South San Francisco T1-11 Siding

T1-11 is the grooved plywood panel siding many homeowners recognize from mid-century houses, additions, garages, and backyard sheds. It went up quickly and cheaply, and it gave a rustic, vertical-groove look. Decades later, the panels tend to fail in predictable ways: swollen bottom edges, peeling face veneer, and grooves that hold water.

Bay Area Siding Company is a Larkspur-based siding and window contractor that works across South San Francisco. That includes the postwar tracts in Buri Buri, Winston Manor, and Westborough, the older homes of Old Town, and hillside streets like Sterling Terrace and Serra Highlands. We handle T1-11 in both directions:

• Replacement. We take failing panels off and re-clad in a more durable material.
• Reinstallation. We install new T1-11 where the rustic panel look is what you want and the parcel allows it.

Talk to someone who works with T1-11: call (415) 619-5167 or request a bid.

Is your T1-11 done, or just overdue for paint?

T1-11 depends on its paint or stain to stay dry. Once that coating fails, especially along the bottom edge and in the grooves, moisture gets into the plywood or OSB, and the panel starts breaking down from the inside. Not every tired wall needs replacing, though. Here is roughly how we separate the two situations.

Probably still maintainable:

• Faded or chalky finish on panels that are still flat and firm
• Small checks in the face veneer with no soft spots underneath
• Isolated damage on one panel, such as where a sprinkler or downspout hit it

Usually past saving:

• Bottom edges that have flared, swollen, or crumbled, often where the panel sits close to soil, a patio, or a roofline
• Face veneer lifting or peeling away in sheets (delamination)
• Panels that flex, sound hollow, or let a screwdriver sink in
• Vertical seams that have opened up, or caulk that fails again right after you redo it
• Staining, musty smells, or damp drywall on the inside of the wall

South San Francisco is a windy, marine-influenced part of the Peninsula, and summer tends to be the windiest stretch. Exposed walls take a lot of moving, damp air and wind-driven rain. That doesn't mean the climate "causes" T1-11 to fail. But a panel whose finish has lapsed has less margin on a wall that sees steady weather, so keeping up with sealing matters more here. If you're seeing several items from the second list, repainting generally won't fix it.

The structural detail that makes T1-11 different

This is the part of a T1-11 job that surprises homeowners, and it's why an honest assessment comes before any quote.

On many homes and outbuildings, T1-11 was installed as a combined sheathing-and-siding panel nailed directly to the wall framing. There is no separate layer of plywood or OSB behind it. In that case, the T1-11 may also be doing structural work, helping brace the wall against racking.

That leads to two very different tear-off scenarios:

• T1-11 installed over existing sheathing. Removal stops at the sheathing. We inspect the substrate, install a new water-resistive barrier (WRB) and flashing, and put the new cladding on. This is a conventional re-side.
• T1-11 serving as the sheathing. Removing it exposes the framing. New structural sheathing has to go on first, and any bracing role the old panels played has to be restored. Only then do the WRB, flashing, and new siding go up. It's a bigger scope, but it's the correct way to do the work.

We confirm which situation you have during the on-site inspection, often by opening a small area. If the job involves new structural sheathing, we confirm the requirements with the City's Building Division before work begins. The two scenarios carry different costs, so if you're comparing bids, it's worth asking each contractor which one they priced.

Replace it with what?

Most South City owners with failing T1-11 move to a material that asks less of them. The common routes:

• Fiber cement. It's noncombustible and resists rot, pests, and moisture, and it holds paint well. Manufacturers offer vertical panel products, so you can keep a grooved or board-and-batten look. It's heavier and needs skilled installation. If you're set on the brand, see our James Hardie siding page for South San Francisco.
• LP SmartSide. This engineered wood has a real-wood texture and is lighter and easier to work than fiber cement. It's the most natural visual swap for owners who like the look of T1-11 but not its upkeep. It still needs periodic painting.
• Vinyl. It's budget-friendly and never needs repainting. The profile choices differ from the vertical-groove look, and it reads less like wood.
• Wood siding. Cedar or redwood board-and-batten gives a natural result, but it has the highest maintenance of these options.

Reinstalling T1-11 can still make sense on a detached shed, a workshop, or a matching repair where the rest of the building stays in panel siding. If you choose it, the details matter:

• Seal all edges, especially the bottom, before installation.
• Keep proper clearance from grade and hard surfaces.
• Flash at horizontal joints.
• Put the panels on a real maintenance schedule.

For a whole-house comparison of materials, our South San Francisco siding replacement page covers the broader decision.

Permits and fire-zone questions in South San Francisco

Who issues the permit. Siding work in South San Francisco goes through the City's Building Division, part of Economic and Community Development. Re-siding is among the projects the City processes as over-the-counter permits. We prepare and pull the permit and coordinate inspections. If new structural sheathing is part of the scope, we raise it with the Building Division during permitting.

Fire Hazard Severity Zones. CAL FIRE issued updated Fire Hazard Severity Zone maps for South San Francisco in 2025, and the City is incorporating them into its building-code process. Parcels near San Bruno Mountain and the Sign Hill open space are the most likely to carry a designation. A designated parcel can bring California Building Code Chapter 7A into play for exterior wall coverings, and Chapter 7A favors noncombustible or ignition-resistant materials. T1-11 is a combustible wood panel, so on a designated parcel it may not be the right choice for re-cladding a home, and fiber cement becomes the more practical path. Whether any of this applies depends on your specific parcel and the scope of work. We check your address with the City before recommending a material rather than assuming either way.

How a T1-11 project usually runs

1. Site visit. We walk the walls, probe suspect areas, and determine whether your panels sit over sheathing or serve as the sheathing.
2. Written scope. You get the materials, any sheathing work, and the flashing plan in writing.
3. Permit. We file with the Building Division.
4. Removal and inspection. We address anything we find behind the panels.
5. Install. WRB, flashing at windows, doors, and transitions, then new cladding installed to the manufacturer's instructions.
6. Finish, cleanup, and walkthrough. We review the finished walls with you.

If your windows are also original single-pane units, replacing them during a re-side lets both jobs share the flashing work. See window replacement in South San Francisco.

What drives the cost

We don't publish prices because T1-11 jobs vary more than most re-sides. The main drivers are:

• Sheathing. Whether new structural sheathing is needed is the largest variable.
• Size. The wall area and the number of stories.
• Material. Your replacement material and profile.
• Openings. How many windows and doors need flashing and trim.
• Hidden damage. The extent of any water damage found at removal.
• Access. Hillside or tight-lot access, common on sloped streets.
• Fire-zone requirements. Any requirements that apply to your parcel.

FAQs

Can you just put new siding over my old T1-11? We don't recommend it. Covering failing panels hides whatever moisture damage exists, and if the T1-11 is your sheathing, it leaves a deteriorating structural layer in the wall. Removing it lets us inspect the wall and build a proper WRB and flashing system.

Will removing T1-11 mean opening my walls to the framing? Only if the T1-11 was installed directly on the framing as the sheathing layer. If there's sheathing behind it, removal stops there. We confirm this at the site visit before quoting.

My South San Francisco home is near San Bruno Mountain. Can I still use T1-11? Possibly not for the house itself. If your parcel carries a Fire Hazard Severity Zone designation under CAL FIRE's 2025 maps, Chapter 7A requirements may apply to the wall covering. We verify your parcel with the City's Building Division and will steer you toward a compliant material, such as fiber cement, if needed.

Can I get the grooved T1-11 look without the maintenance? Yes. Fiber-cement and engineered-wood manufacturers make vertical panel products with a similar appearance and better moisture resistance.

Ready to deal with that swollen, peeling T1-11? Call Bay Area Siding Company at (415) 619-5167 or request a bid. You can browse finished exteriors in our project gallery and see the rest of the Peninsula communities we serve.