siding typically lasts 30 to 50 years when installed to spec with proper clearance, flashing, and fasteners. Houston’s humidity and storm-driven rain shorten that lifespan when the original installer cut corners on housewrap or grade clearance, and we regularly replace planks on additions that are only 8 to 12 years old. Call (940) 289-7658 and we’ll tell you whether your product is worth repairing or replacing.
Yes, isolated sections of failed HardiePlank can be cut out and replaced with matching HZ5 planks, and this is the most common repair we do in Houston. The key is matching the exposure and the ColorPlus color code, and we stock the common profiles on our trucks. Call (940) 289-7658 for a written repair quote.
A standard 400-square-foot code-compliant ADU with James Hardie exterior typically takes 10 to 16 weeks from permit to final inspection in Houston. A smaller addition of 200 to 300 square feet usually runs 6 to 10 weeks. Weather and permit office backlogs at the City of Houston are the variables that push these windows out, and we’ll tell you the schedule before you sign anything.
We stock HardiePlank HZ5 lap siding in the two most common Houston exposures, 4×8 HardieSoffit panels, and 4/4 and 5/4 HardieTrim boards at our Houston yard. Special-order colors and exposures arrive in 7 to 10 business days. We also work with KraftMaid, Cambria, and Silestone for interior finishes on Houston ADU projects, so the exterior and interior specs stay documented from day one.
Written by Curtis Lane, Owner, Bradford Remodeling Co Houston at Bradford Remodeling Co Houston, serving Houston and nearby areas.
