Platform Framing

A floor, then walls

Platform framing builds a house one story at a time. The first floor’s joists sit on the foundation, and the subfloor covers them all the way to the outside edges, making a flat platform. The walls are framed lying down on that platform: a bottom plate (sole plate) and top plate nailed to precut studs, with window and door headers in place. Each wall is tipped up, its bottom plate nailed down through the subfloor into the framing, then plumbed and braced. Once all the walls stand, a second top plate goes on, lapping the first at corners and intersections to tie the walls together ¹.

The second floor (or the ceiling joists and roof) then sits on top of those walls, and the process repeats.

The layers, bottom to top

From the foundation up ^{1 2}:

1. Foundation wall with anchor bolts cast in.
2. Sill plate, bolted down (pressure-treated where it touches concrete).
3. Floor joists, usually 2-inch-thick lumber, 8 to 12 inches deep, 16 inches on center, with a rim joist (band joist) around the outside edge. Today often engineered I-joists.
4. Subfloor sheathing, now plywood or oriented strand board.
5. Wall framing: bottom plate, studs (2×4 or 2×6, commonly 16 inches on center), and a double top plate.
6. Ceiling joists and rafters, or prefabricated trusses.
7. Sheathing on the outside of walls and roof, which also braces the frame.

The UDC’s Wisconsin rules for each layer, joist bearing, notching, stud spacing, headers, rafter ties, and bracing, are in SPS 321.22 to 321.27 and come up in the rest of this module ².

Words of the trade

• On center (o.c.): the distance from the center of one stud or joist to the next. 16 and 24 inches o.c. put a framing member under every edge of 4-by-8 sheets.
• Nominal size: lumber is named before it’s dried and planed. A 2×4 actually measures 1 1/2 by 3 1/2 inches; a 2×10 is 1 1/2 by 9 1/4.
• Bearing wall: a wall that carries load from above; a partition that carries only itself is non-bearing.
• Crown: the slight edgewise bow most boards have. Joists go in crown up, so loads flatten them ¹.
• Moisture: framing lumber should be dry, no more than 19 percent moisture, or it shrinks and twists in the walls ¹.

Balloon framing

Before platform framing took over, many houses were balloon framed: long studs ran continuously from the sill to the rafter plate, two stories tall, and the second-floor joists hung on a board let into the studs (a ribbon) ¹. Balloon framing was used where masonry veneer or stucco needed the wall to shrink less in height ¹, but the long studs got scarce, and walls framed on a deck are faster and safer to build.

If you own an older Wisconsin farmhouse, check: an open stud bay in a balloon frame is a chimney from the basement to the attic. Fire, and mice, travel straight up unless the bays are fireblocked at each floor.

What to take from this

Platform framing builds each floor as a deck to the outside edges, then frames walls flat on it, raises and braces them, and laps a second top plate over the corners. The stack runs foundation, sill, joists and rim, subfloor, bottom plate, studs, double top plate, ceiling joists and rafters (or trusses), sheathing. Know o.c. spacing, nominal sizes, crown-up joists, and dry lumber, and fireblock the open bays of a balloon-framed house.