What is a subfloor?

What is a subfloor made of?

A subfloor is made of wood: lumber boards in many older homes, and plywood or oriented strand board (OSB) panels in most newer ones. Seattle's 2021 residential code lists the lumber grades and structural panels allowed, and the Wood Handbook describes the move from boards to panels: In Seattle, “Subflooring shall be not less than utility grade lumber, No. 4 common grade boards or wood structural panels as specified in Section R503.2.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29. “Older homes have board sheathing but newer homes generally use panel products.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Common sheathing materials include plywood and OSB, which are available in a number of types to meet various sheathing requirements.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29.

How do the Wood Handbook and Seattle's code describe a subfloor?

A subfloor is the layer of wood panels or boards laid over the floor joists, underneath the finish floor you walk on. The USDA Forest Products Laboratory's Wood Handbook describes it, and Seattle's 2021 residential code sets which structural panel grades may be used for it: “Floor sheathing, or subflooring, is used over the floor framing to provide a working platform and a base for the finish flooring.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. In Seattle, the residential code says: “Where used as subflooring or combination subfloor underlayment, wood structural panels shall be of one of the grades specified in Table R503.2.1.1(1).” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29.

What is the purpose of a subfloor?

A subfloor gives the builders a working platform while the house is framed, and later gives the finished floor a solid base. The Wood Handbook explains that walls are usually framed flat on the subfloor and then tilted up into place: See the USDA Forest Service, Forest Products Laboratory passage quoted above. “Wall framing is erected over the platform formed by the first-floor joists and subfloor.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “In most cases, an entire wall is framed in a horizontal position on the subfloor, then tilted into place.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29.

How thick is a subfloor?

There is no single subfloor thickness: the code ties it to how far apart the joists are and to each panel's span rating. In Seattle's 2021 residential code, the Performance Category on the panel's grade mark counts as its thickness, and subflooring may be left out where joists are no more than 16 inches apart and 1-inch tongue-and-groove strip flooring is laid across them: In Seattle, “The Performance Category value shall be used as the “nominal panel thickness” or “panel thickness” wherever referenced in this code.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29. “Panels in PS 2–10 are designated by application (wall, roof, sub-floor, or single floor) and by span rating.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. In Seattle, “Subflooring shall be permitted to be omitted where joist spacing does not exceed 16 inches (406 mm) and a 1-inch (25 mm) nominal tongue-and-groove wood strip flooring is applied perpendicular to the joists.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29.

Is a subfloor considered structural?

Yes, a subfloor is part of the floor structure, so plywood and other panels used for it must meet national product standards. The Wood Handbook says model building codes require U.S. Department of Commerce standards for subflooring plywood, and Seattle's 2021 code names PS 1 and PS 2 and requires a grade mark on each panel: In Seattle, “Wood structural panel sheathing used for structural purposes shall conform to CSA O325, CSA O437 DOC PS 1 or DOC PS 2.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29. “Model building codes in the United States stipulate that plywood used for structural applications such as subflooring and sheathing must meet the requirements of certain U.S. Department of Commerce standards.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. In Seattle, “Panels shall be identified for grade, bond classification and Performance Category by a grade mark or certificate of inspection issued by an approved agency.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29.

Does a subfloor have to be tongue and groove?

Not always. A footnote to the subfloor span table in Seattle's 2021 residential code calls for tongue-and-groove joints or blocking at unsupported panel edges, unless a qualifying layer goes on top, such as 1/4-inch wood panel underlayment or 3/4-inch wood finish flooring laid at right angles to the supports. For board subflooring, end joints must land over supports unless end-matched lumber is used: In Seattle, “Unsupported edges shall have tongue-and-groove joints or shall be supported with blocking unless minimum nominal 1/4-inch-thick wood panel-type underlayment, fiber-cement underlayment with end and edge joints offset not less than 2 inches or 1-1/2 inches of lightweight concrete or approved cellular concrete is placed over the subfloor, or 3/4-inch wood finish flooring is installed at right angles to the supports.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29. In Seattle, “End joints in lumber used as subflooring shall occur over supports unless end-matched lumber is used, in which case each piece shall bear on not less than two joists.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29.

Does a subfloor need to be pressure treated?

Seattle's 2021 residential code calls for treated wood in two subfloor cases: plywood subflooring in wood floors built on the ground, and the bottom of a wood structural floor that sits closer than 18 inches to exposed ground in a crawl space, where the wood must be preservative-treated or naturally durable: In Seattle, “Framing materials, including sleepers, joists, blocking and plywood subflooring, shall be pressure-preservative treated and dried after treatment in accordance with AWPA U1 (Commodity Specification A, Special Requirement 4.2), and shall bear the label of an accredited agency.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29. In Seattle, “Protection of wood and wood-based products from decay shall be provided in the following locations by the use of naturally durable wood or wood that is preservative-treated in accordance with AWPA U1.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29. In Seattle, the locations include: “In crawl spaces or unexcavated areas located within the periphery of the building foundation, wood joists or the bottom of a wood structural floor where closer than 18 inches (457 mm) to exposed ground, wood girders where closer than 12 inches (305 mm) to exposed ground, and wood columns where closer than 8 inches (204 mm) to exposed ground.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29.

Can a subfloor get wet?

A subfloor can get wet, but the panel type matters and wet subflooring has to be dried. The Wood Handbook recommends exterior-type panels with water-resistant adhesive where moisture is likely, including a subfloor left open to the weather during construction, and the EPA's mold cleanup guide for schools and commercial buildings says a wet subfloor under carpet must be cleaned and dried: “Exterior-type panels with water-resistant adhesive are desirable in locations where moisture may be a problem, such as floors near plumbing fixtures or situations where the subfloor may be exposed to the weather for some time during construction.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “The subfloor under the carpet or other flooring material must also be cleaned and dried.” Source: U.S. Environmental Protection Agency, read 2026-10-29.

What is combination subfloor underlayment?

Combination subfloor underlayment is the code's name for a structural panel used as both the subfloor and the underlayment layer beneath the finish floor. Seattle's 2021 residential code sets grades and maximum spans for these panels, including sanded plywood, and separately requires particleboard floor underlayment to be at least 1/4 inch thick: In Seattle, “Where sanded plywood is used as combination subfloor underlayment, the grade, bond classification, and Performance Category shall be as specified in Table R503.2.1.1(2).” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29. In Seattle, “The maximum allowable span for wood structural panels used as subfloor or combination subfloor underlayment shall be as set forth in Table R503.2.1.1(1), or APA E30.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29. In Seattle, “Particleboard floor underlayment shall conform to Type PBU and shall be not less than 1/4 inch (6.4 mm) in thickness.” Source: City of Seattle, Department of Construction and Inspections, read 2026-10-29.

What is a subfloor in a mobile home?

In a manufactured (mobile) home, the subfloor falls under HUD's federal Manufactured Home Construction and Safety Standards. Under 24 CFR 3280.305(g), floors must carry a live load of at least 40 pounds per square foot, and wood subfloors in kitchens, bathrooms, laundry areas and other wet areas must be moisture resistant or sealed: “Floor assemblies shall be designed in accordance with accepted engineering practice standards to support a minimum uniform live load of 40 lb/ft 2 plus the dead load of the materials.” Source: U.S. Department of Housing and Urban Development (eCFR), read 2026-10-29. “Wood, wood fiber or plywood floors or subfloors in kitchens, bathrooms (including toilet compartments), laundry areas, water heater compartments, and any other areas subject to excessive moisture shall be moisture resistant or shall be made moisture resistant by sealing or by an overlay of nonabsorbent material applied with water-resistant adhesive.” Source: U.S. Department of Housing and Urban Development (eCFR), read 2026-10-29. “Wood panel products used as floor or subfloor materials on the exterior of the home, such as in recessed entryways, must be rated for exterior exposure and protected from moisture by sealing or applying nonabsorbent overlay with water resistant adhesive.” Source: U.S. Department of Housing and Urban Development (eCFR), read 2026-10-29.

Does a subfloor need an expansion gap?

Yes, panel subfloors are normally laid with a small gap at the edges and ends. The Wood Handbook advises leaving about 1/8 inch (3 mm) between abutting panels so they can swell and shrink as their moisture content changes: “About 3 mm (1/8 in.) of space should be left between the edges and ends of abutting panels to provide for dimensional changes associated with moisture content.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29.

Sources

• 24 CFR 3280.305 Structural design requirements (Manufactured Home Construction and Safety Standards) — U.S. Department of Housing and Urban Development (eCFR); read 2026-10-29.
• Mold Remediation in Schools and Commercial Buildings Guide: Chapter 4 — U.S. Environmental Protection Agency; read 2026-10-29.
• Wood Handbook: Wood as an Engineering Material (FPL-GTR-282) — USDA Forest Service, Forest Products Laboratory; published 2021-03; read 2026-10-29.
• 2021 Seattle Residential Code, Chapter 3: Building Planning — City of Seattle, Department of Construction and Inspections; read 2026-10-29.
• 2021 Seattle Residential Code, Chapter 5: Floors — City of Seattle, Department of Construction and Inspections; read 2026-10-29.