Softwood plywood is usually made either of cedar, Douglas fir or spruce, pine, and fir (collectively known as spruce-pine-fir or SPF) or redwood and is typically used for construction and industrial purposes.
The most common dimension is 1.2 by 2.4 metres (3 ft 11 in × 7 ft 10 in) or the slightly larger imperial dimension of 4 feet × 8 feet. Plies vary in thickness from 1.4 mm to 4.3 mm. The number of plies—which is always odd—depends on the thickness and grade of the sheet. Roofing can use the thinner 5⁄8-inch (16 mm) plywood. Subfloors are at least 3⁄4 inch (19 mm) thick, the thickness depending on the distance between floor joists. Plywood for flooring applications is often tongue and groove (T&G); This prevents one board from moving up or down relative to its neighbor, providing a solid-feeling floor when the joints do not lie over joists. T&G plywood is usually found in the 1⁄2-to-1-inch (13 to 25 mm) range.