Shed Base: Types, Thickness and How to Build

A shed base is the flat, firm layer your shed stands on: compacted gravel, a concrete slab, concrete deck blocks, paving slabs, a plastic grid or a timber frame. The right shed base keeps the floor level, keeps the wood off wet ground and lets water drain away from the building.

This guide covers sheds on both sides of the Atlantic. US builders mostly pour slabs or lay gravel pads with skids, and UK gardens are full of paving slab bases and plastic grid kits, so each section says where a method or a material is regional. The calculator below works in feet and inches or in metres. If you are building on gravel in the US, the gravel shed base guide goes deeper on pad size, stone and cubic yards.

Which shed base is best for your shed

Pick the base by the shed, the ground and the work you are willing to do. A light storage shed on flat, well-drained ground can sit on almost any of these. A heavy workshop, a log cabin or a shed on soft ground narrows the list.

Gravel pad

A pad of compacted crushed stone, with pressure-treated skids or beams on top, is the standard US base for a wood shed. Our shed designer builds its skid floor on this idea and sizes the pad 12 inches past the shed on every side, 4 inches thick. The gravel drains, the skids carry the load, and there is no concrete to pour.

Concrete slab

A slab is the most permanent base and the only one that doubles as the floor of a workshop or garden room. It is also the most work: excavation, formwork, a compacted sub-base, a damp-proof membrane in UK practice, and enough concrete to fill it in one go.

Concrete deck blocks

Precast deck blocks hold treated beams off the ground, and the floor frame sits on the beams. The designer’s block floor uses 4x4 pressure-treated beams, with beam lines no more than 48 inches apart and blocks close enough for each 4x4 to pass its span check, never more than 48 inches. The blocks need firm, level ground under each one.

Paving slabs

In the UK a paving slab base is the classic DIY option: slabs bedded on sand and cement over a compacted sub-base. Slabs of 600 x 600 mm and 450 x 450 mm are the sizes the calculator counts by default.

Plastic grid kits

A plastic shed base is a set of interlocking grid tiles, filled with gravel and laid on a membrane. Kits are sold by shed size, and some are sold as an eco shed base because rain drains straight through. The ground under the grid still has to be level and firm.

Timber frame

A timber shed base is a grid of pressure-treated bearers. It lifts the floor off the ground and is easy to level on packers, which makes it popular on uneven plots and as a frame over a plastic grid.

What is the cheapest shed base

The cheapest shed base is usually the one with the least material and the fewest tools. We do not print prices here, because they change by region and by week, but the shopping lists tell the story.

The short shopping lists

• Gravel pad: stone, landscape fabric, a rented plate compactor, treated skids.
• Plastic grid: the grid kit, a membrane, gravel to fill the cells, and hardcore under it if your kit asks for it.
• Deck blocks: blocks, treated beams, and a level.

The long shopping lists

• Paving slabs: a compacted sub-base of MOT Type 1 (the UK’s standard crushed-stone sub-base), sand and cement for the bed, and the slabs.
• Concrete slab: timber for the formwork, sub-base, a membrane, and concrete by the bag or by the truck.
• Timber frame: treated bearers, fixings, and something solid under them.

Cheap now or cheap later

A cheap base that sinks or holds water costs more than a proper one, because the fix means emptying and lifting the shed. Spend on the ground work under any base: stripping the turf, compacting the sub-base and getting it level.

How big should a shed base be

Match the base to the shed floor or make it slightly bigger. Makers do not agree on the exact margin, so check your shed’s instructions first.

Same size as the floor or slightly bigger

These are the rules we read in September 2026:

• Power Sheds advises a base “the exact same size as the shed floor”, and notes that some companies prefer one slightly smaller “so water cannot pool around the base or floor”.
• Shire Garden Buildings marks out a base with the same measurements as the floor, plus the thickness of the formwork timber.
• Heidelberg Materials UK adds 100 mm to the width and the length “to allow a small edge around the base”, and Waltons says at least 100 mm wider and longer.
• B&Q says “roughly the same size or slightly bigger”, and Tiger Sheds goes furthest, at 30 to 40 cm beyond the shed on each side.
• In the US, the Duramax manual for one resin shed asks for a base at least 4 inches larger than the shed, and our designer sizes a gravel pad 12 inches past the shed on every side.

Why the edge matters

Forest Garden explains that the lowest cladding board overlaps the floor on the outside, to keep rain out of the joint. On a slab that runs well past the walls, rain running off the cladding lands on the concrete and can sit against the bottom board. That is our reading of the “slightly smaller” advice, and why a small edge or none at all suits hard bases. A gravel pad drains through, so its wider margin does not hold water in the same way.

Level, not sloped

Dunster House, a UK garden building maker, asks for a base that is “level with no run-off”, because the building handles the water. Level matters for the amount of concrete too: Heidelberg notes that a 1 cm error over 25 m² adds a quarter of a cubic meter of concrete.

How the calculator works

The calculator multiplies the base area by the thickness of each layer and, for blocks, slabs, grids and bearers, counts pieces from the size you enter. It adds nothing for waste or compaction.

The numbers it starts from

• Concrete: 4 inches on 4 inches of gravel in feet, the Elgin, Illinois example below; 75 mm on 75 mm of MOT Type 1 in metres, from B&Q and Shire.
• Bags: 0.45 cubic feet for a 60 lb bag and 0.60 for an 80 lb bag, from the QUIKRETE Concrete Mix data sheet; 0.01 m³ for a 20 kg bag, from the Wickes listing of one ready-to-use concrete. Metric bag yields differ by product, so the field is editable.
• Weights: MOT Type 1 at about 2 tonnes per m³ (Mainland Aggregates) and sharp sand at about 1.6 tonnes per m³ (Smith & Sons, Bletchington).
• Deck blocks: the designer’s own layout, the same one its floor plans use.

A worked example in each system

A 10x12 ft slab 4 inches thick is 120 sq ft times 1/3 ft, or 40 cubic feet, which is 1.48 cubic yards. At 0.60 cubic feet a bag, that is 67 bags of 80 lb mix, or 89 bags of 60 lb mix at 0.45. A 3 x 2 m slab 75 mm thick is 0.45 m³, or 45 bags at 0.01 m³ a bag, on 0.45 m³ of MOT Type 1 that weighs about 0.9 tonnes.

Ordering

Suppliers expect you to order a little over the finished volume. Cemex UK puts it as “a small contingency” and gives no percentage, while Tiger Sheds suggests rounding up by 10 to 15%. Give your supplier the size and depth and let them advise.

How thick should a concrete shed base be

For an ordinary storage shed, about 3 to 4 inches of concrete on a compacted sub-base of about the same depth. Heavier buildings need more. The exact numbers depend on where you are and who you ask.

In the US

There is no single national number for a shed slab. The City of Elgin, Illinois, in its shed quick guide, accepts “a concrete slab at least 4 inches thick” on a minimum 4 inch compacted gravel base, and the Duramax manual mentioned above asks for a slab of 4 to 5 inches. Bagged concrete mix is sold in 60 and 80 lb bags; QUIKRETE’s data sheet says its Concrete Mix is for pours 2 inches thick or more and lists floor slabs and patios among its uses. For a large slab, a ready-mix truck delivers by the cubic yard.

In the UK

B&Q tells you to dig 150 mm, for “75mm of concrete and a 75mm sub base” of MOT Type 1. Shire recommends the same 75 mm of concrete on 75 mm of compacted hardcore (broken stone or rubble used as a sub-base). Heidelberg Materials UK says at least 75 mm of concrete on 75 to 100 mm of compacted hardcore. Tiger Sheds asks for 100 to 150 mm under log cabins and garden rooms. Bagged ready-to-use concrete comes in 20 and 25 kg bags, and ready-mixed concrete is ordered in m³.

The mix

Heidelberg says 1 part cement to 5 parts ballast (sand and gravel mixed) “is usually perfectly adequate for shed bases”. B&Q gives a richer mix: 1 part cement, 2 parts sand and 3 parts aggregate, or 1 part cement to 4 parts ballast. If you order ready-mixed concrete, tell the supplier it is an external shed base and let them name the mix: Tarmac lists shed bases among the uses of its general-purpose mixes but says those should only be used outside where the concrete will be completely covered.

The damp-proof membrane

UK guides lay a damp-proof membrane (DPM), a heavy plastic sheet, over the sub-base before the concrete goes in. Waltons turns it up at the edges “to create a tray-like effect”, and Heidelberg overlaps and tapes any joints. B&Q says the membrane helps prevent condensation inside the shed and stops the concrete drying too quickly.

Can a shed sit on gravel, slabs, soil or grass

Gravel and slabs, yes, with the right preparation. Bare soil and grass are where most bases fail.

On gravel

Yes, as a compacted pad of crushed stone with the floor frame on skids or beams, not with the floor itself on the stones. That is the US standard, and the gravel shed base guide covers the stone, the depth and the build. In the UK, plastic grid kits filled with gravel are the common version: one seller, Garden Buildings Direct, specifies gravel at least 10 mm across for its grid.

On paving slabs

Yes, when the slabs are laid as a base, on a compacted sub-base and a sand and cement bed. Loose slabs set on turf move and settle unevenly, and the shed follows them.

On soil or grass

Treat bare soil and grass as the starting point, not the base. The makers disagree here: ProBASE says its grids can go directly on grass or soil over a membrane, while The Garden Range, which also sells plastic bases, recommends not installing directly onto grass or soil and asks for 75 to 100 mm of levelled hardcore first. Prince William County, Virginia, lets some sheds skip a permanent foundation only when the supports sit level on firm soil and any wood touching the ground is treated for ground contact. Strip the turf, build on firm subsoil, and when in doubt, put compacted stone under the base.

Plastic shed base grids: pros and cons

A plastic grid shed base is a quick, light base for a garden shed, and its quality depends on what is under it.

Sizes and counts

Grid sizes vary by brand. ProBASE grids measure about 500 x 500 mm, four to a square meter, and one seller gives the EcoBase Fastfit panels as roughly 500 x 500 mm as well. Kits are sold by shed size, so the calculator’s grid count is for checking a kit or topping one up: type the tile size from your pack.

What goes under them

At least a weed membrane, and on most ground a layer of levelled hardcore. Tiger Sheds says to allow 8 to 10 kg of gravel per EcoBase panel to fill the cells, and to check your kit for the exact figure.

Pros

• Light to carry, no mixing, and a small base can go down in a day.
• Rain drains through the gravel-filled cells instead of pooling.
• Easy to lift and reuse if you move the shed.

Cons

• The grid spreads the load but does not fix bad ground: it needs a level, firm bed.
• Brands disagree on whether soil or grass is acceptable underneath, which leaves you reading the small print.
• For heavy buildings, a slab is the more certain choice.

Timber frame shed base

A timber base is a set of treated bearers laid across the shed’s floor joists, on level supports.

Bearer sizes and spacing

The sizes we found range from 70 x 47 mm (Forest Garden’s ready-made base) through 75 x 50 mm (Waltons) to 75 x 75 mm (Shire). Waltons spaces bearers 400 to 600 mm apart, running perpendicular to the floor joists, and 16 to 24 inches apart in its wooden base guide. Shire sets them at 90 degrees to the joists and a maximum of 600 mm apart. The calculator evens out the spacing across the width so no gap is wider than the one you enter.

Treatment for ground contact

Every bearer needs preservative treatment rated for ground contact. In the UK that is Use Class 4: the Wood Protection Association’s “Make sure it’s 4” campaign covers wood in contact with the ground or frequently wet, and Forest Garden’s base is sold as “UC4 Pressure Treated for ground contact”. In the US the equivalent is AWPA Use Category UC4A, ground contact.

Skids and beams, the US version

The US equivalent is the skid floor: 4x6 treated skids on edge, running front to back under the floor frame. The designer uses one skid line for about every 4 feet of width, so an 8 ft wide shed gets three. The shed on skids guide builds the floor on top of them.

How to lay a paving slab shed base

This is the UK method, from the guides of Marshalls (for patios), Shire and Power Sheds. The calculator gives the slab count and the sub-base.

What the layers look like

Marshalls lays a compacted sub-base of 100 mm and a 40 mm bed of three parts sand to one part cement. Shire uses a dry mix of 1 part cement to 8 parts building sand, about 40 mm deep. Power Sheds digs about 100 mm in all: 50 mm of aggregate, 20 mm of dry mix and 30 mm of slab.

Step by step

1. Mark out the base with pegs and string, the size of the shed floor or your chosen margin. Measure both diagonals and adjust until they match.
2. Strip the turf and topsoil down to firm ground, deep enough for the sub-base, the bed and the slabs.
3. Spread and compact the sub-base of MOT Type 1 or hardcore, in layers, with a plate compactor.
4. Mix the bed of sand and cement and rake it level to the depth your guide gives.
5. Lay the slabs from one corner, tapping each down and checking level in both directions as you go.
6. Check the whole base with a long level, and fill the joints if your slabs call for it.

How to lay a concrete shed base

This is the method in the Heidelberg, Waltons and B&Q guides, with the US numbers alongside.

Before you dig

In the US, call 811 a few business days before you break ground: it is the free national before-you-dig service that gets buried lines marked (Common Ground Alliance). In the UK, check for buried services before digging too. Work out the volume with the calculator and decide between bags and a delivery before you build the formwork.

Step by step

1. Mark out the base with pegs and string, square it by the diagonals, and remove the turf and topsoil.
2. Dig to depth: slab plus sub-base, for example 150 mm for B&Q’s 75 mm plus 75 mm, or 8 inches for 4 inches on 4 inches.
3. Build level formwork. Heidelberg uses 25 mm timber planks and stresses that the formwork must be level, because the slab will follow it.
4. Lay and compact the sub-base of gravel or MOT Type 1.
5. Lay the membrane (UK practice): Heidelberg spreads a layer of building sand over the hardcore to protect the membrane, then turns the membrane up against the formwork.
6. Pour, spread and strike off the concrete level with the top of the formwork, working in one go.
7. Let it cure. Heidelberg says the slab can be walked on within 48 hours but ideally keeps the formwork in place for at least five days before building the shed. Waltons notes that concrete takes around 28 days to reach full strength.

Common shed base mistakes

Most base problems show up a season later as a sticking door, a damp floor or a shed that rocks.

Building on turf or topsoil

Grass and topsoil compress and hold water. Every guide above starts by removing them down to firm ground.

A base that is not level

A slope that looks harmless adds concrete, racks the shed and jams the door. Check level across both directions and both diagonals before the concrete sets or the shed goes on.

The wrong size

A slab much wider than the shed can hold rain against the bottom boards, and one smaller than the floor leaves the edges unsupported. Follow your shed maker’s rule.

Untreated wood on the ground

Bearers and skids that touch the ground rot unless they are treated for ground contact: UC4 in the UK, UC4A in the US.

Skipping the sub-base

Concrete or slabs laid straight on soil crack and sink. The compacted layer of stone is what spreads the load.

Permits and planning permission

Base rules depend on the country and often on the town. These are the rules we read in September 2026; confirm them where you live.

In the US

The 2021 International Residential Code, section R105.2, exempts one-story detached accessory structures up to 200 sq ft from a building permit. That is a model code: states and towns adopt it with changes, and the same section says the exemption does not authorize work that breaks the code or other local laws. Some towns set a lower threshold: Elgin, Illinois, defines a shed as an accessory structure of no more than 150 sq ft. Zoning rules such as setbacks can apply at any size, so call your local building department and read our shed permit basics.

In England

The Planning Portal says outbuildings are permitted development, with no planning permission needed, within limits. Among them: nothing forward of the wall forming the principal elevation, single storey, eaves no higher than 2.5 meters, overall height up to 4 meters with a dual-pitched roof or 3 meters with any other, and no more than 2.5 meters high within 2 meters of a boundary. Any outbuilding within the grounds of a listed building needs planning permission, and designated land such as national parks and conservation areas has extra limits. The base itself is a hard surface, which the government’s technical guidance places under Class F of the permitted development rules; the condition about porous surfaces only applies to more than 5 m² between the front of the house and a road. For building regulations, the Planning Portal says they will not normally apply to an outbuilding under 15 m² with no sleeping accommodation.

In Wales, Scotland and Northern Ireland

The Welsh Government also treats outbuildings as permitted development within limits, including a maximum height of 2.5 meters within 2 meters of a boundary, and uses the same 15 m² building regulations rule. In Scotland, mygov.scot says most sheds do not need planning permission, as long as they meet conditions such as a maximum height of 4 meters. In Northern Ireland, check with your council before you build.

Where to go next

With the base chosen and the quantities worked out, the next job is the floor. The shed foundation guide compares skids, blocks and concrete for US sheds, the anchoring guide covers holding the shed down on each kind of base, and the shed designer lays out the floor frame, walls and roof for your size.