How to Build a Shed Foundation

Most shed problems that show up years later start at the bottom: a door that binds because one corner settled, a floor that stays damp because it sits on dirt, a frame that was never square so the roof sheathing never lines up. None of that is hard to avoid. A good shed foundation needs three things: it must be flat, it must keep the wood dry, and it must give you a square frame to build on.

This guide covers the two floating foundations our designer draws: pressure-treated skids on a gravel pad, and beams on concrete deck blocks. The numbers come from the same engine as our shed designer, which works them out for any size.

Skids, blocks or concrete

All three work. The choice comes down to your ground, the size and weight of the shed, and what your building department accepts. For slab thickness, paving slabs, plastic grids and timber frames, with quantities in feet or metres, see the shed base guide.

Skids and blocks are called floating foundations: the shed rests on the ground instead of being anchored below the frost line. They move a little with the seasons, and a light timber building shrugs that off. The model code agrees for small sheds: section R403.1.4.1 of the 2021 International Residential Code does not require frost protection for a detached light-frame accessory building up to 600 sq ft with eaves no higher than 10 ft. Your jurisdiction sets its own frost depth and can ask for more. If you want to add plumbing or plan to heat the building, talk to your building department before you pick one. Our permit basics guide explains what to ask.

Pick and prepare the site

Before you buy any lumber, walk the spot after a heavy rain. Water that pools there will pool under your shed. Look for:

• Drainage. High ground is better than low ground. If the only spot is low, raise the pad with more gravel and slope the soil around it away from the shed.
• Room to work. Leave enough space around the shed to stand and put up siding and trim, and later to paint it.
• Setbacks. Towns set minimum distances from property lines and often from other buildings. Check them now, not after the pad is down.
• Utilities. Call 811 a few business days before you dig. It is free, and it keeps your shovel away from buried lines.

Plan the gravel pad one foot bigger than the shed on every side. For an 8x10 shed that is a 10x12 pad, and at 4 inches deep it takes about 1.5 cubic yards of gravel. A 12x16 shed needs a 14x18 pad and about 3.1 cubic yards. Buy crushed stone with angular faces, not round pea gravel. A U.S. Forest Service design guide puts it plainly: crushed rock compacts relatively well, while round gravel does not, because the stones roll against each other.

Build a skid foundation

Skids are long pressure-treated timbers, usually 4x6, that run front to back under the shed. Set them on edge, with the 5-1/2 inch face vertical: that is the stiff way, and it lifts the floor frame further off the gravel. Use lumber treated for ground contact (AWPA use category UC4A), not the above-ground grade. The model code requires ground-contact treated wood wherever wood touches the ground under a building people occupy (section R317.1.2), and it is the right grade for skids either way.

Space the skids so the floor joists never span more than about 4 ft between supports. On an 8 ft wide shed that means three skids, about 46-1/4" apart. A 12 ft wide shed gets 4 skids about 46-13/16" apart. At that spacing a 2x6 joist at 16 inches on center is loafing: the designer checks it at a 50 psf floor load against the span divided by 360, the deflection limit the model code sets for floors, and a 2x6 is good for about 8' 7-13/16".

1. Mark the footprint. Drive stakes at the four corners of the pad, one foot outside the shed on every side, and run string between them.
2. Strip and dig. Remove sod and topsoil inside the strings until you reach firm subsoil, and rake the bottom flat.
3. Lay fabric and gravel. Roll out landscape fabric, then spread crushed gravel in two layers, compacting each one, until you have the 4 inches of compacted gravel the designer plans.
4. Level the pad. Screed the gravel with a straight 2x4 and check it with a level in both directions until it is flat.
5. Set and level the skids. Place the pressure-treated skids on edge at the spacing from your plan, then level each one along its length and against its neighbors.
6. Build the floor frame. Nail the joists between the two rim joists at 16 inches on center, then set the frame on the skids.
7. Square the frame. Measure both diagonals and rack the frame until they match, then toenail or strap the frame to every skid.
8. Glue and fasten the subfloor. Run construction adhesive on the joists and screw down 3/4 inch tongue-and-groove plywood, long edges across the joists.

For the 8x10 the floor frame is 9 joists at 93" long between two rim joists at 10'. The rim joists run the same direction as the skids and the joists run across them. Nail each joist through the rim with three 16d common nails (3-1/2 x 0.162 inch) or four 3 x 0.131 inch nails at each end, which is what the model code's fastening schedule calls for, or use joist hangers if you prefer. The free 8x10 gable shed plan lists every piece and how to cut it from standard lumber.

Build on concrete blocks

On a slope, a block foundation is easier than trying to build a level gravel pad. Solid concrete deck blocks with a notch on top hold 4x4 beams, and the beams take the place of skids. The designer places the beams at the same spacing as skids and puts blocks under each beam as close as the 4x4 needs to pass its span check, never more than 4 ft apart, which works out to 12 blocks for an 8x10.

• Dig out the topsoil under every block and set each one on a few inches of compacted gravel.
• Set the highest block first, then bring the others up to it with more compacted gravel. Avoid stacking loose shims.
• If the low side would need a tall stack of blocks, a gravel pad or a pier is a better choice.
• Lay the beams, check them with a level along their length and across to each other, then build the floor frame on top exactly as for skids.

The concrete block shed foundation guide has the block count and spacing for every size, and the shed floor guide covers the frame that goes on top.

Level and square

A frame that is out of square by half an inch at the floor will be out by the same amount at the roof, and the sheathing will show it. Square it now, while it is easy to push around.

• Diagonals. Measure corner to corner both ways. When the two diagonals are equal, the frame is square. For an 8x10 frame they should both read 12' 9-11/16".
• The 3-4-5 check. From one corner, mark 3 ft along one side and 4 ft along the other. The distance between the marks should be exactly 5 ft.
• Level. Put a 4 ft level on a long straight board and check across the joists in several places. Fix any low skid now, not after the walls are up.

Once it is square, fasten the frame to the skids or beams so it cannot shift while you sheath the floor.

Anchor the shed

A floating shed is heavy, but a strong wind gets under a light one. Ground anchors near each corner, strapped to the floor frame, are cheap insurance, and some places require them: Elgin, Illinois, accepts a gravel base only with code-approved tie-downs, and Prince William County, Virginia, lets a small shed skip a permanent foundation only if it is anchored against wind. The designer adds four to the materials list for skid foundations. With blocks, fasten the beams to the blocks and the frame to the beams. Ask your building department what anchoring they expect in your area.

Ready for the walls? How to frame shed walls picks up right where the subfloor ends. Or open the shed designer to get the foundation, floor and framing for your own size.