Platform House Framing
Category Construction Glossary
Platform 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 framed structure sits atop a concrete (most common) or treated wood foundation. A sill
plate is anchored, usually with 'J' bolts to the foundation wall. Generally these plates must be
pressure treated to keep from rotting. The bottom of the sill plate is raised a minimum 6
inches (150 mm) above the finished grade by the foundation. This again is to prevent the sill-
plate from rotting as well as providing a termite barrier.
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 regular
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. Sheathing has very specific nailing
requirements (such as size and spacing); these measures allow a known amount of shear
force to be resisted by the element. 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. There are also engineered wood panels made for shear and
The floor, or the platform of the name, is made up of joists (usually 2x6, 2×8, 2×10 or 2×12,
depending on the span) that sit on supporting walls, beams or girders. The floor joists are
spaced at (12″, 16″, and 24″ on center) and covered with a plywood subfloor. In the past, 1x
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 in wood framed construction. 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 also used along with the more common stick
framing method. “Trusses” are engineered to redistribute tension away from wall-tie
members and the ceiling members. The roof members are covered with sheathing or
strapping to form the roof deck for the finish roofing material.
Floor joists can be engineered lumber (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.