How to prepare and lay a concrete slab for a garden office?

The base for your garden office has to be stable and level. It’s not worth cutting corners.

Before you start marking out the base, you need to equip yourself with a ball of string, wooden pegs, a hammer or mallet, a spade, a wheelbarrow, a tamper, a broom, some damp-proof membrane and some polythene or hessian sacking.

Preparing the base

Step 1 – Mark and measure the area

Using pegs and string, mark out the area for the base of the building. Measure lengths between opposite corners to ensure the angles are equal and all the sides of the base are completely straight.

• It is best to keep the base the same size as the footprint of the building.

Step 2 – Dig

Dig out the ground in the marked area to 200mm (around 8″) deep for a small shed. You can then remove the pegs and string line.

• Try to keep the depth consistent all the way across – it will help you achieve a level base.

Step 3 – Lay MOT Type 1 Stone

Shovel a minimum thickness layer of 100mm (4″) compacted MOT type 1 stone (crushed stone 40mm to dust) into the dug-out area.
Compact the stone using a powered wacker plate to form a hard base over which you can spread the concrete layer.

• Compacting the stone will ensure a firm base for the concrete and prevent it from cracking or subsiding later on.

Step 4 – Shuttering

Measure, cut and fit 25mm (1”) timber battons to the base and hold in place with wooden pegs, which sit below the top of the battons. Check the framework is level. The framework has two purposes: to create a strong and stable edge to the concrete, and to ensure it is level

• If the concrete is filled to the top edge of level framework, the concrete will also be level).

Step 5 – Cover with sand and damp-proof membrane

Cover the stone layer with a thin layer of sand (concreting or sharp sand). After this, cover with a layer of 1000g polythene damp-proof membrane to prevent moisture rising through the concrete and into the building’s floor bearers.

• The sand fills any voids which could cause sinking and is used as protection so the stones do not pierce the membrane. The membrane also makes the concrete stronger by allowing it to cure without the moisture being sucked away.

Laying the concrete

Concrete is caustic and can cause burns; wearing the correct personal protective equipment (PPE) is very important.

Keep your arms covered and do not go into the concrete unless you are wearing waterproof footwear such as wellington boots, if the concrete goes over the top then remove them immediately and wash your skin and your boots. We also recommend gloves and safety glasses.

Step 1 – Spread concrete evenly

Spread the concrete evenly and slightly proud of the timber rails. This can then be levelled off with a straight edge timber resting on the rails, using a sawing motion. Repeat tamping from one end to the other.

• Once the surface has been smoothed over, use a stiff broom to lightly brush across the base. This will leave a textured, non-slip surface.

Step 3 – Cover base

If wet weather is forecast, cover the finished base with polythene or a tarpaulin for 24 hours. In hot weather, as soon as the concrete has cured, cover with sacking and keep it damp for a day to prevent the concrete drying too quickly, which could result in shrinkage and cracking.

• Hessian sacking is also very good at preventing frost damage.

Step 4 – Leave to cure

Leave a minimum of three days to cure. This allows the concrete to harden to enable you to walk on it.

When the concrete is 2 days old remove the timber rails – they should lift out fairly easily as only one edge is in contact with the concrete.

• It takes 28 days for concrete to reach its full strength however most of the strength gain occurs in the first 7 days.

Step 5 – Fill in space

If your base is flush with the ground, you will be left with a space running around the edge of the concrete now that the rails are gone. The best way to fill this in is with pipe bedding or gravel. This will help to drain away moisture from the concrete base.

Success – Now all you have to do is sit back and wait for your shed, summerhouse, workshop or log cabin to be delivered.

We deliver our concrete to the following towns and their surrounding areas: Bedale, Boroughbridge, Catterick, Easingwold, Harrogate, Helmsley, Knaresborough, Leeming, Leyburn, Masham, Northallerton, Richmond, Ripon, Pateley Bridge, Tadcaster, Thirsk, Wetherby and York.