How to build a wooden sandpit

Enjoy outdoor fun with a kid’s sandpit

A sandpit makes a fun addition to the garden. Children can build sandcastles, dig holes and enjoy sensory touch-and-feel play. A sandpit is a perfect way to introduce some seaside spirit to the back garden and they can be built in any shape or size to suit your outdoor space.

We’re building a sandpit that is:

made from timber deck boards and decking joists
1metre (m) by 1m square on the inside
sunken into a lawn, with a seating frame that rests on the lawn
fitted with a wooden cover

We’ll run through how to build a wooden sandpit that’ll keep children entertained summer after summer.

How to select a site for a sunken sandpit

We recommend selecting a site that is:

firm and level
dry and has good drainage. The sandpit needs to drain naturally to prevent it becoming waterlogged, so avoid any areas of the garden that are naturally wet or likely to be affected by possible flooding.
half in sunlight and half in the shade. The sunlight helps the sand dry out while the shade provides protection. Avoid sites that are in full, direct sun or in a dark corner or damp area of the garden.
easily viewable and not obstructed so that you can keep an eye on the children as they play

Wherever you locate it, don't forget the suntan lotion or sun hat.

Safety first

Always wear protective safety gear when necessary.
Enlist the assistance of a helper, especially when lowering the finished sandpit into the ground.
Make sure all children are at a safe distance away from the sandpit until construction is complete.
Don't burn the off-cuts of pre-treated timber decking as the smoke and ash produced is toxic. Dispose of them as you would ordinary household waste.

How to select a site for a sunken sandpit

Materials:

Decking boards x length required for the sandpit seating frame. Our inside frame measures 1x1m, and so we're using four 1.8m deck boards. At B&Q, our decking boards range from 1.8 to 4.8m in length – choose how many you need for your size of sandpit.
Treated exterior timber – x length required for the wooden sandpit cover, though deck boards can be used instead
Decking joists – we recommend decking joists that are approximately 40 to 50mm thick to ensure a stable support for the seating frame. We've used two joists at 144mm wide to create a combined joist frame height of 288mm. Choose your preferred joist width to create the desired height of your frame.
50mm decking screws – for the wooden cover
100mm galvanised nails - we recommend decking screws and galvanised nails as they're weatherproof and so won't corrode
20mm galvanised nails
12mm galvanised clout nails
Perforated metal fixing plates – four or eight as required
Weed control fabric or pond liner
Exterior wood filler
Sandpaper
Exterior wood adhesive
Exterior woodcare - to protect and colour the sandpit
Decking end grain protector
Play sand – be sure to opt for play sand rather than building sand. Play sand is softer and safer. It’s non-toxic and non-staining and has been washed, dried and sieved making it much gentler for children.
Cardboard - if necessary, to protect the sandpit from glue drips

Tools:

Handsaw or for a power option, choose a mitre saw (if the joists are thinner than the cutting depth of the mitre saw) or a circular saw
Combi drill
Wood drill bit and countersink bit
Drill driver or electric screwdriver
Builders line and pegs
Garden spade
Lawn edger
Tape measure
Long spirit level
Two quick-release bar clamps
Claw hammer
Paint brush - for the end grain protector and paint
Rafter square - to mark 45 degree angles
Sanding block
Garden fork - if using a pond or plastic liner
Scissors or a utility knife
Work bench
Cloth

Safety kit:

Rigger gloves – for handling timber
Disposable gloves – for applying the end grain protector

Step 1

Measure and mark four deck joists to a length of 1m. Repeat the process for another four joists measuring 1.05m (this extra allows for the overlap of the corner joins). These joists will form two square frames or ‘boxes’ that will sit one on top of the other.

Use a saw to cut the deck joists to size.

Step 2

Treat all cut deck board ends with a decking end grain protector or wood treatment.

If these sawn ends are left unprotected, water will soak in leading to rot and decay.

Wear safety gloves and apply with a paint brush following the instructions on the tin.

Leave to dry.

Step 3

Make two pencil marks on each end of the longer deck joists – one at the top and one at the bottom (3cm from the top and bottom edges). These marks are where the galvanised nails will go to secure the longer joists to the shorter ones creating the frames. Make the marks to align with the centre of the adjoining inner joists. This will total eight marks per frame – two in each corner of the frame.

Use a wood drill bit to drill pilot holes into the pencil-marked areas. A pilot hole provides a guide hole to help direct the nail and prevent the wood from splitting and cracking. The drill bit should be thinner than the nail's shank and drill to the same depth as the nail.

Use a claw hammer to hammer 100mm galvanised nails through the pilot holes to fix the joists together.

Repeat in every corner to create the two joist frames. Our joists are 144mm in height, so together they make a sandpit depth of 288mm.

Step 4

Lay one joist frame in position on the lawn and tap in builders pegs halfway into the soil at the outside corners.

Lift out the frame and join the pegs with builders line or string to outline where the hole will be dug.

Use a lawn edger to cut straight edges along the marked lines and a garden spade to dig the hole to the depth of the finished frame. For this project, measure the height of one frame and double it - as it will be twice the height when the two frames are fixed together. With the hole dug, remove the pegs and string.

Step 5

Away from the hole, stand the two frames on their sides. Stack so that the longer joists on the top frame are on top of the shorter joists on the bottom frame (or vice versa). This creates a sturdier, more robust join - though isn't possible if the sandpit isn't completely square.

Once in position, clamp the two joist frames together with quick-release clamps. Make sure that the frames are flush and square with each other.

Lay a perforated metal plate in the centre of the frame’s side facing upwards. Hammer eight 20 to 25mm galvanised nails (four in each joist) into the metal plate securing it to the side of the frame.

Repeat to secure all four sides of the joist frame, and then remove the clamps and place it flat on the grass.

Step 6

Create the horizontal seating around the top of the sandpit using deck boards.

Measure and mark a 45 degree angle a couple of centimetres in from one end of a deck board using a rafter square. The longest edge will form the outside of the seat around the sandpit (as illustrated). By bringing the measurement in slightly from the end of the board, you'll create a nice pointed tip when the board is cut, producing a better finish when the seat boards are fitted together.

Step 7

Clamp the deck board to a workbench and cut along the marked line with a panel saw (as shown) or use a mitre or circular saw if preferred.

Step 8

Lay one deck board in position on top of the joist frame so the shortest length is flush with the inside of the joist frame. Position the angled end with an inside corner of the joist frame.

On the other end of the board, mark the point where the inside of the board meets the corner of the frame. From this point, measure and mark another 45 degree angle (in the opposite direction to the previous cut). Place it back in the workbench and cut the new mark and then reposition it back on the frame.

Step 9

Repeat Steps 6 and 7 and make another angled cut in a new board.

Lay this second board in place on the joist frame so that its angled end butts up against the first board.

Mark, measure and cut the other end and repeat until all four boards are in place with both ends angled.

Step 10

Lay the seating frame upside down on a solid, flat surface (for example on a patio or piece of plywood - not straight onto grass).

Glue the joints of the deck boards together with a waterproof wood adhesive. If required place some cardboard underneath to protect the surface from any glue drips.

Lay a perforated metal plate on the underside of each corner of the deck board frame. We recommend attaching to the grooved (or more grooved) side of the deck board as this will be the underside of the seat, leaving the smoother finish on top.

Hammer six 20mm galvanised nails into the metal panel positioning three on each side of the board. Wipe away excess glue with a damp cloth and leave to dry.

Step 11

Once the glue has dried, flip the frame over and sit the deck board frame in place on top of the joists frame so the metal panels are on the underside.

Use a wood drill bit to drill pilot holes into the top of the board frame every 30cm around its inside edge, directly into the centre of the joist frame.

Remove the drill bit and replace with a countersink bit – this should be the depth of the screw’s head. Drill into the pilot holes to create countersunk holes. Countersinking sets the screws to a consistent depth without splitting or denting the wood. It also ensures the screws will be flush with the surface of the timber for a tidier finish.

Remove the countersink bit and screw in 75mm deck screws. For a really smooth and neat finish, fill the holes with wood filler and sand smooth.

The frame is now completed.

Step 12

Treat the timber to protect it from the elements and leave to dry.

We’ve opted for a vibrant pink shade of exterior wood paint on our seating frame and a more discrete wood treatment on the joist one.

At B&Q, we have a wide range of exterior wood paints, stains and varnishes. For advice on finding the right outdoor paint, head to our exterior paint buying guide.

Step 13

When the paint is dry, place the frame onto a sheet of weed control fabric or pond liner.

Lap the fabric/liner up to the outside of the frame, just below the seat and trim to fit with a utility knife or heavy-duty scissors.

Secure the fabric/liner to the frame with 12mm clout nails hammered in every 10cm around the frame.

If the liner isn’t wide enough, overlap the lengths by 30cm. There's no need to glue it together, just overlap and tack in place.

Step 14

Add a thin layer of sand (10 to 15mm) into the bottom of the hole. This helps cushion and level the area, while also aiding drainage.

Lower the finished box into the hole with the assistance of a helper.

Step 15

If using a pond or plastic liner - perforate it with a garden fork to create plenty of drainage holes.

This isn’t necessary with weed control fabric as this is porous, allowing rainwater to drain away as well as preventing weed growth.

Step 16

Fill the pit with play sand. Fill to no more than 10cm from the top so there’s room to dig and build sandcastles.

Top tip: Keep play sand safe

Sandpits require a bit of maintenance to help keep them hygienic and safe for children to play in. Top it up with fresh play sand occasionally, as and when required, and replace the sand every year or when it needs it.

It’s best to cover a sandpit when it’s not in use to stop it getting waterlogged, filled with garden debris or fouled by birds and animals.

You can simply cover with a tarpaulin and weigh down at the corners, however we’ve opted for a wooden cover as tarpaulin tends to sag after heavy rain.

This is also a sturdier option that provides air circulation, though it isn’t 100 per cent waterproof. Wooden covers can be quite heavy, especially for a larger sandpit. If building a big sandpit, consider building the cover in sections.

Read on to make a small wooden cover with exterior timber or deck boards.