Domakihub How to Lay a Shed Base: A Solid Foundation Every Time
Choose the right method, prepare the ground properly, and your shed will stand level for decades.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
A shed without a proper base will warp, rot, and become unusable within a few years — a problem that costs far more to fix than to prevent. The video above walks through the practical process visually; this article breaks down the exact steps, material choices, and decisions you need to make before you begin. Whether you are building storage at a home in Athens, a utility room behind a holiday villa in Crete, or a tool shed at a hotel property, the principles are identical.
Why the Base Matters More Than the Shed Itself
Garden sheds and storage buildings are commonly sold as the main item, but the base is what determines their lifespan. A level, well-drained base prevents timber floor joists from sitting in moisture, stops walls from racking out of square, and keeps doors and windows opening correctly. Skipping or rushing this stage is the single most common reason sheds need early replacement.
In Greece the Mediterranean climate adds a specific concern: summer heat and winter rain alternate rapidly, expanding and contracting ground that is not properly compacted. Loose or soft ground shifts seasonally, which cracks concrete slabs and tilts timber frames. Adding a compacted hardcore sub-base is therefore not optional — it is essential.
Which Base Type Is Right for Your Situation
There are three main options. A concrete slab is the most permanent and suits heavy sheds, metal buildings, or any structure that will be used as a workshop. A compacted gravel or hardcore base topped with paving slabs works well for medium-weight timber sheds and drains better than solid concrete. Plastic interlocking grid panels filled with gravel are the fastest to install and ideal where drainage across an entire garden is a concern — they are popular for holiday rental properties where disruption must be minimised.
For very large structures (over approximately 15 square metres), for buildings that include electrical installations, or for any shed that will be attached to a house wall, consult a licensed structural engineer before you begin. In Greece, ancillary garden buildings may require a building notification or small-works permit depending on municipal rules — check with your local planning office (Πολεοδομία) first.
How to Lay a Concrete Shed Base: Step by Step
- 1
Mark out and excavate the area
Measure the footprint of your shed and add 50 mm on each side so the base extends slightly beyond the wall line. Mark the rectangle with string lines and pegs, keeping corners square by checking the diagonal measurements are equal. Excavate to a depth of 150 mm to allow for 100 mm of compacted hardcore sub-base plus a 100 mm concrete slab, for a total finished depth of 200 mm below the original ground level. Remove all topsoil, roots, and organic material — none of these should remain under a slab.
- 2
Lay and compact the hardcore sub-base
Fill the excavated area with MOT Type 1 hardcore (crushed stone or similar compactable aggregate) to a depth of 100 mm. Spread it evenly with a rake and compact it thoroughly using a vibrating plate compactor, which can be hired from any tool hire depot. Work in passes across the full area until the surface does not shift or sink underfoot. This layer is what distributes the load of the slab and the shed into stable ground — do not skip or thin it out.
- 3
Build the timber formwork and lay the membrane
Construct a rectangular frame from 150 mm x 25 mm treated timber boards, fixed at the corners with stakes driven into the ground. Use a spirit level across the top edges of the frame to confirm it is level in all directions — this is the most important accuracy check in the entire job. Once the frame is level and square, lay a heavy-duty damp-proof membrane (thick polythene sheet) over the sub-base, turning the edges up inside the formwork to create a tray. Overlap any joints by at least 300 mm and tape them. The membrane keeps ground moisture from wicking up through the concrete.
- 4
Mix and pour the concrete
For shed bases up to about 10 square metres you can mix concrete on site using a hired mixer: the standard mix is one part cement, two parts sharp sand, and three parts aggregate (known as a C20 or 1:2:3 mix). For larger areas, ordering ready-mix concrete saves time and ensures a consistent mix. Pour the concrete into the formwork steadily, filling to the top of the timber frame. Work the concrete into corners and edges with a shovel, and eliminate air pockets by tamping vertically with a length of timber. Two people make this stage much easier — one filling, one spreading.
- 5
Screed, finish, and cure
Draw a straight-edged piece of timber across the top of the formwork in a sawing motion to level the surface flush with the frame — this is called screeding. Fill any low spots and repeat until the surface is even. For a smooth finish, work a steel float over the surface in circular strokes once the concrete begins to stiffen slightly. Cover the slab with polythene sheeting or damp hessian to slow the curing process and prevent cracking from rapid drying — this is especially important in summer heat. Leave the slab to cure for a minimum of 3 days before removing the formwork and at least 7 days before placing the shed on it; 28 days gives full strength.
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FAQ
- How thick should a concrete shed base be?
- For a standard garden shed, a 100 mm (10 cm) concrete slab over a 100 mm compacted hardcore sub-base is the widely accepted minimum. If the shed will house heavy machinery, a ride-on mower, or act as a workshop, increase the slab to 150 mm. The sub-base thickness should remain at least 100 mm regardless of slab depth.
- Can I lay a shed base directly on existing concrete or paving?
- Yes, if the existing surface is sound, level, and shows no signs of cracking or subsidence. Check it with a spirit level across multiple lines — a tolerance of no more than 5 mm over 2 metres is acceptable. If the old surface is cracked, crumbling, or uneven, it is better to break it up and start from a compacted sub-base than to build on an unstable foundation.
- Do I need a building permit to lay a shed base in Greece?
- It depends on the size of the structure and your municipality. Small ancillary buildings (typically under 15 sq m) may qualify as exempt works under Greek planning law, but rules vary by zone and island. Always check with your local planning authority (Πολεοδομία) before breaking ground. If the shed will have electrical supply or connect to drainage, a licensed electrician or plumber must sign off their respective installations regardless of planning status.