Lumber Sizes and Types Guide

Fresh pressure-treated lumber is saturated with water-based preservative and can have a moisture content of 50–75% or higher. Using it immediately leads to several problems: (1) Boards will shrink significantly as they dry — a 2×6 can lose up to ¼ inch in width, creating gaps in decking or sheathing. (2) Wet lumber is prone to twisting and cupping as it dries unevenly, especially in direct sun. (3) Wet treated lumber is heavier and harder to work with. (4) Paint and stain will not adhere properly to wet wood. Best practice: stack treated lumber flat, with spacers (stickers) between layers, in a covered area for 2–6 months before use. If you must use it immediately, choose kiln-dried-after-treatment (KDAT) lumber, which is dried to ~19% moisture content at the mill.

4/4 (four-quarter) hardwood is nominally 1 inch thick in its rough-sawn state. After surfacing (planing both faces smooth), it typically ends up at 13/16″. If you need a finished thickness of exactly ¾″, you would start with 4/4 stock and plane it down — that's the standard practice for most furniture projects. If you need a full 1-inch finished thickness, you should buy 5/4 (five-quarter) stock, which rough-sawn measures ~1¼″ and surfaces down to approximately 1-1/16″. This is a key difference from softwood construction lumber — hardwood is sold by the quarter-inch of rough thickness, and you must account for the material lost during surfacing in your board-foot calculations and project plans.

No. 3 / Economy grade is explicitly excluded from structural applications by all major US building codes (IRC, IBC). It is only suitable for non-structural uses: temporary bracing during construction, concrete formwork, shipping crates, and stakes. Building codes require a minimum of No. 2 grade for all structural framing members (studs, joists, rafters, beams). Using No. 3 for any load-bearing application — even temporary — is a code violation that could fail an inspection and, in a worst-case scenario, lead to structural collapse. For temporary support structures that must carry loads (e.g., shoring during remodeling), No. 2 is the minimum acceptable grade. Always verify the grade stamp on each piece — it is a legal identifier of the lumber's tested strength properties.

Select Structural (SS) 2×6 Southern Pine as a floor joist at 16″ OC spacing can span approximately 10′9″, while a No. 2 grade 2×6 of the same species at the same spacing spans roughly 9′8″ — a difference of about 13 inches, or roughly 10% less span capacity. The gap widens with longer spans: a SS 2×10 may span 15′6″ while No. 2 spans 13′9″. These numbers vary by species (Douglas Fir-Larch is ~10–15% stronger than Southern Pine across grades) and loading conditions (40 psf live / 10 psf dead is standard residential). For critical applications where every inch of span matters — open-floor-plan headers, long deck beams — stepping up from No. 2 to No. 1 or SS can eliminate a support post. Always consult the current span tables from the American Wood Council or your local building department rather than relying on rules of thumb.

No — and this is one of the most common moisture-related mistakes in basement finishing. Building code (IRC R317.1) requires pressure-treated lumber for any wood that is in direct contact with concrete or masonry, or within 18 inches of exposed ground in a crawlspace. A basement floor slab wicks moisture continuously, and untreated pine in contact with it will begin to rot within 2–5 years, especially if the basement lacks a functioning vapor barrier under the slab. The correct approach: use pressure-treated bottom plates for all basement partition walls, with a sill gasket (foam or polyethylene) between the treated plate and the concrete as a capillary break. If the basement has a history of flooding or high humidity, also use pressure-treated studs for the bottom 12–24 inches of the wall or switch to steel studs, which are impervious to moisture.