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How to build a shed base

Advice on laying timber, plastic, paved and concrete 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.

There are a number of different options to create the necessary flat and level surface, and we're going to take you through four of them. All are ideal to prevent water collecting around the shed floor and damaging it. These four methods are how to lay:

  • a timber base (also known as a portabase)
  • a plastic base
  • a paved base, and
  • a concrete base

Both paved and concrete bases can't be laid straight onto soft ground and so require a sub-base. A concrete base also has a further extra step - constructing the frame or edging that supports concrete while it sets, this is known as formwork. So do bear in mind that these options can take longer to build than the timber and plastic bases. We recommend only using a concrete base for larger sheds and log cabins.

Simply choose the one for you and read through the step-by-step advice on how to lay your chosen shed base.

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Take time to consider a few things before building your shed base.

Planning permission

Sheds are generally classified as temporary structures so you won't usually need planning permission unless you live in a conservation area. But for more information on the up-to-date legislation, check the Government’s Planning Portal website.

Access

You will need access to all sides of the shed during construction and to apply an external paint, stain or varnish if your shed is made of wood. So ensure your base isn't built too close to walls or fencing (these might also need easy access for maintenance).

Electrical supply

If you have plans to add electricity to your shed, consult with a qualified electrician before building the base.

For more on these and other practical considerations about selecting a site for your shed, head to our Sheds buying guide.

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A timber shed base, or portabase, is made from pressure-treated timber and comes with metal spikes which are hammered into the ground to keep the shed base in place. These can be installed on a level lawn or the frame can be placed onto a patio using the metal L-shaped legs (supplied with the kit) to help keep it level. The Shire shed base (shown), has patented spikes that allow the base to be securely fixed onto unlevel ground. The spikes also help to keep the base off the ground, therefore reducing the threat of rot.

Buy as part of a complete installation kit with certain sheds or individually as just the shed base in one of our two sizes - 6x4 and 7x5.

Assembly instructions will vary depending on the type of timber frame and the shed floor to be fitted, so always follow any specific manufacturer instructions.

For this project, we will show how to install the Forest timber shed base on grass.

Safety first

  • Wash hands thoroughly after handling treated timber and especially before eating.
  • Wear protective safety gear when necessary.

You will need:

Materials

  • Timber shed base - as bought individually or supplied with a shed and its fixings

Tools

Protective kit

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Step 1

Spread out the timbers into position, roughly where the shed is to be located.

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Step 2

Make two pencil marks on each end of the frame's two longer beams. These marks are where the screws will go. Make the marks to align with the centre of the adjoining shorter beams. This will total eight marks - two in each corner of the frame.

Use a flat wood bit to drill recesses into each of the eight marks. These should be the depth of the screw's head. Also known as countersunk holes, these recesses ensure the screw heads will be flush with the frame's surface for a neat finish.

Replace your drill bit for one that's thinner than your screw's shank. Line up the adjoining beams and drill a pilot hole through the centre of the recess, from the longer beam into the adjoining shorter beam. A pilot hole provides a guide hole to help direct the screws and prevents the screw from splitting the wood.

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Step 3

With the pilot holes drilled, it's now time to fit the two 100mm screws (as provided with the kit). Use a drill driver to tighten the screws into place.

Once the frame is assembled, evenly space out the remaining short beams down the frame and fix these in place too.

Fitting an OSB floor

If fitting an OSB (Orientated Strand Board) shed floor to a portabase, place the floor onto the frame (before the centre beams are fixed) and mark out where the floor beams meet the frame. Once all markings have been made remove the flooring from the frame and fix the centre beams into position (see diagram below).

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Step 4

Move the frame into its final position and use a square to check the frame is square. Each corner should be 90 degrees.

As a final check, measure the diagonal corners, if they are identical, you know the frame is square.

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Step 5

With the frame in the final position, fix the L-shaped feet to the inside of the frame.

Drill pilot holes with a 2mm drill bit. Don’t position them too far into the corners, you need to allow space for the spikes.

Keep checking that the frame is level.

Screw the three 40mm round head screws (provided as part of the kit) into the pilot holes to secure the L-shaped feet.

Either of the two faces can be used for fixing the foot as this allows a larger range of height difference. Position the feet so that the base is firm and level in all directions.

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Step 6

To secure the base to soft ground, hammer in the spikes at each corner until they are level with the top of the base. Use a club hammer and an off-cut of wood to prevent damage to the spike.

If the base is located on a hard floor (level paving or concrete), only the L-shaped feet are required.

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Step 7

Continue to check the base is level with a spirit level and secure the spikes to the base with the screws provided.

Drill pilot holes using a 2mm drill bit and then secure the screws into the pilot holes.

The shed floor can now be positioned onto the base. There is no need to screw this into place.


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An interlocking plastic shed base is the perfect option for a quick and easy installation. This type of base is made from 100% recycled plastic and the plastic tiles (or grid) fits and locks together to form the base. The grids are free-draining and self-ventilating, ensuring the floor of the shed stays dry.

They can be laid on to a level concrete slab or patio, but a top dressing of sharp sand or builders sand will help to bed in the grids, this sub-base saves on any excavation too.

Below we show how to install a plastic Hawklok shed base onto firm, level grass.

You will need:

Materials

Tools

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Step 1

Measure out the site and hammer a peg into each of the four corners. It’s a good idea to make it larger than the base of your shed to aid drainage. It can be decorated with gravel after the shed is erected.

Run a builders line from each of the four pegs. This will help you visualise the site.

Clear the area.

Use an edger to cut into the lawn for a straight cut, and then use a spade to remove the turf.

Use a straightedge with a spirit level on top to check that the whole area is level, and level out any slight unevenness in the ground.

Alternatively, dig to the required level and compact the area using pea gravel to fill uneven spots. By excavating this area, your plastic base won't be on display.

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Step 2

Once the site is prepared, lay down the membrane. In windy conditions, peg or weigh down the membrane.

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Step 3

Lay out the required number of plastic grids for the shed.

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Step 4

Remove the locking pins from the grids and clip them together.

Once all grids have been clipped together, insert the locking pins into the centre of the grids.

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Step 5

With all the grids in place, the base is now ready for your shed floor.

When your shed is in place, trim any excess membrane with a knife.


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Both paved (pictured) and concrete shed bases require the support of a sub-base. This sub-base is made from an MOT type 1 hardcore (consisting of crushed or broken bricks, blocks and stone) compacted with an earth rammer or a vibrating plate compactor. Then topped with a layer of sharp sand or ballast – called blinding – to fill any gaps.

For a paved base:

This requires 5cm of compact hardcore underneath the paving slabs.

For a concrete base:

This requires 7.5cm of compact hardcore underneath 7.5cm of concrete.

You can dig the base down into the ground (as we have in our example), or have it raised above. If you choose to have your base level with the ground, you will have to excavate the top earth.

For a paved base:

This will be about 12cm deep to allow for the sub-base, mortar and paving slabs.

For a concrete base:

This will be about 15cm deep.

A concrete base needs to be installed well in advance to ensure that it's had enough time to set properly before you start erecting the shed.

We recommend making a concrete base the same size as the base of your building or 25mm smaller on all sides than the base of the shed. This creates an overhang and prevents water accumulating around the timber floor and runners.

You will need:

Materials

Tools

To hire

  • Earth rammer or a vibrating plate compactor
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Step 1

Measure out the site for your sub-base and hammer a peg into each of the four corners.

Run a builders line from each of the four pegs. This will help you visualise the site.

Use a square to make sure the corners are square before you start excavating. As a final check, measure the diagonals to make sure they are the same.

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Step 2

Use an edger to cut into the lawn for a straight cut, and then use a spade to cut the turf into strips, then roll it up and remove.

Save some of the turf in case you need to make good between an existing lawn and the finished base.

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Step 3

Dig the area to the correct depth and hammer in wooden marker pegs (spaced 1m apart) to mark the finished hardcore level across the site.

Create the wooden pegs by cutting one end of 50mm x 50mm pieces of timber into sharp points using a panel saw.

Use a long spirit level to check the level of the pegs.

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Step 4

Tip in enough hardcore to fill your sub-base to just above the top of the pegs.

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Step 5

Compact the hardcore with an earth rammer or a vibrating plate compactor, as used here.

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Step 6

Cover the surface with a thin coat of sharp sand or ballast and rake it level.


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Safety first

  • If you are laying slabs 600mm x 600mm or larger, you must have someone to help you lift and lay them.
  • Always wear protective safety gear when necessary.
  • When using cement, always wear a dust mask, gloves and goggles. Wash your hands after use as wet cement can cause burns.

You will need:

Materials

Tools

Protective kit

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Step 1

Before you begin to lay the slabs, hammer in wooden pegs into the four corners of the site and attach builders lines to mark the final height of the paving slabs.

Create your mortar by either mixing four parts sharp sand to one part cement, or use a pre-mixed alternative. If using a pre-mixed one, add and mix in clean water straight into the tub or bag. It should be of a slightly moist, smooth and damp consistency, not wet or sloppy.

Use the trowel to lay the mortar onto the sub-base. Do enough for one paving slab at a time.

Wet the back of the paving slab with a hand brush dipped into a bucket of water. This dampness improves adhesion and makes it easier to slide the slab into position.

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Step 2

With a helper, carefully lift the first slab and lay it on top of the mortar.

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Step 3

Use a piece of timber and a club hammer or rubber mallet to tap the slab into position. Take great care not to crack the slab.

Cut in the external edges of the paving flush with the slab as you go with a trowel.

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Step 4

Continue to lay the first row of slabs.

Make spacers the same size (between 5 and 10mm wide made from pieces of timber) and use these in all the joints to ensure they're the same size.

Use a long spirit level to check and re-check that the surface is flat and level.

If you put a slab down and it rocks on the mortar, take it up and re-lay it; you're unlikely to be able to correct it by trying to push mortar underneath a slab once it’s in position.

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Step 5

When the first row is complete, lay slabs along the two adjacent outer edges. If you're using a grid pattern, the first and last slabs on alternate rows will be half slabs.

Fill in the central area, working back row by row.

Keep using a spirit level to check and re-check that the surface is flat and level.

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Step 6

Leave the mortar to set for 48 hours before walking on the slabs (or as recommended by the packaging instructions). Only then can the joints be filled. If the slabs are wet or it looks as if rain is imminent, wait for a drier day and cover with tarpaulin to protect against the elements.

Remove the wooden spacers and fill the joints either with more mortar (pictured) or paving grout (also known as dry joining compound).

If using mortar: Fill all the joints with mortar, and once all filled, push the mix down firmly with a trowel or a piece of wood, then brush in more mortar. Repeat this process three or four times to prevent holes from appearing in the mortared joints.

Carefully brush away all the excess mortar – take time over this, as it’s easy to end up with mortar on the slabs.

If using paving grout: Brush it in and compact with a jointing tool. It dries hard in a few hours, so protect it from rain with tarpaulin until then.

For more tips on how to build a shed base with paving slabs, watch our step-by-step video:


Concrete has to be supported by a frame or edging, known as formwork, until it's set. The formwork is constructed directly on a prepared sub-base.

You will need:

Materials

Tools

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Step 1

Set out the site using builders lines and eight wooden pegs.

Create the wooden pegs by cutting one end of 50mm x 50mm pieces of timber into sharp points using a panel saw.

Hammer two wooden pegs into the ground at each corner of the site, a short distance from the base area (as pictured).

Run a builders line from each peg to its opposite and tie it taut. The lines should mark the edges of the concrete which will be on the inside of the formwork.

Use a square to check the corners are exactly 90 degrees.

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Step 2

Saw a plank of rough-sawn timber to length and place it in position at one end of the site.

Hammer a wooden peg into the sub-base at each end, outside the area to be concreted. Do so until the pegs are flush with the top of the plank.

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Step 3

Use a spirit level to make sure the plank is level, then secure each end of the plank to the wooden pegs with 50mm nails.

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Step 4

Attach the last plank and complete the square or rectangle.

Make sure that there are pegs at the corners and that the corners are tight-fitting by nailing the planks together. It doesn’t matter if the planks run past the edge of the square, as they do in the picture.

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Step 5

Hammer in pegs at roughly 1m intervals along the outside of the formwork.

The structure must be solid and square and the correct size for your base as it's very difficult to alter it once you begin concreting.

 


Concrete starts to harden and set about two hours after it has been mixed, so it must be laid, tamped (packed down) and given its finish within that time. Divide large areas into bays or sections which can be completed one at a time before they begin to set. You can walk on the concrete after three days and remove the formwork after five days (but as always, check any product instructions for the mix you are using as there are rapid-setting options available).

Safety first

  • Always wear protective safety gear when necessary.
  • Wet cement can cause burns, so always wash your hands after use.
  • Don't put your hands or a shovel into a mixer while it's running.
  • Don’t allow concrete to dry on your tools. Clean it off with a stiff brush.

You will need:

Materials

Tools

Protective kit

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Step 1

Mix the concrete.

Either mix using one part cement to five parts ballast or use a pre-mixed alternative. If using a pre-mixed one, add the amount of water instructed on the packaging.

How much concrete is needed?

To estimate the amount of concrete required for the shed base, calculate the surface area of the site by multiplying the length by the width (in metres). Multiply that figure by the depth or thickness required (as a fraction of a metre) and you will have the volume in cubic metres. For example, 7.5cm deep would be x 0.075.

Numbers are rounded up and you should add 10% for wastage.

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Step 2

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

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Step 3

Starting in one corner, pour concrete onto the formwork.

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Step 4

Push the blade of the shovel up and down in the wet concrete (particularly near the edges) to get rid of any air pockets.

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Step 5

Use a garden rake or shovel to spread the concrete, leaving it about 18mm higher than the top of the formwork.

Work in manageable sections of about 1m to 1.5m square.

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Step 6

With a helper, compact the concrete using a straight piece of 50mm by 100mm timber that's longer than the width of your site (this is known as a tamping board).

Start at one end and use steady blows of the plank, moving it along the site at a rate of about half its thickness at a time, until you have an even ridged surface.

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Step 7

To remove excess concrete and level the surface, go back to the end where you started and slide the tamping board backwards and forwards in a sawing motion across the site. Moving it all the time away from the starting edge.

Fill any depressions in the surface and repeat the procedure, if necessary, until you have an even surface flush with the top of your formwork.

Continue to pour the concrete, tamp and level until you reach the end of the frame.

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Step 8

At this stage you have a rough tamped surface which can be left as is or given any other finish as required.

When you're happy with the finish, run an edging trowel along the formwork to round off the exposed edges of the concrete to prevent chipping.

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Step 9

Cover the concrete with a plastic sheet to slow drying and prevent cracking.

Raise the sheet on wooden supports so that it does not touch the surface and weigh it down with bricks around the edges.

Let the concrete set completely before installing your shed and remove the formwork with the help of a crowbar.