Floor Framing

Girders and posts

In a basement house, the floor joists usually span from the foundation wall to a girder (beam) down the middle, carried by posts on their own footings ¹. The UDC rules ²:

• Girders and beams are sized from the code’s tables or engineered.
• Built-up wood girders are fitted at posts, with joints tied together to carry horizontal loads, and fastened to the posts with framing anchors or clips. Deep beams (over 11 1/4 inches) need lateral restraint at columns, such as a saddle.
• Posts bear over the middle third of their footing, are restrained top and bottom, are positively attached to the beam, and have a steel bearing plate. An adjustable steel post’s screw must be cast in concrete or permanently disabled after it’s set.

Joists

Joists are chosen for strength and for stiffness, which limits bounce and cracking ¹. The UDC’s span tables in SPS 325 Appendix A give the allowed span by size, spacing, species, and grade for the 40 psf floor live load ². Spacing is most often 16 inches on center ¹.

• Install joists crown up, with the biggest edge knots on top ¹.
• Double joists where a bearing wall above runs parallel to them ². Bearing partitions must sit over beams, girders, or other bearing walls, or within one joist depth of them unless the joists are designed for the load ².
• Joists that meet over a girder overlap at least 3 inches and are nailed together with at least two 12d nails, or are butted and tied with straps, plates, or blocking ². A joist’s tail may not run past the beam by more than its depth ².
• Joists parallel to the foundation wall get blocking in the first space every 32 inches to brace the wall top ².

Openings for stairs and chimneys are framed with doubled headers across the joist ends and doubled trimmers alongside, often hung in metal joist hangers ¹.

Bridging, diagonal 1×3 cross-bridging or solid blocking, goes at midspan to stiffen the floor and spread concentrated loads, no more than 8 feet apart ¹. Subfloor of plywood or OSB is glued and nailed over the top.

Bearing

The ends of the framing need enough to sit on ²:

Notching and boring

Plumbers and electricians cut floor joists all the time. The limits ²:

• Notches in the top or bottom: no deeper than 1/6 of the joist depth, no longer than 1/3 of the depth, and never in the middle third of the span. End notches may be up to a quarter of the depth.
• Holes away from the edges: diameter no more than 1/3 of the joist depth, at least 2 inches from the top and bottom edges and from any other hole or notch. A hole within 2 inches of an edge counts as a notch.
• Beams and girders: no notching or boring at all without engineering.
• Trusses: never cut, bored, or notched ². Engineered joists (I-joists, LVL) follow the manufacturer’s hole charts ².

For a 2×10 (9 1/4 inches actual), that’s a notch of 1 1/2 inches at most near the ends and a hole of about 3 inches in the middle of the depth.

Overhangs

A joist may cantilever beyond the wall below by no more than its own depth unless engineered, or up to 2 feet when it’s an extension of the main joists carrying only a wall and limited roof ².

What to take from this

Joists 16 inches on center span from the foundation to a girder on posts that bear on the middle third of their footings. Size them from the span tables, install crown up, double them under parallel bearing walls, lap 3 inches over girders, and frame openings with doubled headers and trimmers. Give joists 1 1/2 inches of bearing on wood and 3 on concrete. Notch no more than 1/6 the depth and never mid-span; bore no more than 1/3 the depth, 2 inches from the edges; never cut trusses or beams.