Concrete Block Shed Foundation: Layout and Block Counts

Last updated October 15, 2026

A concrete block shed foundation is a grid of solid blocks set on compacted crushed stone, one under each end of every skid and one at least every 4 feet between. A 10×12 shed on four skids needs 16 blocks and a 12×24 on four skids needs 28. Level the blocks, then lay the skids on top.

How many blocks does a concrete block shed foundation need?

3DSHEDPLANS plans build the floor frame on pressure-treated 4×4 skids that run the long way, with 2×6 floor joists on top. In our current plan builds, the number of skids depends on the shed’s depth: 2 skids for a 4 ft deep shed, 3 for 6 to 8 ft, 4 for 10 to 12 ft and 5 for 14 to 16 ft. Older plans can differ, so count the skids on your plan’s floor page first.

The block count is simple: skids times blocks per skid. Put a block under both ends of each skid and space the rest evenly, no more than 4 feet apart. For heavy storage, tighten that to about 3 feet.

Shed size	Skids	Skid length	Blocks at 4 ft max	Blocks at 3 ft max
4×8 lean-to	2	8 ft	6	8
8×8	3	8 ft	9	12
8×12	3	12 ft	12	15
10×12 or 12×12	4	12 ft	16	20
10×16 or 12×16	4	16 ft	20	28
10×20 or 12×20	4	20 ft	24	36
12×24	4	24 ft	28	40
16×24	5	24 ft	35	50

These counts are a layout rule of thumb for storage sheds on firm, well-drained soil, not an engineering rating. Soft clay, a loft, a riding mower or a full rack of firewood all call for the tighter column. For 20 and 24 ft skids, the 3 ft column includes one extra block under each skid joint. Buy a few spares, plus one extra block for every low-side spot where you stack two.

Where the skid joints go

Home centers rarely stock 4x4s longer than 16 feet, so skids on a 20 or 24 ft shed are two pieces butted together, with a 3 ft treated 2×4 scab nailed on each side. Each skid has one joint, staggered at least 4 feet from the joint on the next skid: at 8 or 12 feet on 20 ft skids, and 8 or 16 feet on 24 ft skids. A 4 ft block grid puts a block right under each of those joints. At 3 ft spacing the joints fall between blocks, so add one under each joint.

Which concrete blocks to buy for a shed

Solid cap blocks (4 by 8 by 16 inches nominal). The usual pick. They are solid all the way through, about 3 5/8 inches tall, and a 3 1/2 inch wide 4×4 skid sits on one with room to spare. Set each one with its 16 inch length running along the skid, so the skid bears on the full length of the block.
Hollow concrete blocks (8 by 8 by 16 inches nominal). Usable when the cores stand vertical, so the load runs down the webs, with a solid cap block on top so the skid does not bear over the holes. Never lay them on their side. They stand about 7 5/8 inches tall, so use them everywhere or not at all.
Precast deck blocks. Pyramid-shaped blocks with a socket for a 4×4 post or a slot for a 2x joist on edge. A flat 4×4 skid does not seat properly in either, so they suit post-and-beam floors better than skid floors.
Patio pavers. Thin pavers are useful as shims under a block, not as the support itself.

Avoid broken landscape blocks, bricks and mixed leftovers.

As a rough estimate, solid cap blocks cost a few dollars each at a home center; prices vary by region, so check your local store.

How to build a concrete block shed foundation step by step

Before you dig, check your local permit office about shed size, placement and foundation rules, which vary from town to town. Call 811 a few days ahead so buried utility lines get marked. Tools: stakes, string, tape, shovel, hand tamper, a 4 ft level and a long straight 2×4.

Mark the outline. Drive stakes at the four corners of the floor frame and run string between them. Measure both diagonals; when they match, the layout is square.
Mark each block. Measure along the strings and mark the block centers: one within a few inches of each skid end, the rest evenly spaced. The outer skids sit flush with the floor edge, so their centers are 1 3/4 inches inside the string. On a 12 ft deep shed the four skid centers are just under 4 feet apart.
Dig each spot. Cut out a hole about 16 by 24 inches and usually 4 to 6 inches deep, through the sod to firm soil. If topsoil runs deeper, keep digging and add more stone.
Add stone and compact it. Put in 4 to 5 inches of angular crushed stone with fines, and tamp it hard with a hand tamper or the end of a 4×4. Round pea gravel rolls and will not hold a block in place.
Set the highest corner first. Find the high corner of the site and bed that block on its stone so its top sits about 3 to 4 inches above the surrounding soil. This block sets the height for all the others.
Set the other corners to it. Use a straight 2×4 and a 4 ft level, a water level or a laser level. Add or scrape stone under each block until its top matches the first one.
Fill in the middle blocks. Pull a tight string across the tops of the end blocks and set every middle block to that line.
Lay the skids and check. Set the treated 4×4 skids on the blocks, crown up, and check them for level along their length and across all of them. Fine-tune with thin pavers or composite shims, then build the floor frame on top as the plan shows. Skids usually just bear on the blocks; tie-downs do the holding.

Stone quantity: a 16 by 24 inch hole filled about 4 1/2 inches deep holds 1 cubic foot of compacted stone. That means a 12×24 shed on 28 blocks needs about 28 cubic feet, roughly 1 cubic yard. Order 15 to 20 percent extra because loose stone shrinks as you compact it.

Blocks on a slope: how far can you stack?

A block foundation handles a gentle slope well. On the low side you dig shallower holes, add more stone or stack a second solid block, which adds about 3 5/8 inches. Together that covers roughly 4 to 6 inches of fall across the footprint.

Past that point, tall stacks of dry blocks can tip or shift sideways under a load. On steeper ground, use concrete piers, posts on footings or ground screws instead. Our guide to shed foundation options compares those choices side by side.

When you stack two blocks, set them flush, long sides running along the skid, with a bead of construction adhesive between them so the top block cannot slide.

No skids? The perimeter layout

Some guides set blocks only under the rim of a floor frame, at the corners and every 6 to 8 feet. That only works when the rim is a doubled board or beam sized for that span, so follow your plan’s framing. Our plans use skids, so the grid above applies.

Frost, moisture and settling

A block foundation sits near the surface, so it floats on the soil instead of reaching below the frost line. Where the ground freezes, wet soil can expand and lift blocks unevenly, then drop them again in spring. HUD’s guide to frost-protected shallow foundations explains how frost depth and soil type drive that movement.

A small skid shed can usually ride out some movement, because the skids and floor frame move as one unit. Crushed stone under each block and soil graded away from the shed both help. Check the doors and level each spring and re-shim as needed. In very cold climates, read our notes on building a shed foundation in cold weather.

The USDA Forest Products Laboratory Wood Handbook explains that wood which stays wet for long periods is at risk of decay. Blocks lift the skids into moving air, so keep at least a few inches of clearance between the skid bottoms and the soil, and keep grass and mulch away from them.

Use skids treated for ground contact. The AWPA treated wood guide lists ground contact use categories such as UC4A for wood that sits close to the soil or is hard to replace, and a skid under a finished shed is hard to replace. Check the end tag on every 4×4.

Common mistakes with block foundations

Setting blocks straight on sod. The grass rots, the topsoil compresses and the blocks sink unevenly. Always dig to firm soil and add stone.
Spacing blocks too far apart. A skid that spans 6 or 8 feet between blocks can sag under a loaded floor, and the floor feels bouncy.
Missing a skid joint. On a 20 or 24 ft shed, a butt joint that hangs between blocks is the weak spot. Put a block under it.
Shimming with untreated wood. Scrap wood shims rot and crush, and the shed drops. Use pavers, composite shims or more stone.
Mixing block types. Deck blocks, cap blocks and bricks settle at different rates.
Ignoring drainage. If water runs toward the shed after rain, grade the soil away from it before you set a single block.
No tie-downs in windy areas. A shed on blocks is held down only by its own weight. Auger-style earth anchors at the corners, with galvanized straps fastened to the outer skids or rim, keep it in place. Some areas require tie-downs, so check your local permit office.

Blocks or a gravel pad?

Blocks save digging and stone, and they handle a gentle slope better. A full gravel shed foundation gives the skids continuous support and controls weeds and mud around the shed. Many builders combine the two: a compacted gravel pad with solid blocks under the skids on the low side. On a gentle slope blocks are usually less work; on a hillside or in deep frost country, look at piers.

What we checked: skid counts and joint positions come from our plan generator’s cut lists; block sizes are standard nominal concrete masonry sizes; frost, decay and treatment notes follow HUD, the USDA Wood Handbook and AWPA.

Plans that fit

Every 3DSHEDPLANS plan PDF includes the floor framing and a full material list, so you can count the skids before you buy blocks.

The 10×12 gable shed plan sits on four 12 ft skids, the 16-block layout.
The 12×24 gable shed plan is a long shed where block spacing and skid joints matter most.
The 12×24 lean-to shed plan has the same footprint under a single-slope roof, a good fit for a gentle slope behind a garage.
The 12×16 gable shed plan v2 is a 12 ft deep shed, so plan on four skids and a 20-block grid.
For a light, small shed, the 4×8 lean-to shed plan needs only a handful of blocks.

Browse every size on the shed plans page, or start with the gable shed plans or lean-to shed plans. You can also download the free 12×24 gable shed plan PDF to see how the floor and skids are drawn.

Frequently asked questions

How many concrete blocks do I need for a shed?

Count the skids, then put a block under both ends of each skid and at least every 4 feet between. A 10×12 shed on four 12 ft skids takes 16 blocks, a 12×16 takes 20 and a 12×24 takes 28. Add spares and any extra blocks you stack on the low side.

How far apart should blocks be under a shed?

Set them no more than about 4 feet apart along each skid, with a block under each end. Go to about 3 feet for heavy loads like a riding mower or firewood, and add a block under any skid joint that falls between blocks.

Can I use hollow cinder blocks for a shed foundation?

Solid blocks are the better pick. If you use hollow concrete blocks, stand them with the cores vertical and put a solid cap block on top so the skid bears on solid concrete. Laid on their side, hollow blocks can crack under a skid.

Do I need gravel under concrete blocks for a shed?

Yes. Blocks set on sod or topsoil sink and tilt as the soil settles and the grass rots. Dig each spot 4 to 6 inches deep to firm soil, add crushed stone, compact it and set the block on the stone.

How high can I stack concrete blocks under a shed?

Two solid cap blocks (nominal 4 inch, 3 5/8 inch actual) is a sensible limit for a dry stack. With a little extra stone, that covers roughly 4 to 6 inches of fall. More than that, switch to concrete piers or ground screws.