Concrete Block Shed Foundation: How Many Blocks

A concrete block shed foundation is a set of precast blocks that carry pressure-treated beams, with the floor frame on the beams. It is the usual answer when the ground is uneven or you want the floor higher than a skid floor puts it. The count is simple: beam lines times blocks per line, with the blocks close enough for each 4x4 beam to carry its load, and never more than 4 ft apart.

This guide gives the block count for all 20 sizes on the shed designer, explains solid blocks, deck blocks and hollow blocks, and shows what changes in the shopping list against skids. It is the detailed page under the shed foundation guide, the way the gravel shed base guide is for gravel.

What a concrete block shed foundation is

Think of it as the floor frame standing on legs. Beams run front to back, blocks sit under the beams at intervals, and the joists cross the beams. Nothing is poured and nothing is buried.

The designer’s block floor

On the designer, the blocks floor replaces 4x6 skids on gravel with 4x4 pressure-treated beams on precast blocks. The shopping list calls the blocks “precast concrete deck blocks for 4x4 beams”. The beams sit at the same spacing as skids would, and the 2x6 floor joists and 3/4 in plywood subfloor on top are the same either way. Pick the block foundation in the designer to switch any size to it, for example the 10x12 with blocks.

Blocks are not a code footing

The 2021 IRC, R403.1, says exterior walls sit on continuous masonry or concrete footings, crushed stone footings, wood foundations or another approved structural system, with exterior footings at least 12 inches below undisturbed ground. Blocks laid on the surface are not that kind of footing. They are the common practice for small sheds that need no permit, and where a permit is needed the building department decides.

The same code has a frost exception: R403.1.4.1 says protection from frost is not required for free-standing accessory structures of 600 square feet or less, of light-frame construction, with an eave height of 10 feet or less. The model code is a starting point. The edition your town adopted, and its amendments, decide.

Decks.com by Trex goes further for decks. It says a precast deck block is subject to the same requirements as a typical footing, at least 12 inches below grade or below the local frost depth, and that it cannot simply be placed at grade. It also says deck blocks are best suited to climates where frost penetrates the soil no more than 4 or 5 inches. That is one publisher’s view of deck posts. For a shed, your building department’s handout is the authority.

Where blocks suit a shed and where they do not

Common practice is to use blocks on a gentle slope, because each block can be set at its own height. It also lifts the floor: 11-1/2 inches of foundation against 5-1/2 on skids. Blocks are a poor fit where the low side would need a tall stack, where the soil is soft or wet, or where your building department wants a permanent foundation. Poured piers and slabs are covered below and in the shed base guide.

Solid blocks, deck blocks and cinder blocks

Three names get used for the same idea. They are not the same product.

Solid concrete blocks and their real size

NCMA’s TEK 02-01B says concrete masonry units typically have nominal face dimensions of 8 in by 16 in, in nominal thicknesses of 4, 6, 8, 10, 12, 14 and 16 in. ASTM C90 defines the difference between hollow and solid units, and the net cross-sectional area of a solid unit is at least 75% of the gross area. CMHA adds that specified dimensions are typically 3/8 in less than nominal, so the 8x8x16 block you buy is 7-5/8 by 7-5/8 by 15-5/8 in.

The designer’s 11-1/2 in foundation is an 8 in block plus a 3-1/2 in beam (a 4x4 is 3-1/2 in square, as PS 20 sizes it). A real 8x8x16 block is 3/8 in shorter, so measure what you buy and set the height from that.

Hollow blocks, “cinder blocks” and which way the cores face

“Cinder block” is the old name people still use for the standard hollow concrete block. CMHA says ASTM C90 units may be made hollow or solid and reinforced as necessary for the design loads. A hollow block has cores, and in a wall those cores are often grouted where the design calls for it. That is wall engineering.

For a shed, common practice is to set a hollow block with its cores vertical and to cap it with a solid 4 in block or a solid cap so the beam bears on a full surface, not on thin face shells. That is general practice, not a rule we could source. We found no maker or association document on laying hollow blocks with the cores sideways under a shed. A solid block avoids the question.

Precast deck blocks and pier blocks

A deck block is a precast block shaped to hold a post or beam. Oneonta Block Company sells a universal pier block of 12 in by 12 in by 8 in tall that fits 4x4s, 6x6s and 2x6s and weighs about 64 lbs. Decks.com by Trex says the purpose of a deck block is to spread the concentrated load at the bottom of a wooden deck post onto strong soil.

Read what it says about limits, because it was written for decks. Decks.com says pier blocks are best suited to low or ground-level decks and that the small bearing area of a pier block limits how much load they can support. It also says a concrete pier block has no way to mechanically connect to the post. The 12 in by 12 in footprint it gives for a normal deck block is the bearing area you are relying on.

How many concrete blocks for a shed foundation

The designer computes block layout from the shed’s width and depth. Here is the rule, and here it is for every size.

The block-count rule

Beam lines run front to back, so their number comes from the width: the larger of 2 and the width divided by 48 in, rounded up, plus 1. That is the same rule the designer uses for skids. Blocks per line come from the depth and from what a 4x4 can carry between two blocks. The designer works out that allowed gap first, never more than 48 in, then divides the depth minus 3-1/2 in by it, rounds up and adds 1. The total is lines times blocks per line.

The allowed gap is set by the outer beams, the ones under the side walls. On a gable shed they carry half a joist bay of floor, the side wall and half the roof with its snow; the inner beams carry a full joist bay of floor. The designer checks both at 50 psf floor live load plus 10 psf dead, your snow load plus 10 psf roof dead and 10 psf of wall, for bending, shear and a deflection of span/360.

Every size in the tables below follows that rule. The blocks sit centered under the beam, spaced evenly from 1-3/4 in in from each end.

Blocks for every shed size

The first number in the shed size is the width across the door wall. The beam count, blocks and gap are the designer’s results at 30 psf of snow, and the gap is measured center to center of the blocks. The last column is the most the outer 4x4 beam may span at that load; the gap always stays under it.

The four lean-to plans, width by depth. Their side walls carry no roof, so the 4 ft limit decides the gap:

A heavier snow load shortens the allowed gap on a gable shed, and the designer adds blocks to match.

How many blocks for a 10x12 shed

A 10x12 shed has 4 beams, each 144 in long. Its outer beams may span 43-15/16 in at 30 psf of snow, so 4 blocks per beam at 46-13/16 in would be too far apart: the designer puts 5 under each beam, 35-1/8 in apart, 20 in all. The beams are 38-13/16 in apart across the shed. Change the depth to 16 ft and the count goes to 24.

How far apart to space the blocks

There are two spacings, and they are not the same thing.

Between blocks along a beam

The designer sets the gap from a span check on the 4x4 beam itself, and never lets it pass 4 ft. In the table the gap runs from 30-13/16 in to 46-1/4 in. No IRC or AWC span table lists a single 4x4 beam, so the designer computes it the way the NDS does, with the lower of the Spruce-Pine-Fir No. 2 values it uses for the rest of the frame and Southern Pine No. 2 (the species of the treated 4x4s on its price list), reduced for wet service. The 4 ft cap is a planning value, not a code figure. Decks.com, writing about decks, says deck blocks might end up 6 or 8 feet apart and that “there’s no hard-and-fast rule”, with a structural engineer to set it. Follow your building department or the block maker’s instructions where they ask for more.

Between beams

The beams run at the skid spacing: 34-1/4 in for a 6 ft width, 46-1/4 in for 8 ft, 38-13/16 in for 10 ft, 46-13/16 in for 12 ft and 47-1/8 in for 16 ft. Those short spans are why the floor joists are not the weak point. The designer’s 2x6 joists at 16 inches on center are allowed about 8 ft 7-13/16 in at 50 psf live load, and the beams hold them a lot closer than that.

Beams longer than 16 ft

The designer’s boards stop at 16 ft by default. On the 10x20, 12x20, 12x24 and 16x20, the beams are spliced, and each butt joint is centered on a block with a 2 ft 2x6 splice plate on both faces. Never splice between blocks.

Height, shopping list and base

Switching from skids to blocks changes the floor height and three lines of the shopping list.

How high the floor sits

The foundation is 11-1/2 in tall with blocks and 5-1/2 in with skids. Add the 6-1/4 in floor stack and the top of the subfloor is 17-3/4 in above the pad, against 11-3/4 in on skids. The overall height of the 10x12 goes from 143-3/4 in to 149-3/4 in.

That has three side effects. A ramp gets longer: 17-3/4 in at 1:12 is 17 ft 9 in, against 11 ft 9 in. Prince William County, Virginia, exempts sheds from a permanent foundation only if the floor is no more than 18 in above grade, and 17-3/4 in leaves 1/4 in to spare on the pad alone. The shed ramp guide does the sums for both floors.

What changes in the shopping list

On the designer’s 10x12, blocks against skids changes four things:

• Beams. Four 12 ft 4x4 pressure-treated (ground contact) beams replace four 12 ft 4x6 skids. Both are treated to the ground-contact category, AWPA UC4A.
• Blocks. A new line: 20 precast concrete deck blocks.
• Ground anchors. The skid list has 4 ground anchors. The blocks list has none. Blocks hold the frame up and do not hold it down, so plan your own tie-downs; the how to anchor a shed guide covers them.
• Unchanged. The gravel line stays: 2.1 cubic yards of compactable crushed stone for a 4 in pad 1 ft wider than the shed on each side, plus a roll of landscape fabric. The 2x6 joists and rim joists are the same.

Wood joists bear on wood beams here. If you frame a joist or beam directly on a masonry block, the 2021 IRC, R502.6, asks for at least 3 in of bearing on masonry or concrete, against 1-1/2 in on wood. And the joists sit 11-1/2 in above the pad, inside the 18 in in R317.1, so they are pressure treated.

What goes under the blocks

The soil under each block does the work. Decks.com says deck blocks usually sit in direct contact with the soil, so the soil must be strong and compacted, and topsoil is the worst because it is compressible.

Crushed stone, not round gravel

The designer’s pad is compactable crushed stone. A U.S. Forest Service guide says round gravel is a poor choice for parking pads because the rocks roll against each other, and that crushed rock with angular faces compacts relatively well. That guide is about trails, not sheds, so it shows how the stone behaves and not how deep a shed pad should be. The gravel shed base guide sizes the pad and explains the layers.

Drainage and 811

Slope the ground so water runs away from the shed: R401.3 of the model code asks for a fall of at least 6 in in the first 10 ft for a house foundation, and it is a fair reference here. Call 811 before you dig: the Common Ground Alliance says to contact it a few business days before any digging.

Set the blocks step by step

The steps are general practice for a block floor. The maker’s instructions and your building department override them.

1. Call 811, then mark the beams. Stretch strings for the beam lines at the spacing above, and mark each block position.
2. Strip the topsoil. Take out the soft top layer under every block and under the pad.
3. Lay the pad. Fabric first, then crushed stone in thin layers, each compacted, out to 1 ft beyond the shed.
4. Set the high block first. Bed it level, then bring the others up to it with compacted stone. Avoid loose stacked shims; a maker’s instructions decide if shimming is allowed.
5. Check level in every direction. Use a long straight board and a level across each row and across the rows, and check the diagonals for square.
6. Set the beams. Lay the 4x4s on the blocks. Where a beam is spliced, the joint sits on a block.
7. Frame the floor. Joists cross the beams; the how to build a shed floor guide covers the frame.

How many blocks high is stable is a question we cannot answer with a number. The designer models one 8 in block per position, and we found no source for a safe stack. If the low side needs more than one block, common practice is to regrade or choose another foundation.

Poured concrete piers: a different foundation

Poured piers are not deck blocks. They are holes dug to depth and filled with concrete, and they are a permanent foundation. The 2021 IRC, R403.1.4.1, says piers and other permanent supports are protected from frost by extending below the frost line in Table R301.2, subject to the accessory structure exception. Decks.com describes a poured deck pier footing as typically a 24 in hole and says deck blocks are not to be confused with real piers.

The designer draws only skids and blocks. If you want piers, a frost-depth footing or a slab, you plan it yourself with your building department, and the shed base guide covers the slab. One building department, Village of Lombard, Illinois, lists 8 in piers among the accepted foundations for sheds of 80 to 200 sq ft.

Permits and local rules on block foundations

Local rules differ a lot, so read yours before you buy blocks.

What three handouts say

The three building-department handouts we read do not agree, and none names concrete blocks by that name.

• Village of Lombard, Illinois. For sheds from 80 to 200 sq ft it lists floor kits on gravel, a 4 in slab and piers, and says other foundation types go to the Building Division for approval. Sheds not attached to a foundation must be tied down with earth anchors.
• SAFEbuilt, in a Minnesota handout. It says that sheds not placed on slabs get treated skids, that the shed is anchored against uplift and overturning, and that exterior footings sit at least 12 in below undisturbed ground.
• Coon Rapids, Minnesota. A structure over 200 sq ft needs a permanent concrete slab under it, and one of 200 sq ft or less must include ground anchors.

Elgin, Illinois, accepts a gravel base with code-approved tie-downs on sheds up to 150 sq ft. Prince William County, Virginia, exempts sheds up to 256 sq ft from a permanent foundation on five conditions, among them an eave height of 10 ft or less, a floor no more than 18 in above grade, ground-contact treated wood, and anchoring against wind.

What to ask before you build

Ask your building department three things: is a block foundation accepted for my size of shed, is a permit needed, and what tie-downs do you accept. Our shed permit basics explain how the thresholds work.

Where to go next

• Gravel shed base: the pad under skids or blocks, its size, depth and the stone to buy.
• Shed on skids: 4x6 skids on gravel, the other floor the designer draws.
• Shed base: every base type side by side, with concrete slabs and the base calculator.
• How to anchor a shed: tie-downs for blocks, skids and slabs.
• How to build a shed floor: joists, rim and subfloor once the beams are in.

Enter your own width and depth in the shed designer and the block count updates. The 10x12 gable plan has the full cut list, and the how to build a shed guide puts the foundation in order with the rest of the build.