Platform framing · Pascal Architecture

The corner of a two-storey light wood frame, built the way a crew builds it: sill, joists, a floor deck as a working platform, then walls framed flat and tilted up, then the next floor on top.

Most wood houses in North America are framed this way. Each storey is its own box: the studs are one storey tall, and each floor is built as a flat deck on top of the walls below. That deck is the trick. Once it is down, the crew has a level, solid place to stand, to lay out the next walls, and to frame them lying flat before tipping them up into place. This is platform or western framing; the figure builds a corner of it in the order it goes up, then closes it in: sheathing, housewrap, windows and siding outside, insulation and gypsum board inside. Use Show to take any of them away again.

From the sill up

Wood starts on the foundation with the sill plate, a pressure-treated 2×6 laid on a strip of foam sealer that stops air leaking through the joint. Anchor bolts cast into the concrete pass through it and are drawn tight with a washer and nut: at least two per piece of sill, no more than 6 ft (1830) apart and within 12 in (305) of each end, more in high-wind and seismic zones. The top of the concrete stays well clear of the ground, at least 6 in (150) above finished grade, so that the first wood is out of the splash and the soil.

The rim joist stands on edge around the outside, and the floor joists run between, here 2×10s at 16 in (405) on centre. A joist needs 1½ in (38) of bearing on wood or steel and 3 in (75) on concrete or masonry. Where the joists are deep and slender, pairs of diagonal cross bridging at about 8 ft (2440) intervals keep them from rolling over and share a concentrated load with their neighbours.

The platform

The subfloor is ¾ in (19) plywood or OSB in 4 × 8 ft sheets, laid with the long side across the joists so each sheet spans several of them, end joints staggered by half a sheet, with a ⅛ in (3) gap for swelling. Glued as well as nailed, sheet and joists act together as a row of T-beams: the floor is stiffer and quieter, and as a whole it becomes a diaphragm that carries wind sideways to the walls.

Joist	Typical span at 16 in on centre
2×6	up to 10 ft (3050)
2×8	8 to 12 ft (2440 to 3660)
2×10	10 to 14 ft (3050 to 4265)
2×12	12 to 18 ft (3660 to 5485)

A quick first guess for joist depth is the span divided by 16. Floors are usually limited by stiffness rather than strength: the joists must not sag more than 1/360 of their span under live load, or the floor feels springy and plaster cracks.

Sawn joists are only one way to frame a floor. Engineered joists, I-shaped or open-webbed, span further without a girder, stay flat and do not shrink, at a price and with rules of their own about holes and fire. The reference house keeps to lumber, and the reason is its girder:

What are the floors framed with?

Decided by the span · the depth the floor can have · what must run through it · stiffness underfoot · budget and lead time

Sawn joistsThis house
Solid 2×8 to 2×12 joists, cut to length on site and set 16 in apart.
Choose it when spans of up to about 15 ft for a 2×10, which a centre girder makes easy; the crew wants to cut, notch and block on site; ducts and pipes can run along the joists rather than across them.
The catch it shrinks as it dries, about ¼ in per floor with the plates; long, deep, straight joists are scarce and dear; floors near their limit bounce.
The house's joists No. 03 →
Wood I-joists
Engineered joists: an OSB web glued between two flanges of solid wood or LVL, 9½ to 16 in deep.
Choose it when clear spans of 16 to 24 ft with no girder; flat, stiff, quiet floors that do not shrink; long lengths that one person can carry.
The catch their thin webs burn through quickly, so codes ask for board under them in many floors; holes only where the maker's chart allows, never a notch; blocks and web stiffeners at every bearing.
Open-web floor trusses
2×4 chords laid flat with webs between them, joined by toothed plates, made to order 12 to 24 in deep.
Choose it when long clear spans to 24 ft and more; ducts, drains and wires must cross the joists through the open webs; a wide flange to glue the subfloor to.
The catch designed and built before framing, so the plan is fixed early; never cut on site; a deeper floor raises the whole house.

Here The girder cuts the 24 ft depth into two 12 ft spans, well inside what a 2×10 at 16 in carries, so plain lumber is the cheapest floor. The heating branches run between the joists toward the outside walls, along them, so nothing has to cross them.

Walls framed flat, tilted up

A wall is a ladder of studs between a sole plate at the bottom and a top plate at the top. The crew nails it together lying on the deck, often sheathes it there, then lifts the top edge and swings the wall up about its bottom edge, which is what the figure shows. A precut stud is 92⅝ in (2353); with one plate under it and two over it the wall stands 8 ft 1⅛ in, just enough for an 8 ft sheet of drywall and a ceiling.

Stud	Max. unsupported height	Spacing
2×4	14 ft (4265)	16 in (405); 24 in (610) if under 10 ft and carrying only a ceiling and roof
2×6	20 ft (6100)	24 in (610); 16 in (405) when carrying two floors and a roof

Every opening interrupts studs, and their load has to go round it. A header spans the opening and sits on jack studs (also called trimmers), which are nailed to full-height king studs. Short cripples fill in above the header and below the rough sill, on the same 16 in layout, so sheathing and drywall still find something to nail to. Two 2×s with a ½ in (13) plywood spacer make a header exactly as thick as a 2×4 wall. As a rough guide, a pair of 2×6s spans about 4 ft (1220) under one storey; deeper pairs span further, and wide openings are engineered as beams.

The figure frames its corner in 2×4s, the classic wall. The reference house uses 2×6s outside, and the choice is mostly about insulation: a deeper cavity holds more of it, and the climate decides how much is needed. Other walls replace the studs altogether.

What are the outside walls built of?

Decided by the insulation the climate and energy code ask for · wall height and load · labour and speed · fire, insects and wind · budget

2×4 at 16 in
The classic stud wall, 3½ in (89) deep: room for R-13 to R-15 batts.
Choose it when mild climates where R-13 meets the code; interior partitions anywhere; the cheapest, most familiar wall.
The catch too shallow for cold climates without foam outside; studs, plates and headers, about a quarter of the wall, carry heat straight through.
2×6 at 16 inThis house
5½ in (140) deep, holding R-19 to R-21 batts.
Choose it when cold and mixed climates; taller walls; room for pipes, boxes and deeper insulation.
The catch about half as much lumber again as 2×4; the studs still bridge the insulation, so newer codes add foam outside as well; deeper jambs at every window and door.
The house's 2×6 wall No. 18 →
Advanced framing
2×6 at 24 in, in line with the joists and trusses above, two-stud corners, headers only where loads need them.
Choose it when more insulation and less wood: the framing falls from about 25 to 15 percent of the wall; trusses at 24 in already set the module; the plan is drawn to that module from the start.
The catch it must be planned, not improvised; sheathing and board have to span 24 in; crews used to 16 in resist it.
Double-stud wall
Two rows of 2×4 studs set apart, 10 to 12 in overall, filled with dense-packed cellulose.
Choose it when very cold climates or very low energy targets: R-35 to R-40 walls; breaking the thermal bridge without foam.
The catch thick walls take floor area; the outer sheathing runs cold and must be able to dry; airtightness is essential.
Structural insulated panels
Foam 4 to 8 in thick glued between two OSB skins, cut in a factory and set by crane.
Choose it when a fast, airtight, well-insulated shell; simple shapes known early.
The catch changes after ordering are hard; wires run only in factory-cut chases; leaky joints let damp air rot the skins.
Insulating concrete forms
Hollow foam blocks stacked and filled with reinforced concrete; the foam stays on as insulation.
Choose it when hurricane and tornado regions; quiet, massive walls; one system from the footing to the roof.
The catch costs more than wood framing; poured in lifts, with bracing and inspections; openings are hard to change later.
Light-gauge steel
Cold-formed galvanized C-studs screwed together, in the same sizes as wood.
Choose it when termite country; straight walls that never shrink or twist; noncombustible construction.
The catch steel carries heat about 400 times better than wood, so a cavity of batts loses about half its value without foam outside; a different trade and fasteners.

Here Climate zone 5 asks a wood wall for about R-20, or R-13 with R-5 of foam outside: 5½ in holds R-21 batts where 3½ in holds only R-15. Studs at 16 in suit the crew; with trusses at 24 in the house could move to advanced framing for a little more insulation and less wood.

At the corner, three 2×s meet so that both inside faces have wood to nail drywall to. The double top plate is what ties the walls into one frame: the upper plate laps across the joint in the lower one at every corner and intersection, and its own joints are kept at least 4 ft (1220) from those below.

Platform against balloon

Before platform framing, houses were framed as balloon frames: switch the figure to see one. The studs run unbroken two storeys from the sill to the roof plate. The ground-floor joists sit on the sill beside them; the upper joists are nailed to the side of each stud and carried on a ribbon, a 1×4 let into notches in the studs. Because nothing interrupts the cavities between studs, every one is a flue from basement to attic, and code requires firestops, solid blocks across each cavity at every floor.

Balloon framing is rarely used now: long straight studs are hard to get, the frame has to be raised in place without a deck to work from, and the firestops are easily forgotten. It keeps one real advantage. Wood shrinks across its grain many times more than along it, and a platform frame stacks cross-grain wood at every floor: sole plate, two top plates and a joist, about 13¾ in (349). As the lumber dries in service from roughly 19 to 12 percent moisture, that stack can settle around ¼ in (6) per storey. A balloon frame has only its sill and top plates lying flat, so it barely moves, which suits brick veneer and stucco that cannot follow the frame down. In a platform frame the movement is at least the same at each floor, so it can be detailed for.

Platform or balloon framing?

Decided by lumber lengths · how the crew works · fire spread in the walls · movement under brick or stucco

Platform framingThis house
Each storey's walls stand on their own floor deck, one storey high.
Choose it when almost any light wood frame built today; short studs, and a safe deck to build each storey on; plates that stop fire at every floor by themselves.
The catch it stacks flat-lying wood at every floor, so it settles about ¼ in per storey as it dries; brick veneer and stucco must be detailed to let it move.
The house, framed this way No. 03 →
Balloon framing
Studs run unbroken from sill to roof, and the upper floor hangs on a ribbon let into them.
Choose it when repairing a house framed this way; tall walls in a stairwell or gable where a joint at the floor would hinge the wall; under masonry or stucco, where little shrinkage matters.
The catch long, straight studs are scarce; every cavity is a flue unless firestopped at each floor; no deck to work from while raising it.
Post and beam
Heavy timber posts and beams carry the loads; the walls between them only enclose.
Choose it when open plans and exposed structure; few, large members with wide bays between them.
The catch large timbers and joinery cost more; the walls between need their own bracing and insulation.

Here Like nearly every house built now: precut studs, a deck to stand on at each storey, and lap siding that does not mind the frame settling a little.

Closing it in

The frame is not finished until it has its skin, and the skin is put on in an order that makes every layer lap the right way. Sheathing goes on first, nailed to the sill, rims, plates and studs. Here it is in three courses of standing sheets, and the middle course is the important one: it runs from the lower wall, across the second-floor rim joist, into the upper wall, and nailed to all of them it ties the two storeys into one braced shell. Sheathing that stopped at each floor would leave a hinge there.

Housewrap follows, from the bottom up so that each roll laps over the one below, and down over the top of the foundation. Then the windows are nailed through their flanges, the casings and corner boards go on, and siding runs between them, each course lapping the one below. Inside, batts fill the stud bays, foam is fitted against the rim between the joists, and gypsum goes on the ceiling first and the walls after, the wall board holding the ceiling board’s edges up.

Two junctions

Choose At the sill or At the floor to see the corner cut through where the fragment ends. Read each from the bottom up, asking what bears on what and what is fastened to what.

At the sill. The foundation carries the sill on its sealer, and the anchor bolt’s nut clamps them together. The rim joist bears on the outer edge of the sill and the joists beside it; the subfloor is glued and nailed to both; the sole plate is nailed through the subfloor into the rim. Outside, the sheathing laps down past the sill onto the concrete, so water running down the wall never meets the joint between wood and concrete from behind.
At the second floor. The same sandwich again, standing on the lower wall’s double top plate instead of the sill, and wrapped by the one course of sheathing that spans it. Inside, the ceiling board below and the foam against the rim close the floor off.

Switch to balloon framing and look again: the studs run straight past both levels, the joists are nailed to their sides, and firestops have to close the cavity that the platform closes for free.

Vocabulary

Sill plate: The first piece of wood on the foundation, anchored to it; usually pressure-treated.
Rim joist: The joist across the ends of the floor joists, also called a header or band joist.
End joist: The outermost joist, parallel to the others, bearing entirely on the sill.
Subfloor: The structural deck over the joists; the platform the walls stand on.
Sole plate: The bottom plate of a wall, nailed through the subfloor into the frame below.
King stud: A full-height stud beside an opening.
Jack stud: A shorter stud inside the king that carries the header; a trimmer.
Cripple: A short stud above a header or below a rough sill.
Ribbon: In balloon framing, a 1×4 let into the studs to carry the upper floor joists.
Firestop: Solid blocking that closes a concealed cavity so fire and smoke cannot run through it.

The numbers in the drawing

Fragment	4.2 × 3.8 m corner of a two-storey house
Foundation	8 in (205) concrete, top 8 in (205) above grade
Anchor bolts	½ in (13) at no more than 6 ft (1830), 12 in (305) from plate ends
Sill	Pressure-treated 2×6 (38 × 140) on foam sill sealer
Floor joists	2×10 (38 × 235) at 16 in (405), 1×3 cross bridging at midspan
Subfloor	¾ in (19) plywood, 4 × 8 ft sheets, joints staggered 4 ft
Studs	2×4 (38 × 89) at 16 in (405), 92⅝ in (2353) precut
Window openings	3 × 4 ft (915 × 1220) rough, headers of two 2×8s and a ½ in spacer
Floor to floor	8 ft 11 in (2.72 m): plates, studs and a 2×10 floor
Ceiling joists	2×6 (38 × 140) at 16 in (405)
Footing	16 × 8 in (405 × 205) concrete under the foundation wall
Sheathing	7⁄16 in (11) OSB, standing 4 × 8 ft sheets in three courses
Siding	Lap siding, 7 in (178) to the weather, over housewrap
Inside	Batts in the stud bays, foam at the rims, ½ in (13) gypsum board