Concord T1-11 Siding

A lot of Concord's postwar and later tract housing — plus an enormous number of its garages, sheds, and detached workshops — wears T1-11, the grooved plywood or OSB panel siding that went up fast and cheap from the 1960s through the 1980s. It gave a generation of homes their vertical-groove look, and on the flat valley floor you can still spot it block after block off Clayton Road and the older Monument corridor. It also has a finite service life, and Concord's hot, dry summers followed by wet winters test that life hard. Bay Area Siding Company, based in Larkspur, works on T1-11 two ways: refreshing it where it still makes sense, and replacing it with something longer-lasting where it's had enough. This page covers both roads.

Not sure whether to save or replace your T1-11? Call (415) 619-5167 or request a bid.

Why T1-11 fails — and what to look for

T1-11 is a wood-product panel, and its entire survival depends on staying sealed. The moment the paint or stain film breaks down, water reaches the plywood plies or the OSB core, and the decline begins. In Concord, the intense summer UV degrades the finish fastest on the south- and west-facing walls, and the winter rains then find every gap. The tells:

• Swelling, softening, or a spongy feel along the bottom edges of panels, where water collects
• Delamination — the face ply bubbling or separating from the panel
• Grooves darkening or opening up, and paint chalking or peeling
• Fasteners backing out, or a panel pulling loose from the wall
• Dark staining, mushrooming edges, or a musty smell near the base of a wall

A little of this can be repaired. A lot of it — especially delamination and swelling across multiple panels — means the material is past patching.

The two roads from here

Once we've looked at the actual condition, most Concord T1-11 jobs go one of two ways.

Road one — replace it with a longer-lasting material. This is the common recommendation when panels are widely failing, and tearing off tired T1-11 is the ideal moment to correct what's behind it and upgrade the whole wall. Homeowners most often move to:

• Fiber cement — noncombustible, heat-stable, and far more forgiving of Concord's climate; the durable long-horizon choice.
• LP SmartSide — engineered wood that keeps a warm, real-wood texture, if you liked the wood character of the original.
• Vinyl — the budget route where the parcel and look allow it.

If you want the vertical-panel look specifically, both fiber cement and engineered wood come in panel profiles that echo the T1-11 style without the delamination risk.

Road two — reinstall or refresh T1-11. Sometimes staying with T1-11 is the right move: a detached garage, a shed, an ADU, a rustic cabin-style home where the look is the whole point, or a budget that needs the least-cost path. New T1-11, primed on all edges and faces and kept sealed on a maintenance schedule, will serve for years. We'll be candid about the upkeep it demands so you go in clear-eyed.

The fire-edge consideration

One local factor can settle the question for certain addresses. Most of Concord is flat valley floor with no fire-zone designation, but Cal Fire's updated maps, adopted locally in 2025, place High and in spots Very High Fire Hazard Severity Zones along the city's southeastern edges — around and south of Ygnacio Valley Road near the Cal State East Bay campus, Crystyl Ranch, and the Lime Ridge and Thurgood Marshall Regional Park slopes, under the Contra Costa County Fire Protection District (Con Fire). T1-11 is combustible plywood or OSB. On a parcel where California Building Code Chapter 7A governs the exterior wall, reinstalling wood-panel siding generally isn't the appropriate direction — a noncombustible material like fiber cement is. Whether Chapter 7A applies depends on your parcel designation and scope, and we confirm that with the City before recommending. Away from that edge — which is most of the city — keeping T1-11 remains a legitimate option.

What the work involves

For a replacement, we remove the old panels, inspect the sheathing and substrate underneath, confirm the water-resistive barrier, and correct flashing at windows, doors, and penetrations — the layers a T1-11 failure often lets water reach — before new cladding goes up to the manufacturer's spec. (Tear-off exposes the sheathing and WRB, not the framing; studs only enter it if the sheathing itself has to be opened for a repair.) For a T1-11 reinstall, sealing every edge correctly and a proper finish are the whole ballgame. Either way, exterior siding work in Concord is permitted through the City of Concord Building Division, and we handle the permit.

What affects the cost

The single biggest variable is which road you take — a like-for-like T1-11 reinstall on a garage prices very differently from a whole-house conversion to fiber cement. Beyond that: the size and height of the structure, how much substrate or dry-rot repair the tear-off exposes (T1-11 failures usually bring some), the material you move to, trim detail, site access, and any fire-zone assembly requirement. We price after seeing the panels.

FAQ

Can my T1-11 be repaired instead of replaced? Sometimes. Isolated damage — a swollen bottom edge, one bad panel, a section that lost its seal — can be repaired and repainted. Widespread delamination, swelling across many panels, or rot behind the siding usually means replacement is the better spend.

What should I replace failing T1-11 with in Concord? Most homeowners move to fiber cement for its heat stability and fire resistance, or to LP SmartSide to keep a real-wood texture. Both come in panel profiles that preserve the vertical-groove look if you like it. Vinyl is the budget option where the parcel allows.

Is it OK to put new T1-11 back on if my house is near the fire zone? On a Chapter 7A parcel, generally no — it's a combustible wood panel, and noncombustible cladding is the appropriate direction. We check your parcel's designation with the City first. Outside those southeastern zones, a new, properly sealed T1-11 reinstall is a valid choice.

How long does T1-11 last here? It depends almost entirely on maintenance. Kept sealed and repainted on schedule, it can last decades; let the finish fail in Concord's sun and winter rains and water gets into the panel within a few seasons. That maintenance reality is why many owners switch to a lower-upkeep material at replacement time.

Get a T1-11 assessment for your Concord home

Call (415) 619-5167 or request a bid. We'll look at the panels, tell you honestly whether they can be saved, and price both roads so you can decide. Weighing replacement materials? See fiber cement, LP SmartSide, and the full Concord siding replacement overview. We serve Contra Costa County; see finished work in the project gallery.