Every shed, summer house and log cabin needs a base that is flat, firm, level and square. Get that right and the doors hang true, the roof sheds water and the timber stays dry for years. Get it wrong and no amount of adjusting will fix a door that binds or a floor that rocks. The four common bases are a concrete slab, paving slabs, a pressure treated timber frame and a plastic grid filled with gravel.
Bare grass, soil or loose gravel is not a base: it moves under the weight and holds water against the floor.
Which base for which building?
| Building | Floor supplied? | Good bases |
|---|---|---|
| Garden shed | Yes, OSB or tongue and groove | Paving slabs, concrete, timber frame or plastic grid |
| Summer house | Yes | Paving slabs, concrete or a timber frame |
| Log cabin | Yes on our 19mm cabins, on pressure treated bearers | Concrete or paving slabs. Cabins are heavy, so the base must not settle. |
| Timber garage | No: the walls sit on a base rail | A concrete slab, which also becomes the floor |
| Storage boxes, bin and log stores | Varies | Paving slabs or any existing hard, level surface |
Check the "Floor" row in the specification table on each product page before you plan the base. If a building has no floor, the base is the floor.
The four types of base compared
| Base | Best for | For | Against |
|---|---|---|---|
| Concrete slab | Log cabins, garages, workshops, anything large or heavy | The strongest and longest lasting. Stays level. | The most work and cost. Permanent. |
| Paving slabs | Sheds and summer houses of most sizes | A good DIY job. Can be lifted and reused. | Needs a properly compacted bed or slabs will sink and rock. |
| Timber frame on bearers | Small to medium sheds and summer houses, sloping or soft ground | Quick. Lifts the floor clear of the ground so air can circulate. | Must be pressure treated and well supported, or it will sag. |
| Plastic grid with gravel | Small to medium sheds | Light to carry, drains freely, no concrete. | Only as level and firm as the ground you prepare beneath it. |
What size should the base be?
Product pages give two sizes. The external size is the footprint of the walls on the ground. The overall size includes the roof overhang. Build the base to the external size, or a little larger: up to about 50mm all round is plenty.
A base much bigger than the building catches rain, which then splashes back and sits against the bottom boards. If you want a path or a step in front of the door, lay it slightly lower than the base so water runs away.
Worked example. The Shire Haddon 7x5 summerhouse is 2,048 x 1,550mm on the ground and 2,166 x 1,834mm including the roof overhang. The base should be about 2,050 x 1,550mm, and you need a clear space of roughly 3.1 x 2.7m to allow for the overhang and room to work.
Choosing the spot
- Leave about 450mm clear all round. You need it to fit the roof and felt, and to treat the timber each year.
- Keep away from overhanging trees. Dripping branches and leaf litter keep a roof damp, and roots lift bases.
- Think about height. Within 2m of a boundary an outbuilding can be no more than 2.5m high in England, measured from the ground, so a raised base counts. See planning permission for sheds, summer houses and log cabins.
- Check the door swing and the route in. A mower or bike needs a level approach, and double doors need room to open fully.
- Avoid the lowest corner of the garden. Water collects there. If you have no choice, raise the base slightly and add drainage.
How to lay a paving slab base
This is the base most people choose for a shed or summer house. In outline:
- Mark out the base with pegs and string. Measure both diagonals: when they are equal, the corners are square.
- Dig out turf and topsoil to firm ground, deep enough for the sub-base, the bedding layer and the slabs, so the finished surface sits just above the surrounding ground.
- Lay and compact a sub-base of hardcore or MOT Type 1. This is what stops the slabs sinking.
- Bed the slabs on a layer of sharp sand and cement, tapping each one down with a rubber mallet.
- Check level constantly with a long spirit level, along and across the base and corner to corner.
- Leave it to set before building on it.
How to lay a concrete base
For a log cabin or garage, concrete is the safe choice. The usual method is a timber frame (shuttering) set dead level around the excavation, a compacted hardcore sub-base, a damp proof membrane, then concrete poured and tamped level with the top of the shuttering. A slab for a shed or cabin is typically around 100mm thick. A garage that will take a car needs a thicker, properly specified slab, so ask a builder or groundworker.
Let concrete cure before you build: a few days before light foot traffic, and longer before loading it with a heavy cabin. Finish the surface just above ground level so rain drains off.
Timber frame and plastic grid bases
A timber frame base is a grid of pressure treated bearers, levelled on paving slabs, concrete pads or adjustable spikes. Shire lists pressure treated bearers and a frame base with levelling spikes as suitable for its sheds. It is the easiest way to deal with a gentle slope, because you level the frame, not the ground. Support the bearers at close, regular intervals so the floor can't sag between them.
A plastic grid base is a set of interlocking panels laid on levelled, compacted ground over a weed membrane and filled with gravel. It drains well and suits small and medium sheds. The preparation underneath still matters: level and compact the ground first.
Six mistakes to avoid
- Building on grass or soil. It will move, and the floor will rot.
- "Near enough" level. A few millimetres out across the base becomes a door that won't close.
- Out of square. Walls and roof panels won't line up. Check the diagonals.
- An oversized base. Water pools around the walls.
- No air gap. Timber sitting in damp with no airflow rots from below.
- Forgetting the treatment. Dip treated buildings need a preservative coat straight after assembly and every year. Pressure treated ones need less attention, but still benefit from a water-repellent finish.
Storing firewood? A log store needs the same thing in miniature: a hard, level, free-draining surface, with the logs held off the ground. Our guide to the best wood to burn explains why dry logs matter. And if you are planning a garden sauna, the base advice in our garden sauna buying guide is the same.
Common questions
What is the best base for a shed?
For most garden sheds, paving slabs on a compacted sub-base give the best balance of cost, effort and durability. Concrete is the strongest and suits large or heavy buildings. A pressure treated timber frame is quickest and copes with a slope.
Can I put a shed straight on grass?
No. Grass and soil move and hold moisture, so the shed will settle unevenly and the floor will rot. It needs a firm, level base such as slabs, concrete or a supported timber frame.
Should a shed base be bigger than the shed?
Make it the same as the shed's external size, or up to about 50mm larger all round. A base much larger than the building lets rain pool and splash against the bottom boards.
How thick should a concrete shed base be?
Around 100mm of concrete over a compacted hardcore sub-base is typical for a shed, summer house or log cabin. A garage for a car needs a thicker slab specified by a builder.
Does a log cabin need a concrete base?
Concrete is the best choice because a log cabin is heavy and its interlocking walls need a base that will not settle. Well-laid paving slabs on a compacted sub-base also work for smaller cabins.
Do I need a base for a summer house with a floor?
Yes. The floor is not a foundation. It sits on bearers that need continuous, level support from a proper base.
Choose the building, then build the base
Every product page lists the external size, overall size and floor type. If you are unsure which base suits, send us the model and a photo of the spot.
