How Many Concrete Blocks Do You Need for a Shed Foundation?

Space your support blocks 8 feet apart instead of 5, or skip the center row under a 10-foot-wide shed, and the floor starts to bounce by month three and the door binds within a year. Block count is not guesswork. It comes straight from two numbers: how far apart blocks can sit along each beam (4 to 6 feet on center) and how many beam rows the floor needs (two under an 8-foot shed, three once you pass 8 feet wide). Get those right and most sheds land between 6 and 12 blocks. Here is the count by size and the layout math behind it.

TL;DR: - Most sheds need 6 to 12 support blocks: an 8x8 or 8x10 takes about 6, a 10x12 needs 9 to 12, and a 12x16 runs about 12. - Set blocks no more than 4 to 6 feet on center along each beam, and about 12 inches in from every corner. - Any shed wider than 8 feet needs a third beam row of blocks down the center, or the floor sags between the two outer runs. - Use solid concrete blocks or precast deck blocks on 4 to 6 inches of compacted crushed stone. Never lay a hollow cinder block on its face under load.

The Short Answer: How Many Blocks by Shed Size

Most storage sheds need 6 to 12 concrete support blocks, and the exact count comes from the shed’s width and length. An 8x8 or 8x10 shed rides on two beam rows with three blocks each, so six blocks carry it. Once a shed passes 8 feet wide it needs a third beam row down the center, which pushes a 10x12 to nine or twelve blocks and a 10x16 to twelve or fifteen. A 12x16 typically lands at twelve. These are the structural bearing points that hold the floor frame, each spaced 4 to 6 feet from the next.

One caution: small precast leveling blocks are shims, not structural piers. Count those separately, since a level job can burn through far more of them. The full comparison of gravel, concrete slab, and pier bases lives in our how to build a shed foundation guide.

How Block and Pier Spacing Actually Works

Two rules set every block count. First, no support block sits more than 4 to 6 feet on center from the next one along the same beam run. Stretch that gap and the beam flexes between supports, which is exactly what you feel underfoot as a bouncing floor. Second, place a block about 12 inches in from each corner rather than dead on the end, so the beam cantilevers slightly and shares the load instead of splitting at the tip.

From there the layout is just rows times blocks per row. An 8-foot beam needs a block at each end (12 inches in) plus one in the middle: three blocks. A 12-foot beam needs four. Count the beam rows the same way. An 8-foot-wide floor spans fine on two outer beams, but larger sheds wider than 8 feet need a third beam row down the center. Skip that center run on a 10 or 12-foot-wide shed and the floor joists dish in the middle no matter how solid your corners are.

To size any shed, count blocks per beam (length divided by 5, rounded up, then add one), then multiply by the number of beam rows. The floor frame itself carries the load between those points, so a stiff, well-built frame matters as much as the blocks under it. Our guide to how to build a shed floor covers joist sizing and spacing.

Deck Block vs Solid Concrete Block vs Cinder Block

Not every block belongs under a shed, and the gap between the three you will see at the yard is real. Precast deck (or pier) blocks are the best default. They are cast with a notch that cradles a 4x4 post or a beam, so the frame locks in place and cannot walk sideways. Solid concrete blocks are the strongest option, rating roughly 1,800 to 2,500 psi, and make sense where loads run high or you want maximum bearing area.

Hollow cinder block, or CMU, is the weak link. It is far weaker than solid block, and you must never lay it on its face with the cores running horizontal under load, because the thin webs crush. If a hollow block is all you have, set it cores vertical so the load runs straight down the solid webs, but solid or deck blocks are the better call every time.

Blocks work because they carry the shed on points and let you re-level later. When the shed is large, heavy, or permanent, poured concrete piers or footings replace blocks and tie the structure to stable ground below. For how a frost-proof footing differs from a block that floats on the surface, Family Handyman walks through the footing detail.

The Gravel Pad and Drainage Under Every Block

A block set straight on dirt sinks. Under every support point goes 4 to 6 inches of compacted crushed stone, and the type of stone matters. Use angular crushed stone, not rounded pea gravel: the sharp faces lock together when compacted and hold the block steady, while rounded pebbles roll and shift under load like ball bearings.

Compact the stone in lifts, adding it a couple of inches at a time and tamping each layer rather than dumping the full depth at once. Loose fill keeps settling for months, while compacted lifts stop moving. Extend the pad about 12 inches beyond the shed footprint on every side so the edge blocks sit on solid, drained material instead of the soft soil at the perimeter.

The reason is drainage, and good drainage prevents both settling and heave. Crushed stone lets rain drain straight through instead of pooling around the base, so the ground under each block stays firm and does not wash out or turn to mud. A block on well-drained stone holds its level for years, while the same block on bare, wet clay settles unevenly and starts the floor tilting within a season.

Frost Line: When Blocks Alone Aren’t Enough

Block and deck-block bases float. They sit on the surface, so when the ground freezes and heaves they ride up with it and settle back as it thaws, a cycle Fine Homebuilding shows lifting a footing from below. For a freestanding storage shed you can re-level each spring, that is fine and expected. It is why blocks are the easy, forgiving choice for a garden shed in mild climates.

The trouble starts when the shed is permanent, attached to the house, or standing in a cold-winter region where frost drives deep. There, a floating block can heave a full corner up over winter and never come all the way back, racking the walls and jamming the door for good. The fix is footings or piers poured below your local frost line, which anchors the shed to ground that never freezes. A gravel bed under the block improves drainage and slows heave, but it does not make a shallow block frost-proof. Your building department can tell you the frost depth for your area, which ranges from a few inches in the South to 4 feet or more up north.

For most backyard storage sheds the answer stays simple: 6 to 12 solid or deck blocks, none more than 4 to 6 feet apart, a center row once you pass 8 feet wide, all resting on 4 to 6 inches of compacted crushed stone. Nail those numbers and the floor stays flat and the door keeps latching season after season. Ready to match a shed to the base you just planned? Browse our wood storage sheds collection.

FAQ

Can I use concrete blocks as a shed base?

Yes. Concrete blocks make a solid, low-cost shed base as long as you use solid or precast deck blocks, space them no more than 4 to 6 feet apart under the floor beams, and set each on 4 to 6 inches of compacted crushed stone. Add a center row of blocks once the shed is wider than 8 feet. Remember the blocks float, so switch to frost-line footings for a permanent or attached shed in a cold climate.

How many cinder blocks to build a 12x12 shed?

A 12x12 shed needs about 9 to 12 support blocks: three beam rows (two outer plus a center row, since 12 feet is wider than 8) with three blocks per 12-foot row, spaced 4 to 6 feet apart and set 12 inches in from each corner. Skip hollow cinder blocks laid on their face, though. Solid concrete or precast deck blocks carry the load far better and will not crush under the frame.

How many deck blocks do I need for a 10x12 shed?

Plan on 9 to 12 deck blocks for a 10x12 shed. Because 10 feet is wider than 8, run three beam rows (two outer, one center) with three blocks in each 12-foot row at 4-to-6-foot spacing. Heavier loads or a stiffer floor push you toward the 12-block end. Set every deck block on 4 to 6 inches of compacted angular crushed stone so it drains and holds level.

What blocks are best for shed foundation?

Precast deck (pier) blocks are the best default because their notch cradles a 4x4 or beam and locks the frame in place. Solid concrete blocks, rated around 1,800 to 2,500 psi, are the strongest choice for heavy loads. Avoid hollow cinder blocks under load, and never lay one on its face with the cores horizontal. For a permanent or cold-climate shed, poured concrete piers below the frost line beat any surface block.