When framing a building, the bottom plate, as well as the studs, top plate, sills, and ceiling or roof joists, are the essentials to the structural integrity of the building. Also known as a sole plate, bottom plates can be thought of as an extension of the foundation upon which the roof, ceilings, and walls rest. In a wood-frame building, the bottom plates would most likely be wood, though various other materials may be used, such as steel, concrete, or composite materials.
On a straightforward wood-frame building construction project, the sole, or bottom plate is affixed to the subfloor, or rough floor. On an interior-wall project, the vertical studs are fitted, and then nailed or screwed in place through the subfloor and bottom plate, or if that would result in damage to the floor, simply toe-nailed into the bottom plate. The studs are then nailed or screwed into the top plate, which is at ceiling level. Thus, the bottom plate is the foot of the wall, the studs are the ribs, and the top plate is the head of the wall.
As a rule, interior framing in a completed structure must be done vertically; bottom plate nailed to the subfloor, studs nailed or screwed to the bottom plate, then to the top plate. Exterior wall construction is ordinarily accomplished horizontally. Bottom plate, top plate and studs are nailed or screwed together horizontally on the subfloor, and the entire construction then lifted, all as one complete assembly, into the proper position on the subfloor. Roofs and ceilings are generally not yet built, therefore allowing the uninhibited erection of the wall.
In a wood-frame construction, bottom plates, studs, and top plates are commonly Douglass fir, pine, or hemlock. Wood is readily available, comparatively inexpensive, and sturdy enough to ensure reasonable durability of construction. Wood beams, generally larger than the two-by four or two-by-six boards ordinarily used in the construction of wood buildings, may be used as either top or bottom plates for extra load-bearing strength.
Often enough, novices in construction will confuse bottom and top plates with footers and headers. A header is used in the installation or construction of window or door opening, and footer is a term for the bottom of a building’s foundation. Though similar in concept, the terms are differentiated in the interests of clarification. One would not want to construct a window or door frame, or pour a foundation, when a wall frame is what is actually wanted.