How to build a shed base

There are four types of shed base to consider:

• Concrete shed base

• Paved shed base

• Wooden shed base

• Plastic shed base

This guide concentrates on building concrete and paved shed bases on an existing lawn. These are the two most durable foundations and are especially suitable for larger sheds and garden buildings, e.g., a summerhouse. Sheds are classed as temporary structures, so you don’t usually need planning permission.

Steps

1. Planning where to build a shed base

2. What is a sub-base?

3. Laying a sub-base

4. Building a simple formwork

5. Laying a concrete shed base

6. Laying a paving slab shed base

Try to visualise what the finished shed will look like in your garden, and consider the following when deciding its location:

• Don’t place the shed too close to any walls or fences.

• Take account of overhanging branches from trees or bushes.

• Think about easy access to the shed, both during construction and for regular use afterwards.

• Is there going to be an electrical supply to your shed?

• Is the ground underneath level, and away from any ditches or damp areas where groundwater is likely to pool?

Concrete and paved shed bases can be time-consuming to lay down. But the result is a strong, permanent foundation. You need to lay down a sub-base to help support the paving slabs or concrete.

A sub-base is made from MOT type 1 hardcore (crushed stone & bricks) which is then compacted and topped with a layer of sand. Build your shed base slightly larger than the dimensions of the shed itself (approx. 30-40mm on each side).

When you have decided the site for your shed, dig the ground to the following depths:

• Concrete: 15cm depth, consisting of 7.5cm sub-base and 7.5cm of concrete.

• Paving: about 12cm depth, consisting of 5cm sub-base and 7cm of mortar and paving slabs.

• Mark out your site using wooden pegs and string. Use a set square to check the corners are exactly square. Then measure the diagonals, which should be the same length.

• Cut the turf into strips using a lawn edger, then remove using a spade. Roll the turf up and save some rolls in case you need to fill any gaps between the lawn and the finished base.

• Dig your marked area to the required depth, and then peg it out. Pegging out is a great way to ensure you maintain a consistent depth.

• Take a wooden peg and from the top, mark the depth of both the sub-base, and the completed base. Hammer in pegs 1m apart in rows across the base. Make sure the marked line on the peg is level with the earth. Use a long piece of timber and a spirit level to check that all the pegs are level.

• Add the MOT hardcore so that it reaches the top of the pegs.

• Then compact the layer to the right depth using a plate compactor.

• For a concrete base, add a thin layer of sharp sand and rake it level.

• A paving slab base needs a dry mortar mix, consisting of 8 parts dry sand to 1 part cement. Add this mix and rake it level. It should end up being about 4cm deep.

A concrete foundation must be supported by a wooden frame, or formwork, until the concrete has set.

• Measure and cut timber rails to build a frame that will fit around the edge of your base. Use 50mm nails to attach the lengths together. The frame should be 7.5cm high to match the depth of concrete needed.

• Make sure that the corner timbers fit tightly together.

• Add support to the frame by attaching wooden pegs at regular intervals onto the outside of the frame.

• Use a spirit level to check that each side is level.

Concrete starts to harden within two hours after it has been mixed. But some rapid-set mixes are available. Always follow the manufacturer’s instructions carefully. Always wear protective gear for this stage, and work quickly but methodically.

• Calculate how many cubic metres (m3) you need by multiplying the length and width of your base. Then multiply this figure by 0.075 (this figure represents the 7.5cm depth of concrete you need). Add another 10% for wastage.

• Mix the concrete using 1 part cement to 5 parts ballast. For pre-mixed concrete, add the right amount of water according to manufacturer’s instructions.

• Wet the sub-base and formwork using a watering can fitted with a rose.

• Pour the concrete onto the sub-base. Spread it out evenly, making sure to push right into the corners and up against the edges.

• Use a garden rake or shovel to help spread the concrete around.

• Tamp the concrete down using a piece of timber until the surface is completely smooth.

• Finish off along the edges and corners using an edging trowel.

• The concrete should end up being as high as the formwork.

• Cover the concrete with plastic sheeting so it can dry out naturally. Use wooden blocks to raise the sheeting so it doesn’t touch the concrete. 

• Concrete takes about 3 days to dry completely, then you can walk on it. Remove the formwork after 5 days.

Paving slabs are heavy, so it’s best to have someone to help with lifting and placing the slabs. Wear protective gear such as heavy duty gloves and safety goggles.

• Depending on the size of your shed, and the size and shape of the slabs you’re using, it’s helpful to plan your layout first.

• Choose a corner to lay down your first slab. Wet the back of the slab with a brush and water, then place it into position. Bed the slab in by tapping it gently with a rubber mallet. If you don’t want to tap directly onto the slab, then use a block of wood to protect the stone.

• Continue to lay the slabs down in rows or follow the sequence you’ve planned in advance. Use spacers to leave a consistent gap of 5-10mm between each slab.

• Keep checking your slabs with a spirit level to make sure they’re all level.

• If you lay a slab down and it rocks a little, then take it up and re-lay it.

• Once all the slabs are laid, cover them with plastic sheeting and leave for 48 hours to set.

• Remove the spacers and fill the joints with more dry mortar mix. Gently fill the joints, brushing away any excess mortar. Compress the mixture into the joints and keep adding until the joints are full.

Required skills

• Planning and visualising your project effectively

• Comfortable measuring and cutting timber with accuracy to achieve a level base

• Comfortable mixing cement and concrete 

• Necessary strength to lift and place heavy slabs

• Comfortable with using heavy duty machinery, e.g., plate compactor or concrete mixer

Time required

1-2 days depending on the size of the base and the weather. Additional 2-5 days for mortar or concrete to set thoroughly.

Number of people required

2 people 

Tools and equipment

• MOT type 1 hardcore

• Sharp sand 

• Dry sand

• Cement

• Pre-mixed concrete (if preferred)

• 50 x 50mm timber for frames, pegs, and offcuts

• 50mm nails

• Panel saw

• Builders line, or string

• Set square

• Tape measure

• Lawn edger

• Spade

• Garden rake

• Hammer

• Rubber mallet

• Long spirit level

• Plate compactor

• Plastic sheeting

• Watering can with rose attachment

• Wheelbarrow

• Cement mixer

• Shovel (if you prefer to hand mix the concrete)

• Edging trowel

• Crowbar (to remove the formwork)

Personal protective equipment (PPE)

• Dust mask

• Steel toe-capped boots

• Heavy duty protective gloves

• Safety goggles

Please note: this is a non-exhaustive list; be sure to match your personal protective equipment to the job at hand.