James Hardie Junior ADU Conversion: The San Diego Owner’s Guide
We install and replace James Hardie siding, trim, and HardieBacker board as part of junior ADU conversions across San Diego, with most partial replacements running $1,400 to $4,800 installed depending on wall area, access, and what’s underneath. We stock the common James Hardie profiles on our trucks, so a failed section can usually be matched and swapped within a day or two, not weeks. Most homeowners discover a problem the same way: a corner of HardiePlank lap siding has started to swell or crumble at the bottom edge, or HardieBacker in a shower surround has gone soft behind the tile after years of moisture getting past old grout. It rarely fails all at once. It starts small, down near the soil line or behind a shampoo shelf, and spreads from there.
One thing to be clear about: Seedling ADU San Diego is an independent ADU and conversion contractor. We are not affiliated with James Hardie, and we don’t sell their products directly. We buy James Hardie materials through regional suppliers and install them according to the published fastening and clearance specs.
Which James Hardie Models This Applies To
Junior ADU conversions in San Diego typically involve one of three James Hardie product families. The first is HardiePlank lap siding, which comes in 5/16-inch thickness and standard exposures of 4, 5, 6, 7, and 8 inches. The second is HardiePanel vertical siding, a 4×8 or 4×10 sheet product often used on the upper portions of garage conversions. The third is HardieBacker cement board, the 1/2-inch or 1/4-inch tile backer used in kitchens, wet bars, and bathroom builds inside the ADU.
The compatibility gotcha most San Diego owners run into is color and profile matching. HardiePlank comes in Smooth, Select Cedarmill, and Colonial Roughsawn textures, and a color that was installed five years ago has faded under coastal sun. A new board in the factory color won’t blend until it weathers or gets painted. We flag this before ordering, because no one wants a two-tone wall that took two weeks to install.
For aftermarket versus OEM: James Hardie boards are the OEM, and they are also basically the only game. There is no meaningful aftermarket equivalent that carries the same 30-year limited warranty and performs the same in San Diego’s salt air and sun. The one exception is fasteners and flashing, where we’ll often use a higher-spec stainless fastener at coastal properties than what the original installer may have used, especially within a few miles of the water where standard electro-galvanized nails corrode early.
Ben Ortega will tell you the same thing he tells every client: if the board is a James Hardie board, match it with James Hardie. Saving fifty cents a square foot on a no-name fiber cement sheet is not a bargain three years in when it starts cupping.
Signs Your James Hardie Junior ADU Conversion Is Failing
- Swelling or crumbling at the bottom edge of lap siding. This is usually ground contact or splashback. HardiePlank should sit a minimum of 6 inches above grade, and when a concrete walkway or planter was added later, water wicks up and breaks down the board from the bottom. We see this in older Normal Heights and North Park garages where patios were poured after the original build.
- Gaps opening at butt joints. James Hardie specifies a 1/8-inch gap between boards at the ends, caulked and painted. When that caulk fails and San Diego’s winter rains hit the gap, water gets behind the board and starts working on the framing. The board itself may look fine for another year while the studs behind it don’t.
- Soft spots in HardieBacker behind tile. Press on the tile in a shower or above a sink. If it flexes, water has likely penetrated the grout and the backer board underneath is saturated. HardieBacker is water-resistant, not waterproof, and it depends on a proper waterproofing membrane over it. Skip the membrane, and the board fails slowly but surely.
- Rust staining around nail heads or at board corners. With original construction more than ten years old, especially west of I-5, standard nails corrode and leave orange streaks running down the siding. The board is usually fine; the fasteners are not.
- Loose or sagging sections after Santa Ana winds. If a section of lap siding moves when you press on it, the fasteners have let go. This is more than cosmetic. Once wind-driven rain gets behind the board, the framing and insulation become the next problem, and that repair doubles in price.
Replacement Cost & What’s Included
For a junior ADU in San Diego, plan on these installed ranges for James Hardie work:
| Scope of Work | Installed Price Range |
|---|---|
| Replace 1-3 damaged HardiePlank boards (includes paint-to-match on the patch) | $400 – $900 |
| Replace a full wall section (up to 120 sq ft) | $1,400 – $2,800 |
| Re-side an entire junior ADU exterior (350-500 sq ft) | $6,500 – $12,000 |
| Replace HardieBacker in a shower or wet bar surround | $900 – $2,600 |
Every quote is written before work begins. You’ll see the board cost, fastener cost, flashing, disposal, labor, and paint or primer separately. No line item appears after the crew starts that wasn’t in the written price. That’s the way I have always run jobs, and it’s non-negotiable. The work itself is covered by our 365-Day Done Right Promise: if the install is not done right, we make it right.
The range above shifts based on access. A second-story wall off a ladder costs more than a ground-level garage wall. A repair behind an unmovable water heater costs more because the tank has to be disconnected and reconnected, and we’ll bring a plumber on that day. These details are why we walk the property before we quote, not after.
DIY or Call Us?
James Hardie fiber cement is not a dangerous material to work with, but it is an unforgiving one. You can safely check for the symptoms above: press on the boards, look at the nail heads, run a hose at the base and watch where water pools. What you should not DIY is cutting and fastening the board itself. HardiePlank requires carbide-tipped blades, specific fastener spacing and edge clearances per the published installation specs, and correct flashing at every transition. Get the clearances wrong and the board will telegraph seams or crack within a year. Get the flashing wrong and you’ve moved the water problem from the board to the framing. If you’re in San Diego and the wall faces the ocean, the stakes are higher: salt air attacks shortcuts quickly. Call us at (858) 988-6447 for a written estimate, and know the number before the shovel moves.
FAQs
With correct installation, James Hardie products carry a 30-year limited warranty and commonly last that long or longer in San Diego’s climate, though coastal properties within a few miles of the water should plan on fastener replacement at the 10 to 15 year mark. The board usually outlasts the nails holding it.
Small cracks and chips can be patched with a cementitious patching compound and repainted, but any board that has swelled, crumbled, or lost its fastening points should be replaced. Patching over a moisture problem hides it, and the board continues to break down underneath. Call us at (858) 988-6447 and we’ll tell you honestly which path makes sense.
A three-board patch usually takes half a day including paint touch-up. A full wall section runs one to two days. A shower surround HardieBacker replacement typically runs two days, because the tile has to come out, the board has to cure, and the waterproofing membrane has to be applied before new tile goes up. We’ll give you the schedule in writing.
Our crews carry common 5/16-inch HardiePlank in 4-inch and 6-inch exposures in Smooth and Cedarmill textures, plus 1/2-inch HardieBacker sheets, in San Diego. We also work with Abodu, Cover, Dvele, and other prefab brands for whole-unit builds, so if your conversion involves one of those, we can handle the full scope. If your specific color or profile is not on the truck, we order it from our San Diego supplier and have it within a few business days.
If your junior ADU is showing signs of failure, don’t wait until the next rain. Call Seedling ADU San Diego at (858) 988-6447 for a free written estimate. We’ll walk the property, show you exactly what’s going on, and hand you a price before any work begins.
Written by Ben Ortega, Owner, Seedling ADU San Diego at Seedling ADU San Diego, serving San Diego and nearby areas.
