Floor joist calculator

How it is worked out

• Joists run the short way across the floor, at the spacing you choose, between rim boards along both sides.
• A rim board closes the joists’ ends on each side, and one runs across each end of the floor.
• A rim longer than a 20 ft board is laid in two or more lengths.
• A ¾ in subfloor goes over the joists.
• A floor whose joists would be longer than a 20 ft board wants a beam or a wall under its middle, so it is turned down, and we say why.

We lay out and count the sizes you pick, close enough to plan a weekend and price the lumber. Span tables, your local code and, on a bigger job, an engineer settle what size each board has to be.

Questions

How many floor joists do I need?

The floor’s length divided by the spacing, less one, since the rim boards stand where the first and last would. The list counts them.

How far can a floor joist span?

A span table answers that, from the joist’s size, its spacing, the wood and the load. Pick the size your table gives and this lays it out.

Which way do the joists run?

Across the shorter way of the floor, so they are as short as they can be.