What is shiplap?

What is shiplap made of?

Shiplap can be solid wood boards or engineered panels with the same lapped edge. The Wood Handbook says yard boards are sold with a shiplapped joint and that low-density cellulosic fiberboard can be given shiplap edges, and HUD's Rehab Guide lists shiplap-edged plywood siding panels: “Boards are sold square edged, dressed (surfaced) and matched (tongued and grooved), or with a shiplapped joint.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Cellulosic fiberboards are low-density, wet-laid panel products used for insulation, sound deadening, carpet underlayment, and similar applications.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Boards may be given tongue-and-groove or shiplap edges or can be grooved to produce a plank effect.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Panels are available smooth faced or textured, grooved or ungrooved, square or shiplap edged.” Source: U.S. Department of Housing and Urban Development (PATH), read 2026-10-29.

How does the Wood Handbook define shiplap?

Shiplap is a way of milling the edges of boards, not a separate kind of wood. The Forest Products Laboratory's Wood Handbook (FPL-GTR-282, March 2021) defines shiplapped lumber by its lapped edge joint and groups it with other patterned, or worked, lumber: “Shiplapped Lumber—Lumber that is edge dressed to make a lapped joint.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Lumber that has been matched, shiplapped, or otherwise patterned, in addition to being surfaced, is often classified as “worked lumber.”” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29.

How thick is shiplap wood?

Solid shiplap is sold as yard boards with a shaped edge. The Wood Handbook gives the usual board thickness as nominal 1 inch (19 mm) with nominal widths of 2 to 12 inches, and HUD's guide lists hardboard shiplap siding 12 and 16 inches wide among 7/16- and 1/2-inch lap siding: “Boards are stocked in one or more species, usually in standard 19-mm (nominal 1-in.) thickness.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Common widths are standard 38, 64, 89, 140, 184, 235, and 286 mm (nominal 2, 3, 4, 6, 8, 10, and 12 in.).” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Lap siding is available in 7/16 and 1/2 inch thicknesses and in a wide variety of configurations including shiplap siding 12 and 16 inches wide in lengths up to 16 feet with varying profiles, as well as conventional 6, 8, or 12 inches wide lap siding made to look like beveled siding (Fig. 2).” Source: U.S. Department of Housing and Urban Development (PATH), read 2026-10-29.

Can shiplap be used for exterior siding?

Yes. HUD's Rehab Guide, Volume 2: Exterior Walls (August 1999) lists shiplap among the non-bevel solid wood siding types for house walls, and the Wood Handbook notes that drop siding can also be milled with shiplapped edges: “Non-bevel wood siding types include board and batten, drop siding, shiplap, tongue-and-groove, and variations of these types (Fig. 5).” Source: U.S. Department of Housing and Urban Development (PATH), read 2026-10-29. “Drop siding may be surfaced and matched, or shiplapped.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29.

Can shiplap be installed vertically?

Yes. HUD's guide says shiplap can run horizontally like bevel siding, or vertically with blocking between the studs or over horizontal furring strips, and that plywood siding panels, which come square or shiplap edged, are designed to fasten to framing 16 or 24 inches on center: “Many, including tongue-and-groove and shiplap, can be installed horizontally in the same manner as bevel siding, and vertically with blocking between studs, or over horizontal furring strips.” Source: U.S. Department of Housing and Urban Development (PATH), read 2026-10-29. “Depending on the depth of the grooves, the panels are designed to be attached directly to framing members 16 or 24 inches o.c., or over existing flat siding, insulating or solid sheathing, or furring strips against masonry or uneven walls.” Source: U.S. Department of Housing and Urban Development (PATH), read 2026-10-29.

Can shiplap get wet?

What matters is how wet the wood gets and for how long. The Wood Handbook ties serious decay to moisture above the fiber saturation point, about 30%, and says decay is prevented when wood stays below 20% moisture; EPA adds that damp materials dried within 24 to 48 hours usually do not grow mold: “Serious decay occurs only when the moisture content of the wood is above the fiber saturation point (average 30%).” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Only when previously dried wood is contacted by water in the form of rain or condensation or is in contact with wet ground will the fiber saturation point be reached.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Decay can be prevented from starting or progressing if wood is kept dry (below 20% moisture content).” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “If wet or damp materials or areas are dried 24-48 hours after a leak or spill happens, in most cases mold will not grow.” Source: U.S. Environmental Protection Agency, read 2026-10-29.

Can shiplap be used in a bathroom?

Bathroom humidity on its own does not wet wood enough to rot it, according to the Wood Handbook, but it can still allow some mold and stain the finish. The Handbook names bathrooms for extractive stains and suggests extra fungicide in paint for rooms with persistent moisture: “By itself, the water vapor in humid air will not wet wood sufficiently to support significant decay, but it will permit development of some mold fungi.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Extractives can discolor finishes, particularly in humid environments such bathrooms and kitchens (Figs. 16–20, 16–24).” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Many older oil-based paints can be a food source for microorganisms; if you are painting over old oil-based paint, it is recommended that you request extra fungicide from your paint supplier.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “This can also be helpful if painting areas expected to encounter persistent moisture, such as bathrooms.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29.

Can shiplap be painted?

Yes. The Wood Handbook says Select lumber, the grade group that covers siding, paneling and ceiling boards, is meant for natural or paint finishes, and it recommends a stain-blocking primer over knots, cedar and redwood; HUD notes most hardboard siding comes pre-primed and plywood siding must be painted or stained within 30 days: “Select lumber is intended for natural and paint finishes.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “The Select category includes trim, siding, flooring, ceiling, paneling, casing, base, stepping, and finish boards.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “When applying opaque coatings on woods with high extractives content (knots and anything that has good durability, but especially redwood and cedar), use a stain-blocking primer.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Most products are pre-primed and require two coats (4 dry mils) of field painting.” Source: U.S. Department of Housing and Urban Development (PATH), read 2026-10-29. “Plywood siding must be primed and finish painted or stained within 30 days of installation.” Source: U.S. Department of Housing and Urban Development (PATH), read 2026-10-29.

Can you use shiplap on a ceiling?

Building codes rate wall and ceiling boards by how fast flame spreads across them. The Wood Handbook puts most domestic woods at a flame spread index of 35 to 125, says unfinished lumber 10 mm or thicker is generally acceptable where Class C (up to 200) is required, and says most softwoods meet Class B untreated: “Important provisions in the building codes are those that regulate the exposed interior surface of walls, floors, and ceilings (that is, the interior finish).” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “In the building codes, the classes for flame spread index are A (FSI of no greater than 25), B (FSI greater than 25 but no greater than 75), and C (FSI greater than 75 but no greater than 200).” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “The FSI for most domestic wood species is typically between 35 and 125 (Table 18–1).” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Thus, unfinished lumber, 10 mm or thicker, is generally acceptable for interior finish applications requiring a Class C rating.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “Most domestic softwood species meet the Class B flame spread index without treatment.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29.

Sources

• Wood Handbook: Wood as an Engineering Material (General Technical Report FPL-GTR-282) — USDA Forest Service, Forest Products Laboratory; published March 2021; read 2026-10-29.
• The Rehab Guide, Volume 2: Exterior Walls — U.S. Department of Housing and Urban Development (PATH); published August 1999; read 2026-10-29.
• A Brief Guide to Mold, Moisture and Your Home — U.S. Environmental Protection Agency; read 2026-10-29.