Platform framing

From Wikipedia, the free encyclopedia

Platform framing is a light-frame construction system and the most common method of constructing the frame for houses and small apartment buildings as well as some small commercial buildings in Canada and the United States.

The floors, walls and roof of a framed structure are created by assembling (using nails) consistently sized framing elements of dimensional lumber (2×4, 2×6, etc.) at consistent spacings (12″, 16″, and 24″ on center), forming stud-bays (wall) or joist-bays (floor). The floors, walls and roof are typically made torsionally stable with the installation of a plywood or composite wood “skin” referred to as sheathing. Spacing the framing members properly allows them to align with the edges of standard sheathing. In the past, tongue and groove planks installed diagonally were used as sheathing. Occasionally, wooden or galvanized steel braces are used instead of sheathing.

The framed structure sits atop a concrete (most common) or treated wood foundation.

From the foundation, a framer will bolt or otherwise connect a sole or sill plate. Then, depending on the design of the structure, a framer may install a floor, short “pony walls”, or full height walls.

The floor, or the platform of the name, is made up of joists (usually 2×8, 2×10 or 2×12, depending on the span) that sit on walls and, usually, a beam at the center of the house. The joists are spaced at (12″, 16″, and 24″ on center) and covered with a plywood subfloor. In the past, planks set at 45-degrees to the joists were used for the subfloor.

Where the design calls for a framed floor, the resulting platform is where the framer will construct and stand that floor’s walls (interior and exterior load bearing walls and space-dividing, non-load bearing “partitions”). Additional framed floors and their walls may then be erected to a general maximum of four. There will be no framed floor in the case of a single-level structure with a concrete floor known as a “slab on grade”.

Stairs between floors are framed by installing stepped “stringers” and then placing the horizontal “treads” and vertical “risers”.

A framed roof is an assembly of rafters and wall-ties supported by the top story’s walls. Prefabricated and site-built trussed rafters are more common and structurally superior to the hand cut form. “Trusses” are engineered to redistribute tension away from wall-tie members and the ceiling members of a “scissor truss” support vaulted ceilings. The roof members are covered with sheathing or strapping to form the roof deck for the finish roofing material.

Floor joists are commonly of an engineered form (trussed, i-beam, etc.), conserving resources with increased rigidity and value. They allow access for runs of plumbing, HVAC, etc. and some forms are pre-manufactured.