James Hardie New Detached ADU Nationwide

James Hardie New Detached ADU: The San Bernardino Owner’s Guide

We stock and install James Hardie-compatible new detached ADU systems in San Bernardino, with typical installed project ranges running $180,000 to $280,000 depending on size and site conditions. Most homeowners discover their James Hardie ADU needs attention when moisture readings spike behind the fiber-cement panels, or when the integrated trim system shows hairline separation after a hot San Bernardino summer followed by winter rain. We carry the full HardiePanel HZ5 series spec, the HardieTrim NT3 profiles, and the moisture-management flashings that San Bernardino’s inland-heat climate demands - all under our single-contract design-build model with a written price before any work starts, per Haven Standard Clause 1.

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Which James Hardie Models This Applies To

James Hardie manufactures two distinct product lines for detached structures in California: the HZ5 series, engineered for wet-freeze climates but frequently spec’d in San Bernardino by architects unfamiliar with inland-heat cycling, and the HZ10 series, which is what we specify for every new detached ADU we build here. The difference matters. HZ10 fiber-cement panels carry a higher acrylic-resin content that resists the thermal expansion we see when San Bernardino summer surfaces hit 140°F. You can identify your existing panels by the stamp on the back face - HZ10 or HZ5 - visible if you remove a single panel at a corner. HardiePanel comes in 4×8, 4×9, and 4×10 sheets; HardiePlank lap siding runs 5/16-inch thick in 6.25-inch and 7.25-inch reveals. The trim system is equally specific: HardieTrim NT3 for corners and band boards, HardieSoffit for vented eave closure. We do not source through big-box channels. Our Hardie materials arrive factory-direct with the full 30-year product warranty intact, and we register that warranty in the homeowner’s name at final inspection. Ellery ADU Studio San Bernardino is an independent design-build studio, not affiliated with James Hardie.

Signs Your James Hardie New Detached ADU Is Failing

• Panel edges swelling or “fuzzing” at cut lines. This indicates moisture wicking through unsealed or improperly primed cuts. James Hardie panels must be field-primed with an alkali-resistant primer within 90 days of cutting; in San Bernardino’s low humidity, contractors often skip this, and the panel core degrades in two to four years.
• Trim separation at mitered corners, especially on south and west faces. The NT3 trim expands and contracts with temperature swings. Without proper gap allowances at installation, the miter joints open. We’ve measured 3/16-inch gaps on three-year-old ADUs near Perris Hill that were tight at final walkthrough.
• Interior humidity spikes or musty odor in finished ADU spaces. Hardie’s integrated moisture-management system relies on a drainable housewrap and flashings at every penetration. When the kickout flashing above a roof-to-wall intersection is omitted or the horizontal lap flashing is reverse-lapped, water enters the cavity and the fiber-cement panels act like wicks.
• Paint failure or chalking before year seven. James Hardie prefinished panels carry a 15-year finish warranty, but only when paired with the specified touch-up system. We’ve seen San Bernardino ADUs where painters used standard exterior latex on ColorPlus panels; the finish chalks in 18 months and voids the warranty.
• Soffit vent blockage or pest intrusion at eave lines. HardieSoffit panels require specific vent strip profiles and insect screening. Blockage traps attic heat, accelerating panel movement and fastener fatigue. We find this most often on ADUs where the original builder substituted generic vent strips to save $80 in material.

Replacement Cost & What’s Included

A full James Hardie panel replacement on an existing detached ADU in San Bernardino typically runs $8,500 to $16,000 for a 400-square-foot wall area, including removal, substrate inspection and repair, factory-direct HZ10 panels with ColorPlus finish, NT3 trim, proper flashings, and reinstallation to James Hardie’s published best practices. Trim-only replacement on a standard 24-foot eave line runs $2,200 to $3,800. What drives the range: substrate condition behind the panels (we open one panel at estimate to check), the number of penetrations requiring custom flashing fabrication, and whether the original installation used the specified corrosion-resistant fasteners - San Bernardino’s Santa Ana wind cycles will pull standard fasteners in five to seven years. Every replacement quote includes the Haven Standard documented photo record: before, during, and after, with moisture-meter readings at each phase. Our 365-Day Done Right Promise covers the installation; the James Hardie product warranty covers the materials.

DIY or Call Us?

There’s a safe middle ground here. A homeowner can verify their panel series stamp, check for obvious flashing gaps with binoculars, and confirm whether paint is chalking by rubbing a dark cloth on the surface. What you cannot safely assess: the substrate condition behind the panels, the integrity of the moisture barrier, or whether fasteners are backing out. Removing a single HardiePanel requires supporting the adjacent panels, cutting corrosion-resistant fasteners without damaging the underlying structure, and reinstalling with proper clearances - a 3/16-inch gap at vertical joints, 1/8-inch at horizontal laps. The panels weigh 2.3 pounds per square foot; a 4×10 sheet is unwieldy on a ladder. More critically, improper reinstallation voids the 30-year product warranty and creates latent moisture damage that we find two years later when the repair cost has tripled. If you’re seeing the symptoms above, call (909) 359-6463. We’ll open one panel during a free estimate, show you what’s behind it, and give you a written price before any work starts.

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