T-111 (often pronounced "T-one-eleven") is a brand name that has become a generic term for plywood or OSB siding with vertical grooves. It was immensely popular from the 1960s through the 1990s due to its low cost and ease of installation (it provides both the sheathing and the exterior finish in one).

If you are looking at a house with T-111 siding, look closely at the bottom edges of the sheets and around the windows. Poke them with a screwdriver. If the wood is soft, budget for a full siding replacement (often with fiber cement) in your offer, as patch repairs on T-111 rarely stop the spread of rot.

Factory-primed T-111 often fails within 3–5 years. Peeling starts at groove edges, then spreads. Why? The grooves hold moisture longer, and the thin face veneer expands/contracts at a different rate than the backing. Once water gets under the coating, the sun bakes it into vapor that blows the paint off.

This is the most common and severe issue. T-111 is made of wood veneers (plywood) or wood strands (OSB) glued together.