Shed floor joists: what size and spacing do you need?

Shed floor joist span chart

The clear span is the open distance between supports, skid to skid, not the size of the shed. These spans are for No. 2 lumber, the grade sold for framing, and take the shorter of southern pine and spruce-pine-fir so they work with whichever your store stocks. The floor carries 50 lb per square foot of stored things plus 10 lb of its own weight, and may bend no more than 1/360 of its span.

The building code's tables for rooms in a house use 40 lb per square foot. We size shed floors for more, because a shed holds mowers, bags of concrete and full shelves. At the house figure the spans are longer:

What our plans use, size by size

Our joists run left to right across treated 4x6 skids that run front to back. The engine adds skids until the gap between them is 6 feet or less, so a 10x12 has 3 skids and a clear span of 4' 6" against the 8' 6" a 2x6 is allowed.

2x6 or 2x8? It depends on the skids

Many shed floors are framed on two skids, one under each side wall. Then the joists span almost the whole width, and the joist size has to grow with it. The same sheds with our plans' extra skids keep 2x6s:

So a 10-foot-wide shed on two skids needs 2x8 joists, and a 12-foot one 2x10s; with a third skid in the middle, both are fine with 2x6s. On a 10x12, the third skid costs about $31 at our list prices, against about $35 more for 2x8 joists instead of 2x6s, and it also supports the middle of the floor, where the heavy things stand.

Can you use 2x4 floor joists?

A 2x4 at 16 inches spans 5' 6" at our storage load, and 6' 0" at 12 inches. That can work on paper for a very small shed on closely spaced supports, but our plans never use 2x4 joists: a 2x4 floor feels bouncy, gives little wood to nail the floor panel to, and leaves no room for error when a skid settles. We start at 2x6.

16 or 24 inches on center?

Spacing is set by the floor panel as much as the joist. Our plans use 16-inch spacing under 23/32-inch tongue-and-groove plywood, glued and screwed, which gives a solid floor for wheeled loads. At 24 inches a 2x6 spans 6' 11" instead of 8' 6", and the floor panel itself must be rated for that spacing: check the span rating stamped on the panel.

Do shed floor joists need to be treated?

Yes, for a shed on skids or blocks. The wood-preserving industry's standard, AWPA U1, says above-ground wood must be treated for ground contact (Use Category 4A or higher) when it is less than six inches above the ground on a permeable base, when air cannot circulate under the construction, or when debris can build up against it. A shed floor on skids usually meets the first of these and often the other two: in our plans the joists sit 5-1/2" above the gravel on skids (1' 1-1/2" on blocks), with little air moving under the floor.

That is why our plans list ground-contact (UC4A) treated lumber for the skids, rim joists, band joists and floor joists, and untreated framing from the floor up. Use hot-dip galvanized nails and connectors rated for treated lumber, and seal cut ends of treated lumber with a copper naphthenate end-cut preservative.

Framing the floor

1. Set the rim joists along the sides and the band joists across the front and back, then square the box by measuring its diagonals.
2. Mark the joists at 16 inches on center and set each one crown up, with three 16d galvanized nails through the rim at each end.
3. Glue and screw the 23/32-inch plywood floor with its long edges across the joists, end joints staggered and landing on joists. The panel adds 3/4" to the floor height.
4. Tie the floor to the skids with hurricane ties at the corners, and anchor the shed to the ground.

The plans' cut list gives every joist length and the order to build in. Price the floor for your own size in the free materials calculator.

How we worked out the spans

Each span is the shortest of three limits for a simply supported joist under a uniform load: bending, shear at the supports and deflection, using the American Wood Council's design values for No. 2 southern pine and No. 2 spruce-pine-fir (with the 15% repetitive-member increase that joists at 24 inches or less get), the method behind the building code's span tables. The same formula reproduces the code's published rafter tables to within about an inch, which our tests check. For anything outside these tables, or a floor that must carry a vehicle, ask an engineer.

Sources

• American Wood Council, 2024 Span Tables for Joists and Rafters: published spans by species, grade and load
• American Wood Council, online span calculator help: how spans come from bending, shear and stiffness
• California Residential Code (2021 IRC text), Section R502 Wood floor framing: the residential floor joist span tables
• AWPA Standard U1-25, Use Category System (excerpt): when above-ground wood must be treated for ground contact
• American Wood Protection Association, information for homeowners: ground contact needs UC4A or higher