Shed Base Preparation – How to prepare your garden for a shed

Advice on laying suitable bases.

A firm, level base should be the starting point for any shed or garden building. Without this the structure is likely to be assembled improperly – screw holes will not line up correctly, doors may not fit their doorways and the quality and service life of your shed could be greatly reduced.

A vital part of the preparation for installing any shed is to create a strong and level base for it to sit on top of, if the base is not level you may find that vital screw holes or interlocking parts will not line up correctly making the final assembly difficult and unstable, especially in high winds.

When it comes to shed bases, there are several options available to you, these include

1. Concrete Base

2. Slabbed Paving

3. Timber Floor Kit

4. Eco Base Fast Fit

Creating a base out of paving slabs can be a pretty simple option, but it can be hard to achieve a flat surface if you’re trying to create one on top of grass or soil. The latter option being a concrete base, which can be a cheaper alternative, especially on larger size buildings and are almost always the strongest, durable and long-lasting.

If creating a concrete base sounds like the best solution for you, this is our ‘how to’ guide…

Building a Concrete Base

Step 1:

Before you build your shed base you will need to choose a suitable location that gives you enough distance from any fences or hedges to provide easy access to all sides of your shed. Try to choose a site where the land slopes away for better drainage and don’t forget that, in addition to everyday tools such as spade, mallet and tape measure, you’ll need some durable plastic sheeting to provide a moisture barrier, wooden pegs or stakes and timber to create a frame.

Step 2:

Level the area to be used with a rake and spade, removing all vegetation and roots. When measuring out the area for a shed base we recommend adding a 5cm (2ins) lip around the outside of the building, then use pegs and string to mark out the area. As the base will require a 7.5cm (3ins) layer of compacted hardcore then a 7.5cm (3ins) layer of concrete, the area within the pegs now needs to be excavated to a depth of least 6 inches. Hammer small stakes into the ground so that they are just above the top of the excavation. Nail strips of plywood or timber flush with the top of the stakes along the edge of the excavation, this forms what is called ‘shuttering’, a sort of mould for the concrete. There should also be extra stakes or pegs at a regular distance of 8 – 10 inches to keep the plywood in place when the shuttering is filled with gravel and concrete. Make sure that the distance between the two diagonals is the same and use a spirit level to ensure a 100% level base. 

Step 3:

Add the first layer which will be the hardcore and thoroughly compact it. You will want to spread a thin layer of sand over the top, and a damp-proof membrane (sized to the entire surface area the shed will occupy) should be placed on top of the sand and should be at least 2ins above the surrounding land area. Metal or single-walled vinyl clad sheds will suffer from condensation if any dampness is present and that’s why plastic sheeting is so important. If you don’t have any damp proofing, any moisture in the foundations, will enter the shed and react with the warmer air to cause condensation. The condensation cycle is difficult to stop, therefore, it is vital to prevent it as far as possible in the first place. .

Step 4:

You will then be able to mix the concrete using one part cement to five parts ballast and only a small amount of water as you want the cement to be on the dry side. Dry-mixed concrete can be purchased which you just have to add water to, making sure you follow the instructions on the bag. Once mixed you will want to spread the concrete evenly into the framework, for best results allow the concrete to go slightly higher than the framework as this will make levelling the concrete a lot easier as you can get rid of any excess. Using a straight edge of timber resting on the framework, you then use a sawing motion over the entire surface of the concrete to create a smooth level surface.

Step 5:

Once you have finished levelling the concrete it would be a good idea to check the weather forecast for any bad weather. If rain has been forecast you will want to cover the concrete with polythene for 24 hours, if warm weather is forecast then cover the entire surface with wet sacks and keep damp for 24 hours. Using these methods ensures that the concrete does not shrink or crack. The foundations should be left to ‘cure’ for at least seven days if the weather is warm and dry, a little longer if the atmosphere is damp, always bearing in mind that the concrete needs to be dry to avoid establishing a cycle of condensation inside your shed if it is made from metal or plastic.

Building a Slabbed Base

Step 1:

To prepare a paving slab base for a garden building you will need: a tape measure, four wooden pegs, a ball of string, spade, a large bucket or similar for mixing cement in, garden rake, rubber mallet, tri-square and a spirit level. 

You will also require some building sand, also called sharp sand, and a 25kg bag of standard all-purpose cement. (The sand to cement ratio required will be 8:1.)

Step 2:

We recommend using paving slabs measuring 600mm x 600mm x 50mm (2′ x 2′ x 2″) for reasons of both strength and convenience – this slab size will divide evenly into all shed length and width sizes.

Step 3:

Using the tape measure, mark out the area for the base, making it 5cm (2″) larger than your garden building on each side. Hammer in the pegs and run the string between them, then make sure the corners are completely square by measuring them with the tri-square to ensure they are at a 90° angle to each other.

Step 4:

Dig out the ground in the marked area to around 65mm (2.5″) deep and remove the pegs and string line. You can make the hole a little shallower than this if you would prefer the base of your building to stand slightly raised from the ground to help with drainage.

Step 5:

Stir together one part cement to eight parts building sand for a dry sand and cement mix. 

You don’t need to add water – the damp in the atmosphere will create the curing needed for the cement content. 

Step 6:

In the excavated area, spread the mixture evenly to a depth of about 40mm (1.5″) and rake thoroughly, ensuring it’s completely level.

Step 7:

Starting from a corner and working outwards, carefully lay out the paving slabs and tap each one down with a rubber mallet to make sure it’s secure.

Step 8:

This is the most important part of the process! Check that the slabs are square and firmly butted together, and use a spirit gauge to make sure that they are absolutely level – otherwise your building will eventually warp. Finally, brush off any excess sand and cement mix which could make the base uneven.

Using a Timber Floor Kit

The Timber Flooring Kits are pre-measured, pre-cut and pre-treated providing long term protection from insect and fungal attack.

The timber flooring kit will allow the shed to be raised off the ground and is ideal for sloping sites.

The Timber floor kit is constructed from 16mm thick tongue and groove boards treated in a wax and water based preservative and are mounted on to 45mm x 45mm sawn bearers. Nails are included for assembly, plus screws and washers to fix to the base rails of the shed.

A plastic sheet membrane must be inserted below the floor to ensure there is no dampness. Pegs for securing to soil bases are not included. Nor is the plastic sheet membrane.

Instructions can be found here

Using an Eco Base Fast Fit

A quick-to-assemble, easy-to-use and environmentally-friendly base for garden buildings. Use EcoBase Fastfit as an alternative to a concrete slab or slabbed paving.

EcoBase Fastfit comes with a permeable membrane, cut to the size of the building’s foundation to suppress the growth of weeds as well as easy-to-follow assembly instructions.

To watch our short installation video, please click here.

Instruction leaflet can be found here click here

The information given above is for guidance only. If you are unsure how to complete a base please call us or alternatively get a specialist builder to install one.

Shed Master accepts no responsibility for any loss or damage from the advice provided. If in doubt contact a specialist builder.