How to Build a DIY Sandpit With Lid That Stays Dry

Key Takeaways

• The best sandpit is built in five layers: 100mm of clean gravel, a heavy duty membrane that lets water through, a timber frame, 300mm of play sand, and a lid that sheds rain.
• 300mm of play sand is the minimum depth for children to dig and mould in. 100 to 150mm is a safety level for a very young baby, and anything deeper than 300mm is personal choice.
• A sandpit for two children is about 1.5m square, and the most common regret is building it too small.
• A sunk sandpit only works on ground that drains. On clay a raised frame over a gravel layer is the safest build.
• New UC4 treated softwood or larch is the timber for a sandpit. Creosote sleepers and arsenic treated (CCA) timber are barred from use round children.
• A sandpit lid needs a slope of about 1 in 45 so rain runs off it instead of pooling on top.

I remember a mother of two asked me to sort out a sandpit their children had barely used since the summer. Their father had built a sturdy frame from railway sleepers on the lawn, and followed some online advice to line the bottom with plastic before filling it with sand. After a few weeks of wet weather, the sand had turned grey and claggy, there was a musty smell, and water was sitting in the bottom. When I lifted a sleeper to get to the corner of the liner, it was obvious that the water had nowhere to go.

Everything about it was sound, but the ground beneath wasn’t prepared to cope with rainwater. I removed the spoiled sand and plastic, and prepared a free-draining base before refilling the pit with fresh sand. The difference was straightforward but important. A sandpit needs to shed excess water, freely, not trap it underneath. Getting the basics right at the start saves a lot of pain later.

Where Should a Sandpit Go?

A sandpit should go where you can see it from the kitchen, in part shade, and never at the lowest point of the garden. A pit you can see from the window gets used on a drizzly afternoon while you are cooking. One at the far end of a 30m garden gets used on warm dry days and sits under its lid the rest of the year.

Part shade is the sweet spot: full sun bakes the sand and the children, and full shade keeps it damp, which is where the smells start. Afternoon shade from the house, a fence or a shade sail suits most gardens. Keep clear of big trees, because leaves, bird mess and roots all end up in the sand. The low point is the one position I rule out on every site. Water runs to it and sits in it, and a sandpit built there is a pond with sand in it. Think about the route in as well: the bags of sand and gravel have to get from the drive to the pit, so measure the side gate before you settle on the spot.

How Big and How Deep Should a Sandpit Be?

A sandpit for two children is about 1.5m square, with 300mm of play sand in it and walls that stand 100mm above the sand. That gives two children room to dig without sitting on each other, and enough depth to build with. 300mm is the depth a child can dig a proper hole in and mould a castle from. 100 to 150mm is a safety level for a very young baby who mostly sits and pours, and it is fine to start there and top up as they grow. Deeper than 300mm is your choice.

How big should a sandpit be is simpler: decide what you think, then make it half as big again. By the time there are two children and a friend round, 1.5m square is the minimum, and 2m by 1.5m is where most families end up.

Table 1: sandpit sizes and play sand at 300mm deep

With 300mm of sand the walls want to be 400mm tall: two 200mm x 100mm sleepers, each laid on its long edge so it stands 200mm tall, with the second row sitting on top of the first. The 100mm gap is what holds the sand in once the digging and throwing starts.

What to Order Before You Start

Put your inside measurements into the sandpit calculator here to find out exactly what you need, from the play sand, gravel and membrane to the timber and screws, and get an instant price estimate.

Sunk, Raised or Half and Half?

A raised sandpit, a timber frame sitting on the ground, works on any ground, but only sink a sandpit into the ground, fully or one sleeper deep, where the ground drains freely. Here is the test. Dig a hole the depth of your pit where it will go, tip in a full bucket of water, and come back the next day. If the water has gone, the ground drains and you can build in the ground. If it is still standing, a pit dug there will hold water every time it rains, which is what clay does.

Table 2: which sandpit build suits which ground

How to build a sandpit in the ground, then, starts with the bucket. If it emptied, a sunk pit is lovely: a toddler can step over the edge and the ground is the wall. If the bucket sat there, build up. I have dug out more than one sunk pit that had turned into a grey bog, and the fix was always the same: fill the hole, build a frame on top, gravel first.

What Wood Should You Use for a Sandpit?

New pressure treated softwood rated UC4, or untreated larch, is the wood for a wooden sandpit. UC4 is the treatment class for timber in the ground, the grade sold for fence posts and playground equipment. Larch lasts in ground contact with no treatment, and it is what I would choose if the budget stretches.

Two timbers I would never put round children. The first is creosote treated railway sleepers, the old black ones that smell of tar on a hot day. Creosote has been barred from garden sale since 2003, and older creosoted timber must not be used in toys, playgrounds or anywhere it touches skin often. The second is timber treated with CCA, an arsenic based preservative, barred from home use in 2004. Reclaimed timber of unknown age can be either, so for a sandpit I buy new.

Seal every cut end with two coats of end grain preservative, because the treatment only goes a few millimetres in. Never put a cut end in the ground. Sand the top edges round and recess every screw head, because a splinter from the rim is the sandpit injury I have seen more than any other.

How to Build a DIY Sandpit Step by Step

Here is how to make a sandpit as a raised frame, the build that works on any ground. The other two use the same steps with a deeper dig.

1. Mark out and strip the turf

Mark the outside of the frame with pegs and string, add 100mm all round, and strip the turf and topsoil inside that line.

2. Dig 100mm for the gravel

Dig out 100mm below the stripped surface and level the bottom by eye, and do the bucket test now if you have not already. For a half and half pit, dig 300mm instead: 100mm for the gravel and 200mm for the bottom row of sleepers, which sits down in the hole on the gravel.

For a fully sunk pit, dig 400mm (100mm of gravel and 300mm of sand), keep the sides straight so they do not crumble in, and run the membrane up the soil sides to the top. Edge a sunk pit with a 150mm treated board round the top, set 50 to 75mm into the ground so the rest stands proud and holds the grass back.

3. Lay 100mm of clean gravel

Spread 100mm of clean 20mm gravel and rake it level. It gives rain somewhere to go under the sand, and 100mm is enough because nothing heavier than a child stands on it.

4. Build the frame

Cut two sleepers to the inside length plus twice the sleeper thickness (1.7m for a 1.5m pit with 100mm thick sleepers) and two to the inside width (1.5m). The short ones fit between the ends of the long ones, so every corner is flush with square cuts and no angles to cut. Lay the first row on the gravel, square it by measuring both diagonals until they match, and fix each corner with two M10 x 150mm galvanised coach screws, pre drilled so the timber does not split. Lay the second row turned round, so the long sleepers sit on the sides where the short ones were, and screw down through it into the row below. Sink every screw head just below the surface of the wood. Drilling a slightly wider, shallow hole first (called countersinking) lets the head sit flush or lower. Then sand the top edges and corners of the frame until they feel round, so there is nothing sharp to cut a knee or give a splinter.

5. Lay the membrane up the walls

Lay the membrane over the gravel and up the inside of the walls to the top, like lining a bowl, and peg it through the gravel every 500mm with any join overlapped by 150mm. If you have a staple gun this is where one would come in handy. Taking it up the walls stops sand leaking out at the bottom of the frame and keeps wet sand off the timber.

6. Fill with play sand

Tip in play sand to 300mm, 100mm below the top of the frame, and rake it level. Keep any spare for topping up. Use play sand, never builder’s sand, which can hurt eyes, stain and it turns to sludge when wet, or silver sand, which often stains and is not nearly as gentle.

7. Fit the lid

Build the lid last, once the sand is in. The section How Do You Build a Sandpit Lid? further down has the sizes and the diagrams.

What to Put Under a Sandpit

What to put under a sandpit is 100mm of clean gravel, then a heavy duty membrane that lets water through, then the sand. Gravel for drainage, membrane to keep sand and soil apart. Nothing else.

A plastic sheet or a pond liner is the common mistake, and the difference between fabric, membrane and plastic is the same under a sandpit as under gravel. It keeps the soil out, but it keeps the rain in too, and the sand at the bottom goes sour. Our Heavy Duty Weed Membrane is woven, so it lets water straight through and still keeps the soil out. The other mistake is membrane with no gravel under it on heavy ground: the water gets through the fabric, the clay refuses it, and the sand sits wet anyway. On a sandpit you are renovating, torn fabric or a plastic liner comes out. Clean gravel can stay and be topped up.

Sandpit Drainage: Clay, Low Spots and Old Ponds

Sandpit drainage comes down to where the water goes once it is through the sand. On free draining ground it goes into the soil. On clay it has nowhere to go, so the raised frame over 100mm of gravel is the whole answer: the gravel gives water a space below the sand and the frame keeps the sand above the wet ground. Digging deeper into clay does the opposite. Every extra 100mm of hole is another 100mm of bath.

In a low spot the water also arrives from the rest of the garden, so move the pit uphill if you can, or raise it and shape the ground so water passes by. If the whole area stays wet, a French drain can take the water away before it reaches the pit. An old pond makes a tempting ready dug hole, but take the liner out. A liner left in turns the sandpit back into a pond, and a few holes punched through it are rarely enough. Then treat it as a sunk pit, bucket test first.

How Do You Build a Sandpit Lid?

A sandpit lid that is easy to build and strong enough to sit on is two lift-off halves of 18mm exterior plywood, braced underneath with battens no more than 400mm apart, the same spacing a house floor uses.

For a 1.5m pit, cut each half 1.7m by 0.85m so the pair covers the outside of the frame. Underneath, screw a 47 x 75mm batten on edge along each long side and 47 x 47mm battens across between them, no more than 400mm apart. Set the outer battens 100mm in from the edges so they drop just inside the walls and the lid cannot slide off.

Screw a 38mm thick packer along the top of the back wall so the lid slopes about 1 in 45 towards the front, and the rain runs off instead of sitting on it. A 50mm strip of ply screwed along the edge of one half overlaps the other and keeps the rain out of the joint, so lift that half off first. Each half weighs about 22 to 28kg, depending on the plywood, which keeps it in place in most weather as long as the ply stays flat, so two handles on top for lifting are all it needs. Turn buttons (small swivelling catches) on the outside of the frame are optional, for a very exposed garden.

If you would rather use a cover than a lid, use a heavy PVC-coated cover with eyelets that hook onto the frame. Put a prop in the middle so rain runs off instead of pooling. Lift it off on dry days so the sand can air. Shade mesh keeps cats and leaves out, but it lets the rain through. Our guide to garden play areas has more on keeping cats and dogs off a play area.

Next Steps

The three things we sell for this job are the Play Sand, the gravel under it and the membrane between them, and all three are delivered nationwide. Our Play Sand is washed, rounded and certified to BS EN 1177:2018 at 300mm. The drainage layer is one bag of 20mm gravel, and the lining is our Heavy Duty Weed Membrane on the 14m x 1m roll. Put your own sizes into the sandpit calculator higher up the page to find out exactly what you need and get an instant price estimate. If you would like to talk with us, call Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.

Have you ever built your own sandpit? What did you put under it, and how deep did you fill it? Let us know how it has stood up to a British winter.

FAQs

How deep should a sandpit be?

A sandpit should hold 300mm of play sand, with the walls about 100mm taller than the sand. 100 to 150mm is a safety level for a very young baby, and you can top up to 300mm as they grow.

What kind of wood should I use for a sandpit?

New UC4 pressure treated softwood or untreated larch is the right wood for a sandpit, with every cut end sealed. Creosote sleepers and old CCA treated timber are barred from use round children, so reclaimed timber of unknown age stays out.

Should a sandpit be in sun or shade?

A sandpit is best in part shade, with afternoon shade from the house, a fence or a shade sail. Full sun bakes the sand and full shade keeps it damp, and damp sand is what smells.

What do you put in the bottom of a sandpit?

The bottom of a sandpit is 100mm of clean gravel with a heavy duty membrane over it, run up the inside walls. The gravel drains the rain away and the membrane keeps the sand and the soil apart while letting water through.

How often do you change the sand in a sandpit?

Sandpit sand in a home garden is changed only when it is fouled, or when a dry and a rake no longer bring it back, and it is topped up whenever it drops below 300mm. A lid kept on when nobody is playing is what makes the sand last.

How do you make a sandpit cover?

A sandpit cover is easiest made as two lift-off halves of 18mm exterior plywood on battens no more than 400mm apart, sloped about 1 in 45 by a packer on the back wall so rain runs off.