Lafayette T1-11 Siding

If your Lafayette home was built or remodeled in the mid-century era, there's a fair chance it wears T1-11 — the grooved plywood or OSB panel siding, run vertically, that was everywhere on ranches, sheds, and tract homes of that period. It gave a lot of Lafayette houses their plain, woodsy look. It also has a well-known weakness: T1-11 lives and dies by its paint and sealant, and on Lafayette's damp, shaded lots that finish tends to lose the fight. Bay Area Siding Company works with T1-11 both ways — replacing panels that have run their course, and reinstalling fresh T1-11 where a homeowner genuinely wants that rustic panel look. This page covers both.

Got T1-11 that's failing — or that you want to keep? Call (415) 619-5167 or request an estimate.

How T1-11 fails, and why Lafayette is hard on it

The Lafayette siding-replacement conversation often starts with T1-11 that's delaminating at the bottom edge — and that's exactly the failure mode this material is prone to. Because it's a wood-based panel, once water gets past the finish it wicks in, and the plywood plies or the OSB core start to swell, split, and separate. On the north and downhill walls of a canopy-shaded hillside lot, where the panel stays wet longest, you'll see it first:

• Swelling, splitting, or a spongy feel at the bottom course and around penetrations
• Plies visibly separating (plywood) or the core crumbling (OSB)
• Paint that won't hold — blistering and peeling within a season or two in the shade
• Dark staining, soft spots, or a musty smell where moisture has gotten behind the panel

A panel or two can sometimes be swapped. When it's showing up across several elevations, the material has reached the end of its service life and it's time for a decision.

The usual Lafayette move: replace T1-11 with something more durable

Most homeowners who are re-siding away from failing T1-11 don't put T1-11 back — they upgrade to a cladding that handles Lafayette's moisture and, on the hillside parcels, its fire exposure:

• Fiber cement — noncombustible, rot- and moisture-resistant, and the common upgrade on wooded fire-zone lots. See also James Hardie.
• LP SmartSide engineered wood — keeps a warm wood look and a similar panel or lap option, treated against moisture and termites, tougher than the old T1-11.

The fire point is real here: a large share of Lafayette sits in mapped Fire Hazard Severity Zones — mostly High, with a smaller Very High area on the slopes. Both T1-11 and engineered wood are combustible; on a designated parcel a noncombustible fire-rated assembly may be required, which is one more reason many hillside re-sides move to fiber cement. Whether it applies to your parcel depends on its designation and the scope of work, and we confirm that with the City of Lafayette and the Contra Costa County Building Inspection Division rather than assume.

When reinstalling T1-11 makes sense

There's still a legitimate case for new T1-11: it's budget-friendly, it goes up quickly, and for a shed, an outbuilding, an ADU, or a home where the owner wants to preserve that specific rustic mid-century panel look, it's the honest choice. If we reinstall it, the key is doing the finish and detailing right — proper sealing at the grooves, edges, and penetrations, correct flashing above and below, and a maintenance plan to keep it painted — because that's the only thing that makes T1-11 last. On a fire-zone parcel we'll flag the combustibility question before you commit.

What's behind the panels matters more than the panels

Whether you replace T1-11 or reinstall it, the wall underneath is where the real work is. We remove the old panels to the sheathing so we can inspect and correct the water-resistive barrier and the flashing — the layers that decide whether the next siding lasts. On lots where T1-11 has been wicking water for years, we often find a WRB that needs replacing or sheathing that's taken on moisture; we correct that before new cladding goes on. (Framing is exposed only if the sheathing itself has failed — a larger scope we'll flag if we find it.) Re-siding of this scope generally requires a building permit through the City of Lafayette Planning & Building Department, with inspection contracted to the Contra Costa County Building Inspection Division; we handle it.

Cost drivers

We price from the house. The drivers are whether you're replacing with a new material or reinstalling T1-11, how much substrate repair the failing panels have caused, the size and story count of the exterior, hillside access, and any fire-zone assembly detailing on a designated parcel. A walkthrough is the only honest number.

FAQ

Can you just replace a few bad T1-11 panels? Sometimes — if the damage is isolated. But when swelling, delamination, or paint failure shows up across several walls (common on shaded Lafayette lots), you're usually looking at a whole-wall decision rather than a patch, and it's often the moment to upgrade the material.

Should I put T1-11 back or switch materials? It depends on the home and the parcel. On a wooded hillside fire-zone lot, many owners move to noncombustible fiber cement; for a rustic look outside the fire zones, or for a shed or ADU, reinstalling T1-11 or moving to engineered wood can make sense. We'll lay out the options for your situation.

Is T1-11 allowed on a Lafayette fire-zone parcel? T1-11 is combustible, so on a High or Very High FHSZ parcel it may not be the right choice and a noncombustible fire-rated assembly may be required depending on your parcel and scope. We confirm the requirement with the City and county before recommending it.

Do I need a permit to re-side over failing T1-11 in Lafayette? Generally yes for a re-side of this scope. Permits are issued by the City of Lafayette Planning & Building Department with inspection contracted to the Contra Costa County Building Inspection Division. We handle the submittal.

Let's sort out your T1-11

Call (415) 619-5167 or request an estimate. We'll assess the panels and the wall behind them and tell you honestly whether to patch, replace, or reinstall. See finished work in the project gallery, explore the Contra Costa service area, or read about window replacement in Lafayette.