Guide
Shed foundation options: gravel pad, concrete blocks or skids?
For a wood shed, the two simplest foundations are pressure-treated 4x6 skids on a compacted gravel pad and the same skids carried on solid concrete blocks, and those are the two our plans draw. A 10x12 shed needs a 12 x 14 foot gravel pad, 4 inches deep, which takes about 2.5 cu yd of 3/4-inch crushed stone (3.4 tons).
A concrete slab or poured piers are the heavier-duty options: worth it for a big workshop, or where your town asks for footings below the frost line. Below, the gravel, blocks and skids for every size, how to build the pad, and what changes on a slope.
- 10x12 on skids
- 12' x 14' pad, 2.5 cu yd of stone, 3 PT 4x6 skids
- 10x12 on blocks
- 12 piers of 2 blocks, 0.75 cu yd of stone under them
- Gravel pad
- 4" deep, 12" past the shed on every side
- Frost footings
- not required by the model code for a small light-frame shed (see below), unless your town says so
The four common shed foundations
- Skids on a gravel pad (our default). Treated 4x6 beams lie on edge on 4 inches of compacted crushed stone; the floor frame sits on the skids. Stone drains, so the wood is not standing in water, and the shed can be moved later. The floor ends up about 11-3/4" above the gravel.
- Concrete blocks. The same beams sit on piers of two solid 4x8x16 blocks, no more than 48 inches apart, each pier on 6 inches of compacted gravel. Good for uneven ground, because each pier can be shimmed to level, and it lifts the floor 8" higher for air under the shed.
- A concrete slab. A slab becomes the floor. The model residential code calls for at least 3-1/2 inches of concrete on a 4-inch base of sand, gravel or crushed stone, and a slab needs forms, a reinforcement plan and dry weather to pour. Our plans do not draw slabs.
- Piers or posts below the frost line. Concrete piers or helical piles reach stable soil below the depth the ground freezes. This is what a town asks for when it wants frost-protected footings, and it suits big or tall sheds.
How much gravel for a shed base, size by size
The pad runs 12 inches past the shed on every side, so rain off the roof lands on stone, not soil: a 10x12 shed gets a 12 x 14 foot pad. Every figure below comes from the materials list of that size's plan, from 1 cu yd for a 4x8 up to 5.75 cu yd for a 16x20.
| Shed | Gravel pad | Stone for the pad | Skids | On blocks instead |
|---|---|---|---|---|
| 4x8 | 10' x 6' x 4" | 1 cu yd (1.4 tons) | 3 PT 4x6 (4' 0") | 6 piers, 12 blocks, 0.5 cu yd |
| 6x8 | 10' x 8' x 4" | 1.25 cu yd (1.7 tons) | 3 PT 4x6 (6' 0") | 9 piers, 18 blocks, 0.75 cu yd |
| 8x8 | 10' x 10' x 4" | 1.5 cu yd (2.0 tons) | 3 PT 4x6 (8' 0") | 9 piers, 18 blocks, 0.75 cu yd |
| 8x10 | 10' x 12' x 4" | 1.75 cu yd (2.4 tons) | 3 PT 4x6 (10' 0") | 12 piers, 24 blocks, 0.75 cu yd |
| 8x12 | 10' x 14' x 4" | 2 cu yd (2.7 tons) | 3 PT 4x6 (12' 0") | 12 piers, 24 blocks, 0.75 cu yd |
| 10x10 | 12' x 12' x 4" | 2.25 cu yd (3.0 tons) | 3 PT 4x6 (10' 0") | 12 piers, 24 blocks, 0.75 cu yd |
| 10x12 | 12' x 14' x 4" | 2.5 cu yd (3.4 tons) | 3 PT 4x6 (12' 0") | 12 piers, 24 blocks, 0.75 cu yd |
| 8x16 | 10' x 18' x 4" | 2.75 cu yd (3.7 tons) | 3 PT 4x6 (16' 0") | 15 piers, 30 blocks, 1 cu yd |
| 10x14 | 12' x 16' x 4" | 2.75 cu yd (3.7 tons) | 3 PT 4x6 (14' 0") | 15 piers, 30 blocks, 1 cu yd |
| 12x12 | 14' x 14' x 4" | 3 cu yd (4.1 tons) | 3 PT 4x6 (12' 0") | 12 piers, 24 blocks, 0.75 cu yd |
| 10x16 | 12' x 18' x 4" | 3.25 cu yd (4.4 tons) | 3 PT 4x6 (16' 0") | 15 piers, 30 blocks, 1 cu yd |
| 12x14 | 14' x 16' x 4" | 3.25 cu yd (4.4 tons) | 3 PT 4x6 (14' 0") | 15 piers, 30 blocks, 1 cu yd |
| 12x16 | 14' x 18' x 4" | 3.75 cu yd (5.1 tons) | 3 PT 4x6 (16' 0") | 15 piers, 30 blocks, 1 cu yd |
| 10x20 | 12' x 22' x 4" | 3.75 cu yd (5.1 tons) | 3 PT 4x6 (8' 0", 12' 0", 2' 0") | 18 piers, 36 blocks, 1.25 cu yd |
| 12x18 | 14' x 20' x 4" | 4 cu yd (5.4 tons) | 3 PT 4x6 (6' 8", 7' 4", 10' 8", 11' 4", 2' 0") | 18 piers, 36 blocks, 1.25 cu yd |
| 14x16 | 16' x 18' x 4" | 4.25 cu yd (5.7 tons) | 4 PT 4x6 (16' 0") | 20 piers, 40 blocks, 1.25 cu yd |
| 12x20 | 14' x 22' x 4" | 4.5 cu yd (6.1 tons) | 3 PT 4x6 (8' 0", 12' 0", 2' 0") | 18 piers, 36 blocks, 1.25 cu yd |
| 16x16 | 18' x 18' x 4" | 4.75 cu yd (6.4 tons) | 4 PT 4x6 (16' 0") | 20 piers, 40 blocks, 1.25 cu yd |
| 16x20 | 18' x 22' x 4" | 5.75 cu yd (7.8 tons) | 4 PT 4x6 (8' 0", 12' 0", 2' 0") | 24 piers, 48 blocks, 1.5 cu yd |
Crushed stone is sold by the cubic yard or by the ton. Our conversion uses 1.35 short tons per cubic yard for 3/4-inch stone; suppliers' figures vary with the rock, so order by their number. Compare both foundations for your size in the free calculator: change the foundation to concrete blocks and the materials list changes with it.
Building a gravel pad, step by step
- Call 811 first. Free, and it marks buried lines before you dig.
- Mark the pad 12 inches bigger than the shed each way, and check it is square by measuring both diagonals: they match when it is square.
- Strip the turf and topsoil to firm soil, sloping the bottom slightly so water drains away from where the shed will stand.
- Lay landscape fabric (optional in our plans) to keep weeds and soil out of the stone.
- Spread the stone in two layers and compact each one with a plate compactor or hand tamper until it stops moving underfoot, then level it with a straight board and a level.
- Set the skids in line, check them level along their length and across, and build the floor frame on them.
- Anchor the shed. Our plans add four auger anchors strapped to the frame, because a shed on skids can slide or lift in high wind.
Concrete blocks: spacing and leveling
On blocks, each skid becomes a beam carried by piers. Our plans put a pier at each end of every beam and in between at no more than 48 inches, two solid blocks high, on a 6-inch compacted gravel pad that is 4 inches bigger than the pier all around. A 10x12 has 3 beams and 12 piers: 24 blocks in all.
Set the highest pier first, then bring the others up to it. Composite shims between the block and the beam take care of the last fraction of an inch; for more than that, add gravel under the pier rather than stacking loose pieces.
Building a shed foundation on a slope
Every base has to end up level, so on sloping ground the low side is built up (or the high side dug down) by the drop across the shed. Measure the slope with a long straight board, a level and a tape: the drop over the board's length, divided by its length, is the slope.
| Across the shed | 2% slope | 5% slope | 8% slope | 10% slope |
|---|---|---|---|---|
| 8 ft | 2" | 4-3/4" | 7-3/4" | 9-1/2" |
| 10 ft | 2-1/2" | 6" | 9-1/2" | 12" |
| 12 ft | 3" | 7-1/4" | 11-1/2" | 14-1/2" |
| 16 ft | 3-3/4" | 9-1/2" | 15-1/4" | 19-1/4" |
| 20 ft | 4-3/4" | 12" | 19-1/4" | 24" |
A few inches can be made up with deeper gravel on the low side or a taller pier. Much more than that, dig the high side into the slope so the base is not resting on a deep pile of loose fill, and keep water from running under the shed. Our plans assume a level base; how you level the ground is up to the site.
Frost, codes and permits
The International Residential Code asks for foundations that are protected from frost, but makes an exception for free-standing accessory buildings of light-frame construction up to 600 square feet with an eave height of 10 feet or less (Section R403.1.4.1), one reason skids and blocks are common under sheds. Towns can and do change this, so ask your building department whether yours wants frost footings for the size you plan.
A slab, if you pour one, must be at least 3-1/2 inches thick by the same code (Section R506.1).
| Shed | Concrete |
|---|---|
| 8x10 | 1 cu yd |
| 10x12 | 1.5 cu yd |
| 12x16 | 2.5 cu yd |
| 16x20 | 4 cu yd |
Read do I need a permit for a shed? before you start: setbacks from the property line decide where the pad can go.
Questions people ask
How much gravel do I need for a 10x12 shed?
About 2.5 cu yd of 3/4-inch crushed stone (3.4 tons) for a 12 x 14 foot pad, 4 inches deep after compacting. On concrete blocks it needs only 0.75 cu yd, under the piers.
How big should a shed base be?
12 inches bigger than the shed on every side, so a 10x12 shed sits on a 12 x 14 foot pad and an 8x8 on a 10 x 10.
How deep should a gravel shed base be?
Our plans use 4 inches of compacted crushed stone under skids, and 6 inches under each concrete block pier.
Can I put a shed on concrete blocks?
Yes. Our plans carry treated 4x6 beams on piers of two solid 4x8x16 blocks, no more than 48 inches apart; a 10x12 needs 12 piers (24 blocks).
Do I need a concrete slab for a shed?
Not for a wood shed on skids or blocks. A slab suits a heavy workshop or a garage-style floor; if you pour one, the model code asks for at least 3-1/2 inches of concrete on a 4-inch gravel base.
Do I need frost footings for a shed?
Often not: the International Residential Code exempts free-standing light-frame accessory buildings up to 600 square feet with eaves no higher than 10 feet. Some towns require footings anyway, so ask yours.
Price your base
Open the free materials calculator for your size: it lists the gravel, skids, blocks and anchors with the rest of the shed. The 10x12 gable shed plans show the foundation layout and every piece.
Sources
- California Residential Code (2021 IRC text), Section R403.1.4.1 Frost protection: the 600 square foot accessory building exception
- California Residential Code (2021 IRC text), Section R506 Concrete floors on ground: 3-1/2 inch slab and 4 inch base course
- Designing Buildings, Converting between weight and volume for bulk construction materials: crushed stone at 1,602 kg per cubic meter, 1.35 short tons per cubic yard
- American Wood Protection Association, information for homeowners: ground contact needs Use Category 4A (UC4A) or higher
- Call 811: free marking of buried utility lines before you dig
Our plans and these guides follow common light-frame building practice and are a guide, not drawings stamped for your site: check your local codes, permit rules and HOA before you build.