Shed Bases

A shed base without the concrete

Flat, firm and free-draining in an afternoon. IBRAN grids give a shed base that keeps the floor dry and the building square, with no mixing, no formwork and no waiting for it to cure.

Why the base matters more than the shed

Sheds almost never fail because of the shed. They fail because of what is underneath.

Set a timber building straight onto soil, slabs bedded in sand, or a few blocks at the corners, and two things happen. The ground moves, so the frame racks, the door stops shutting and the roof lets water in. And moisture sits against the bearers, because nothing underneath is draining, so the floor rots from below while the walls are still perfect.

A concrete slab fixes the movement but introduces its own problem: it is impermeable, so water arriving on it has to go somewhere, and it usually ends up as a puddle round the edge that keeps the timber wet anyway. It is also a day's work, a mixer, a delivery of ballast, and a base that is there forever whether you want it or not.

A grid base solves both. Confined stone gives a firm, level platform that does not settle unevenly, and the whole thing stays fully permeable, so rain drains straight through and air circulates under the building rather than moisture collecting against it.

Where this works well

Garden sheds, from 6x4 upwards
Workshops and hobby buildings
Bike and mower stores
Log stores and bin stores
Plastic and metal sheds
Chicken coops and animal housing
Oil tank and equipment standings
Replacing a failed slab or paving base

At a glance

Sub-base	75-100mm compacted MOT for a typical garden shed. More for a workshop or heavy contents
Infill	6-14mm graded blend, angular and clean, filled level with the cell tops
Total dig	Around 110-140mm, depending on grid depth and sub-base
Time	An afternoon for a typical shed base, one person
Tools	Spade, rake, hand saw, and a compactor if you are hiring one
Drainage	Fully permeable. Water drains through rather than pooling round the building
Design life	25 years, subject to documented installation
Material	100% recycled polypropylene, made in the UK

Why it beats the alternatives

Against a concrete slab: no mixing, no formwork, no curing time, no muck away, and no permanent structure left behind if the shed goes. It is also considerably cheaper for a typical garden building once you count the ballast, cement and hire.

Against paving slabs: slabs bedded on sand settle individually, and once one drops the whole building follows it. A grid base spreads load across the full footprint, so the platform stays flat as one piece.

Against timber bearers on blocks: the blocks sink into soft ground at different rates, and the gaps between them collect leaves and damp. A continuous base does not.

Against doing nothing: this is the most common shed base in Britain and the reason so many shed floors are soft in the corners.

Which grid depth?

The number on a grid is a decision about how deep the confined stone layer is. For garden buildings the loads are modest and spread through bearers, so the shallow answer is usually right.

IBRAN®-X30 for most garden sheds. The shallowest dig, the least stone and the least spoil to get rid of, which matters when the only route to the back garden is through the house.
IBRAN®-X40 for larger workshops, heavy contents, or where the base also takes a wheelbarrow or mower route up to it.
IBRAN®-X50 where a vehicle will drive onto the base, such as a garage-style building or a ride-on mower store with regular access.

Specifying more depth than a shed needs is stone you have paid to bury and carry down the garden.

Choosing your stone

Use a 6-14mm graded blend, angular and clean. A shallower cell wants a finer top size to bed in completely, and a blend packs tighter than a single size because the smaller stone fills the gaps between the larger.

That density is what gives you a flat, stable platform. A fully bedded cell puts load through the stone and into the ground beneath; a part-filled cell puts it through the plastic instead, and under a bearer carrying a corner of the building that shows up as a dip.

Avoid rounded pea shingle, which rolls rather than interlocking, and anything carrying fines or dust, which binds the surface and stops it draining. The drainage is half of what you are buying.

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How it goes in

Mark out the base. Make it larger than the shed footprint by at least 100mm on every side, so rain running off the roof lands on stone rather than on soil next to the timber.
Check the ground falls away from the building position rather than towards it, and clear any turf, roots and soft soil down to firm ground.
Excavate to your total depth: sub-base plus grid depth. For a typical garden shed that is around 110-140mm.
Lay a woven geotextile across the dig, lapping joins by at least 300mm and taking it up the sides. This stops the sub-base mixing into the soil beneath and keeps the base performing.
Add the sub-base and compact it. 75-100mm of MOT is ample for a garden shed, compacted in one or two layers rather than tipped and levelled.
Blind with a thin layer of grit sand and screed it level. Check it with a straight edge and a spirit level in both directions: this is the layer that decides whether your shed door shuts.
Push-fit the grids together across the base, cutting the perimeter to line with a hand saw.
Fill the cells with a 6-14mm graded blend, level with the cell tops, and brush it in so no cell is left hollow.
Set the shed on bearers across the base and check it is square and level before fixing anything together.

Full step-by-step detail is in our installation guides.

Make the base bigger than the shed

This is the single most useful thing to get right, and it costs almost nothing at the time.

A shed roof throws water off its edges. If the base stops at the shed footprint, that water lands on soil, splashes soil back up against the boards, and keeps the bottom of the building permanently damp. Extend the base 100mm or more beyond the shed on every side and the runoff lands on free-draining stone instead, where it disappears.

The margin also gives you somewhere to stand while you build, somewhere for the door to swing over, and a clean strip to walk round the building for maintenance rather than treading a muddy path into the lawn.

If the shed has a door on the long side, take the base out further on that side still. It becomes the threshold you stand on every time you open it.

Keeping the floor dry

Rot in a shed floor almost always starts underneath, and it starts because moisture has nowhere to go.

Set the building on bearers rather than directly on the base, so there is an air gap under the floor. Run the bearers in the direction that lets air pass through, keep vegetation cut back from the sides, and resist the temptation to pile logs, bags or bin bags against the walls, which traps damp exactly where the timber is most vulnerable.

With a permeable base, water that gets under the shed drains away and air moves through. That combination is what keeps a timber floor sound for decades rather than years.

Sloping gardens

Few gardens are flat, and a shed base has to be.

For a gentle slope, dig into the high side rather than building up the low side. Cut-and-fill is fine for the last little bit, but material you have added is always less stable than ground you have not disturbed, so take more off the top than you put on the bottom.

On a steeper slope you will need a retained edge on the high side, and it is worth stepping back and checking whether the shed wants to be somewhere else. A level spot ten feet away is usually cheaper than a retaining structure.

Get it right first time

The one thing worth knowing: level and square, checked twice.

Almost every complaint about a shed comes back to the base not being flat. The door binds, the windows will not shut, the roof panels do not meet, and the building racks slowly out of shape.

Screed the blinding layer properly, check it with a straight edge in both directions and diagonally, and check the diagonals of your marked-out area match before you lay a single grid. Ten minutes with a spirit level and a tape saves a decade of a door that never quite closes, and unlike everything else on this list it cannot be fixed once the shed is up.

Ask our engineer

Questions about depths, base sizing, sloping ground or which system fits your building? Ask below, or get in touch directly.

Common Questions

Things that frequently come up when carrying out this project.

Is a grid base as good as a concrete shed base?

For a garden building, better in most respects. Confined stone gives a firm level platform that spreads load across the whole footprint, and unlike concrete it stays fully permeable so rain drains through rather than pooling round the building and keeping the timber wet.

There is no mixing, no formwork, no curing time and no permanent structure left behind if the shed goes.

How deep do I need to dig for a shed base?

Around 110mm to 140mm in total for a typical garden shed, made up of 75mm to 100mm of compacted MOT sub-base plus the grid depth. Larger workshops or heavy contents need more sub-base.

It is a shallow dig, which matters when the spoil has to be carried through the house to get out of the garden.

How big should a shed base be?

Larger than the shed footprint by at least 100mm on every side. A shed roof throws water off its edges, and if the base stops at the footprint that water lands on soil and splashes back against the boards. Extending the base means runoff lands on free-draining stone instead.

Take it out further still on the door side, since that becomes the threshold you stand on.

What size gravel should I use for a shed base?

A 6mm to 14mm graded blend, angular and clean. A blend packs tighter than a single size because the smaller stone fills the gaps between the larger, and that density is what gives a flat stable platform.

Avoid rounded pea shingle, which rolls rather than interlocking, and anything carrying fines, which binds the surface and stops it draining.

Which grid depth do I need for a shed?

The 30mm grid suits most garden sheds: the shallowest dig, the least stone and the least spoil. Use 40mm for larger workshops, heavy contents, or where a wheelbarrow or mower route runs up to the base.

Use 50mm only where a vehicle will drive onto it, such as a garage-style building.

Do I still need bearers under the shed?

Yes. Set the building on bearers rather than directly on the base so there is an air gap under the floor. Rot in a shed floor almost always starts underneath, and it starts because moisture has nowhere to go.

A permeable base drains water away and bearers let air move through, which is the combination that keeps a timber floor sound.

Can I build a shed base on a slope?

Yes. For a gentle slope, dig into the high side rather than building up the low side, since material you have added is always less stable than ground you have not disturbed.

On a steeper slope you will need a retained edge on the high side, and it is worth checking whether a level spot a few feet away would be cheaper than a retaining structure.

How long does a shed base take to lay?

An afternoon for a typical garden shed, working alone. The grids push together by hand with no tools or fixings, and the base is usable immediately: there is no curing time, so the shed can go up the same day.

Most of the work is the digging.

Do I need planning permission for a shed base?

Garden buildings are usually permitted development subject to size, height and position limits, and a permeable base does not add to surface water run-off in the way an impermeable slab does.

Rules differ across the UK and are tighter for listed properties and conservation areas, so check with your local planning authority if you are unsure.

Can I put a grid base over an existing slab or paving?

Yes, provided the existing surface is sound and water has somewhere to go. Fill any holes, check your finished levels, and use the shallowest grid to keep the build-up down.

If the slab is badly broken or holds water, it is usually better to lift it, since a base that cannot drain gives up the main advantage of the system.