T1-11 Siding: Cost, Lifespan, Problems & Replacement

T1-11 (also written T-111 or T1 11) is exterior-grade plywood or OSB sold as siding panels. The standard sheet is 4 x 8 ft (9 and 10 ft lengths exist for taller walls), 19/32 in. thick (sold as 5/8 in.), with vertical grooves routed 4 or 8 in. on center and shiplap edges so neighbouring panels overlap. The grooves give it a board-and-batten-like rhythm.

It became common on 1970s and 1980s houses and is still the default on sheds, garages and barns, because a single panel is both sheathing and siding. That is the source of its low price — and of its main weakness, since there is often no house wrap or separate sheathing behind it.

Plywood vs OSB T1-11

Plywood T1-11 (fir or southern pine, often rough-sawn) holds up better when wet. OSB versions are cheaper, but raw OSB edges swell badly once water gets in. Treated engineered-wood panels are a different product — see engineered wood siding→.

Sheet price depends mostly on plywood vs OSB, grade, and whether it is pre-primed. Installed cost adds labor, trim, flashing and finishing. Big-box prices move with the lumber market, so check a current sheet price before budgeting a large job.

Planning ranges, not quotes. For a shed, material dominates; for a house, labor, trim and paint do.

Maintained — painted or stained every 3–5 years, with sealed edges and clearance above grade — T1-11 typically gives 20–30 years. Neglected, it fails from the bottom up: the finish goes, the lower edge wicks water from splash-back, the outer veneer delaminates, and rot follows in as little as 10–15 years.

• The bottom edge. End grain at the bottom of every panel wicks water, especially within 6 in. of soil or mulch. This is where rot almost always starts.
• Delamination. Once water reaches the glue lines, the face veneer bubbles and peels away in sheets.
• Grooves hold water. The routed grooves have thinner wood and trap moisture and dirt, so paint fails there first.
• Horizontal joints. Where one panel sits on another, a Z-flashing is needed. Without it water runs straight into the joint.
• No drainage plane. Used as sheathing and siding in one, there is often no wrap behind it to stop water that gets past the face.

Soft spots, bulging panels, and peeling in the grooves are the signs to check. See siding problems→ for how to probe for rot.

Repaint (sound panels)

If the panels are firm and the face isn't delaminating, a professional repaint runs about $1,300–$3,900 for an average home. Wash, let it dry, scrape, prime bare wood, back-brush the grooves, and seal the bottom edges. Painting siding→ covers prep and product choice.

Repair (a few bad panels)

Rotted panels can be cut out and replaced, usually with new flashing at the bottom. The hard part is matching groove spacing and texture on older panels; new panels are typically painted the whole wall to blend. Expect it to cost like other siding repairs→.

Replace (failing across whole walls)

When delamination and soft spots show up on several walls, replacement is usually the better money. Because T1-11 is often the sheathing too, a proper replacement adds sheathing and house wrap before the new siding. Planning ranges for an average home, including tear-off:

Sheathing and wrap are extra where the T1-11 was the only layer. Keeping the grooved look? Engineered wood panels come with 8 in. on-center grooves; board and batten→ is the other vertical option.