How to Build a Shed Base – DIY Guide
To build a shed base, excavate the area to 150mm depth, compact a 100mm hardcore layer, then lay a 75–100mm concrete slab or paving slabs on a sand and cement bed. Keep it level throughout. A properly built shed base prevents rot, movement, and a door that won’t shut.
Lay and Compact the Sub-Base
Tip a 100mm layer of hardcore or compacted gravel into the excavated area and spread it evenly with a spade. Use a plate compactor — or a heavy hand tamper on smaller areas — to compact the material firmly in passes, working from the edges inward. Check for level regularly with a spirit level across a straight length of timber. A well-compacted sub-base stops the base from sinking or shifting over time, which is the single most common cause of shed doors jamming and floor frames rotting.
Build and Peg Timber Shuttering
Cut timber shuttering boards to form a frame around the perimeter of your base. Drive timber pegs into the ground on the outside of the shuttering every 600mm to hold the boards upright and in position. Use a spirit level and builders square to confirm the frame is perfectly level on all four sides and square at the corners — a 3-4-5 triangle check works well here. The shuttering contains the concrete or screed and gives you clean, straight edges on the finished slab.
Lay a Damp-Proof Membrane
Roll a damp-proof membrane over the compacted sub-base, lapping it up the inside faces of the shuttering by at least 50mm. Overlap any joins by a minimum of 300mm and tape them securely. This layer is non-negotiable — without it, ground moisture will wick up through the base, rot your shed floor, and void most shed manufacturer warranties. If you are laying paving slabs rather than pouring concrete, a weed-suppressing membrane under a sand bed will also help manage drainage.
Pour and Level the Concrete Slab
Mix concrete to a 1:2:3 ratio (1 part cement, 2 parts sharp sand, 3 parts aggregate) or use a ready-mix bagged product for smaller bases. Pour the mix into the shuttered area, working from one end to the other. Use a screeding rail across the top of the shuttering to drag the surface flat and level. Tamp down any air pockets as you go. For slabbed bases, bed each slab on a full mortar bed (5 parts sharp sand, 1 part cement) and check level after every slab with a spirit level — just as you would when laying paving slabs for a patio.
Allow to Cure and Check Level Before Installing the Shed
Leave a concrete slab to cure for a minimum of 48 hours in dry conditions before placing any load on it — 72 hours is better. Mortar-bedded slabs need at least 24 hours per slab before loading. Once cured, remove the timber shuttering and check the overall level one final time with a long spirit level diagonally across the base. Any low spots on a concrete slab can be built up with a sand and cement levelling screed at this stage. Only once the base is fully set, level, and dry should you position and fix your shed.
Frequently Asked Questions
Do I need planning permission to build a shed base in my garden?
In most cases, no — a standard garden shed is considered permitted development in England and Wales, provided it meets size and height restrictions and is not in a designated area. Check the current rules on the Planning Portal before starting, particularly if your property is listed or in a conservation area.
What is the best shed base — concrete slab or paving slabs?
A poured concrete slab is the most durable option for larger sheds or workshops, as it spreads load evenly and is very stable. Paving slabs on a sand and cement bed are quicker, easier to level individually, and more forgiving for a first-timer — either works well provided the sub-base is properly compacted.
Can I build a shed base on sloping ground?
Yes, but it requires more preparation. You will need to excavate the uphill side deeper and build up the downhill side using compacted hardcore or a retaining edge, so the finished surface is level. For significant slopes, concrete shuttering needs to be stepped or the base raised with concrete blocks. Our guide to laying a patio covers managing slope in more depth.
How thick should a concrete shed base be?
For a standard garden shed, a finished slab thickness of 75–100mm is sufficient. For a larger workshop or if you plan to store heavy equipment or vehicles, increase this to 100–150mm and consider adding steel reinforcing mesh to the slab before pouring.
How long should I leave a concrete shed base before putting the shed on it?
Allow a minimum of 48–72 hours of dry curing time before placing any load on the slab. Concrete reaches its working strength within 24–48 hours but continues to cure and harden over 28 days — avoid heavy point loads on the base for the first week if possible.
Before pouring, coat the inside faces of your timber shuttering with a thin layer of old engine oil or a release agent — the boards will strip cleanly without pulling chunks from the slab edge. It also means you can reuse the shuttering timber rather than breaking it off and wasting it.
Sources
- Planning Portal — Sheds and Outbuildings: permitted development rules — planningportal.co.uk
- HSE — Safe use of cement and concrete: health risks and precautions — hse.gov.uk
- Which? — How to build a shed base — which.co.uk
This guide is for general information only. Always work safely and follow manufacturer instructions. DIYnut accepts no liability for injury or damage arising from DIY work.
