Lumber categories: SOFTWOODS come from coniferous (cone-bearing) trees; faster-growing; generally less dense; less expensive; used primarily for construction (framing, sheathing) and some furniture; common species: PINE (yellow pine for framing, white pine for trim/furniture), FIR (Douglas fir is primary framing lumber in West; very strong relative to weight), SPRUCE (SPF is Spruce-Pine-Fir generic framing grade; also for musical instruments), CEDAR (red cedar, white cedar; naturally rot-resistant; siding, outdoor furniture, fence posts), REDWOOD (rot/insect resistant; outdoor use; less available). HARDWOODS come from deciduous (leaf-shedding) trees; slower-growing; generally denser; more expensive; used for FURNITURE, FLOORING, CABINETRY; common species: OAK (red oak, white oak; strong, common; white oak is rot-resistant; flooring, furniture), MAPLE (hard maple, soft maple; flooring, cabinets), CHERRY (rich color, ages well; furniture), WALNUT (dark, fine grain; high-end furniture), BIRCH (cabinets, plywood); EXOTIC tropical hardwoods like teak, mahogany, ipe, ebony are very expensive.
Engineered lumber and sheet goods: PLYWOOD is multiple thin wood veneers glued with grains alternating perpendicular (cross-grain); very stable, strong relative to weight, available in 4 by 8 foot sheets, thicknesses 1/8 inch to 1 inch plus; grades: A (best, smooth, sanded), B, C, D (rough, knots); face/back grades combined like A-B, A-C, C-D (construction grade). OSB (Oriented Strand Board) is wood strands compressed with adhesive; structural sheathing for floors, walls, roofs; cheaper than plywood. MDF (Medium-Density Fiberboard) is finely ground wood plus adhesive; smooth flat surface; great for painted projects; no grain, won't warp, but heavy and creates dangerous dust when cut. PARTICLEBOARD is coarser version of MDF; cheap cabinet construction. HARDBOARD (MASONITE) is thin smooth panels. LVL (Laminated Veneer Lumber) is for headers and beams; very strong. PSL (Parallel Strand Lumber, like Parallam) is similar to LVL but using strands. GLUE-LAM (Glued Laminated Timber) is beams made by laminating layers. I-JOISTS (TJI) for floor joists; engineered shape (top and bottom flanges with OSB or plywood web). LUMBER DIMENSIONS nominal vs actual: construction lumber sold by NOMINAL size (e.g., 2 by 4) which is BEFORE drying and planing; actual dimensions are SMALLER. Standard examples: 2 by 4 equals 1-1/2 by 3-1/2 inch; 2 by 6 equals 1-1/2 by 5-1/2 inch; 2 by 8 equals 1-1/2 by 7-1/4 inch; 2 by 10 equals 1-1/2 by 9-1/4 inch; 2 by 12 equals 1-1/2 by 11-1/4 inch; 4 by 4 equals 3-1/2 by 3-1/2 inch; 1 by 6 equals 3/4 by 5-1/2 inch. Important: design and measure with ACTUAL dimensions, not nominal. Hardwood lumber sold by quarter-inch thickness: 4/4 equals 1 inch (about 7/8 inch finished); 8/4 equals 2 inches (1-3/4 inch finished).
Moisture content: wood swells when wet, shrinks when dry; KILN-DRIED LUMBER (KD) has been dried to 10-19 percent moisture (construction grade) or 6-8 percent (furniture grade). Pressure-treated for ground contact (chemicals like ACQ resist rot, insects) has green/brown tint; required for deck framing, fence posts. NEVER USE PRESSURE-TREATED indoors (off-gassing of chemicals); never burn it (toxic smoke).