San Anselmo T1-11 Siding

T1-11 is the grooved plywood panel siding you see on many mid-century houses, later additions, garages, studios, and sheds. It is inexpensive, quick to hang, and has a relaxed, rustic look that suits a wooded lot. It also has one serious weakness: it is plywood, and plywood fails when water gets into it.

San Anselmo sits in a shaded, humid part of the Ross Valley with wet winters, which makes that weakness easy to see. Bay Area Siding Company is a siding and window contractor based in Larkspur, a few minutes down the valley. We handle T1-11 in three ways:

• Replace it with a more durable panel or lap product.
• Re-install new T1-11 where the rustic look is the point.
• Repair it when the damage is limited.

Have panels that are swelling, peeling, or soft at the bottom? Request a bid or call (415) 619-5167, and we'll come look at the wall.

What T1-11 is, and why it ages the way it does

T1-11 is a 4×8 (or taller) sheet of plywood or OSB. Vertical grooves are milled into the face so the sheet reads as boards. That design has three consequences:

• The grooves and the bottom edge are the weak points. Grooves collect water. The bottom edge exposes raw end grain that wicks moisture upward if it isn't sealed and held above grade.
• The horizontal seams matter. Where one panel stacks on another, the joint needs flashing (often a Z-flashing). Older and do-it-yourself installs frequently skipped it, so water runs straight behind the siding.
• The finish is doing the protecting. Paint or stain is the main barrier between the wood and the weather. When the finish goes, the plies start to swell and separate.

In a sunny, dry location, a well-maintained T1-11 wall can last a long time. On shaded San Anselmo elevations, where walls stay damp longer after winter storms, the finish tends to break down sooner and moisture stays in the wood longer. That is ordinary wood behavior in a slow-drying environment, not a flaw specific to San Anselmo. In practice, the north walls, the creek-facing sides, and the lowest foot of the wall are usually where trouble shows up first.

Signs your T1-11 is past saving

• Panel edges that are flared, fuzzy, or peeling apart in layers
• A bottom course that is dark, soft, or crumbling where it meets soil, a deck, or a patio
• Paint that fails again within a season or two of repainting
• Open or rusty horizontal seams, or staining that runs down from them
• Grooves packed with mildew that won't clean out
• Visible waviness where panels have swelled against their fasteners

One damaged panel on a garage wall is a repair. Delamination across a whole elevation usually means replacement.

The detail that changes the scope: is your T1-11 also your sheathing?

This matters for your budget more than anything else on this page.

T1-11 comes in structural grades. In many installations, particularly older ones and outbuildings, the panels were nailed directly to the studs with no separate sheathing behind them. The siding is the sheathing, and it may also be part of what braces the wall.

That affects what a tear-off means:

• Where T1-11 sits over separate sheathing (plywood or OSB behind it), removal works like any re-side. The cladding comes off, exposing the sheathing, water-resistive barrier (WRB), and flashing for inspection and correction.
• Where T1-11 is the only layer, removing it does expose the framing. The new scope then usually includes installing sheathing, then a WRB and proper flashing, and then the new siding. Because that sheathing can also play a structural bracing role, we confirm the approach with the Town of San Anselmo Building Division rather than assume.

We find out which situation you have during the consultation, often by checking at an outlet, a window edge, or the attic or crawlspace. That way the bid reflects your actual wall, not a guess.

Your options for replacing T1-11

Most San Anselmo homeowners who call about T1-11 want to keep the vertical, woodsy character without the upkeep. There are good ways to do that.

• Fiber cement panels or board-and-batten. Fiber cement is noncombustible, stable in damp conditions, and resistant to rot and insects. It comes in vertical panel and board-and-batten looks that read close to T1-11. It is heavier and needs careful installation. If you're leaning toward a specific brand, see our San Anselmo James Hardie page.
• LP SmartSide panels. This engineered wood is treated against moisture and insects. It keeps a real-wood texture, is lighter and easier to work than fiber cement, and needs periodic painting. It's the closest like-for-like upgrade for many studios and additions.
• New T1-11. This is a reasonable choice for a shed, a detached workshop, or a building where budget and rustic character come first. We install it with sealed edges, flashed horizontal joints, a WRB behind it, and proper clearance at the bottom. Those are the details that decide whether it lasts.
• A lap or shingle profile. Some owners use the tear-off as a chance to change the look entirely, for example to match a Craftsman main house. That opens up wood, vinyl, and CertainTeed options.

For the general re-side decision and material overview, start with our San Anselmo siding replacement page or the siding replacement hub.

San Anselmo factors that shape a T1-11 job

Hillside parcels and fire exposure. T1-11 is wood, and wood is combustible. Neighborhoods on the wooded slopes, including Seminary Hill, the Bald Hill side, and areas near Sleepy Hollow, have genuine wildfire exposure. Cal Fire has also been updating Fire Hazard Severity Zones for local-responsibility areas across the Ross Valley. Whether California's ignition-resistant exterior provisions (Chapter 7A) apply to your wall depends on:

• your parcel's designation,
• which assemblies you're altering, and
• the scope of work.

Ross Valley Fire and the Town make that determination, and we check it before the material is chosen. Either way, many hillside owners replacing failing T1-11 decide a noncombustible fiber-cement panel is worth considering.

The creek flats. On the lower flats near San Anselmo Creek, where the town has a long flood history, the base of the wall is where T1-11 suffers most. We pay close attention to:

• clearance above grade and hardscape,
• sealed bottom edges, and
• drainage behind the cladding.

If your parcel is in a FEMA special flood hazard area, floodplain rules on materials and improvements can apply below the flood elevation. That is something to confirm with the Town for your property.

Permits. Re-siding in town generally requires a permit through the Town of San Anselmo Building Division. That is especially likely if the sheathing is being added or replaced. We confirm the requirement for your scope and pull the permit.

How a T1-11 job usually runs

1. Walk-through. We look at the panels and identify the wall build-up.
2. Written scope. You get a scope that separates siding work from any sheathing or rot repair.
3. Permit. We handle the permit where required.
4. Tear-off and repair. The old T1-11 comes off, and we repair or add sheathing as needed.
5. Water management. The WRB and flashing go on, with extra attention at windows, doors, and panel seams.
6. Install. The new siding goes up following the manufacturer's instructions.
7. Finish. Trim, paint (if the product needs it), cleanup, and a final walkthrough with you.

If your windows are also aging, doing them during the re-side is often the most efficient time. See our San Anselmo window replacement page.

What drives the cost

We don't post prices because T1-11 jobs vary widely. The main factors are:

• whether the panels double as sheathing, and so whether new sheathing is needed,
• how much hidden rot sits at seams, sills, and the bottom of the wall,
• total wall area and height, plus access on sloped hillside lots,
• the replacement material you choose,
• trim, flashing, and window-opening details, and
• any fire-zone or floodplain requirements tied to your parcel.

FAQs

Can I just repaint my T1-11 instead of replacing it? Yes, if the plies are still tight and the wood is firm. Repainting protects sound panels. It won't fix panels that have already delaminated or gone soft, and paint over swollen wood usually fails quickly.

My San Anselmo home is on a hillside. Can I put new T1-11 back up? Possibly, but it depends on your parcel's fire hazard designation and your scope of work. We confirm the requirements with Ross Valley Fire and the Town of San Anselmo before you commit to a material. Many hillside owners choose fiber cement for its noncombustibility.

Will removing the T1-11 expose my framing? Only if the T1-11 is the sole layer on the wall. If there's sheathing behind it, removal exposes the sheathing and WRB. If there isn't, we add sheathing as part of the job. Either way, we identify which case you have before we bid.

Do you handle just a garage, studio, or addition? Yes. Detached buildings and additions are common T1-11 jobs, and we can match or upgrade them to suit the main house.

Ready to deal with that failing panel siding? Call (415) 619-5167 or request a bid. You can see completed exteriors in our project gallery and learn more about how we serve Marin County.