Preparing A Base

Whatever you are looking to build, getting the foundations right is fundamental to getting the whole building right, this is just a true for a garden building as it is for a house. Whatever type of base you choose to use, the most important point is that it needs to be solid and level. If the base isn’t solid and level, you will find that pressure builds up at certain points and damages the building over time.

Below you will find instructions on how to prepare a concrete or paving slab base. We would recommend a concrete base is prepared for all larger buildings. Some of our sheds offer alternative base options as additional extras such as timber building bases, if you choose to order one of these you will get full instructions with the selected product on how to use it.

Ideally, the site you choose for your shed should be as level as possible to minimise the amount of levelling you will need to do later, your base should also have at least 450mm (18”) of space around all sides for use during assembly and for ongoing maintenance of the building.


Stage 1

Choose the best area for the site of the building, which should be as level as possible to reduce levelling work later. It should give access all round (a minimum 18” or 450mm) for use during the fitting process. Pegs and string should be used to mark out the base area which should be the same measurements as the floor area of the building (including the veranda if applicable) plus the thickness of the timber used in the formwork for the concrete. To make sure the area is square measure the diagonals.

Stage 2

If the base is to go on soft ground or lawn it needs to be dug out following the string lines. To make the base level with the ground surface the hole needs to be 6” or 150mm deep. This is because 3” or 75mm depth of concrete is recommended and this should lay on a 3” or 75mm bed of compacted hardcore.

If you prefer to have a step up to the building with the concrete slab standing proud of the ground the hole needs to be excavated to a depth of 3” or 75mm for the compacted hardcore to come level with the surface. The formwork then needs to be set on top of this to give the required 3” or 75mm depth of concrete. You may wish to do this if the area is prone to forming deep puddles in heavy rain or if you feel rain will splash dirt from the surface on to the building.

Even the soil at the bottom of the hole with a spade and rake. You may now remove the pegs and string.

Stage 3

Make the formwork by measuring, cutting and fitting timber of the right width to provide the depth of concrete you wish (min 150mm) and check the diagonals to ensure the frame is square. Place the formwork in the hole. The top of the formwork must be level to ensure the finished surface of the concrete is level.

Spread a layer of hardcore to the 3” or 75mm depth and ensure well compacted. Even off the surface by covering with a layer of sand.

Stage 4

Mix the concrete by either 1 part cement powder to 5 parts ballast or you can use bags of dry mix concrete to which you just add water. The concrete should be on the dry side and not too sloppy so mix by adding a small amount of water at a time.

The concrete should then be spread over the sand/hardcore evenly until it is slightly above the top of the formwork. Then using a straight edged piece of timber level the concrete to the top of the formwork. This should be done by laying the edge of the timber right across the formwork and then dragging it across the surface of the wet concrete. The best results are obtained by using a sawing motion with the piece of timber as it is dragged across.

Preferably prepare the base when dry conditions are expected but if wet conditions are expected cover the concrete with polythene for 24 hours. On hot days to prevent cracking by rapid shrinkage cover the concrete in base with damp sacks and sprinkle them with water occasionally for 24 hours but do not saturate them.

Allow a minimum of 48 hours to dry completely and then your new base is ready for your garden building to be installed.

Paving Slabs

Stage 1

Site the base in a suitable area as in Stage 1 of the concrete base above. Mark out the base area with pegs and string, which should be the same as the base area of the building including the veranda if applicable. Measure the diagonals to check the corners are square.

Stage 2

For standard slabs dig out the topsoil from the area to a depth of 2.5” or 70mm and ensure the resulting surface is level. The pegs and string may now be removed.

Stage 3

Using 1 part cement to 8 parts building sand to form a dry sand/cement mix. Spread this evenly to a depth of 1.5” or 40mm approx. and use a rake to level it off.

Stage 4

Lay the first slab in 1 corner and tap it down in the centre using a rubber mallet. With a spirit level check it is level. If not tap the relevant side or corner and recheck until correct. Lay the other slabs outward from this first, levelling in the same way as the first slab with the mallet and check the slabs are also level with each other. For best results lay the slabs 1-3mm apart but keep the spaces even. An aid to this would be to put spacers of the correct thickness between the slabs till they are in position and then remove them. To keep the edges of the slabs in line you could use the pegs and string to give you a straight edge to work to.

Stage 5

Check the finished base is level with a long straight edge from corner to corner and measure across the diagonals to check it is square. Put some of the dry sand/cement mix on to the slabs and using a broom brush it in to the gaps between the slabs. Brush away any excess sand/cement mix from the surface of the slabs and your base is ready for your new garden building.