How to pour a concrete slab

tom_builds

Difficulty: Intermediate

A concrete foundation is a durable way to secure a shed to the ground and helps prevent moisture inside.

This guide shows you how to lay a 1400mm x 700mm x 90mm slab for a garden shed but you can adjust this to suit your project and space.

Steps

Step 1

Measure and cut two 700mm lengths and two 1470mm lengths of 90 x 35mm treated Pine to make the short and long sides of your formwork.

1.1 - Measure timber with your tape measure.png 1.2 - Mark a dot at the required length.png 1.3 - Use your set sqaure to mark a line across the length of timber.png 1.4.png

Step 2

Clamp one 700mm length to the inside of one 1470mm length. Clamp a small offcut to one piece of timber and then use the second clamp to secure the other length of timber to the same offcut. Once securely clamped, use your impact driver to drive two screws through the long edge into the short edge. Repeat this process for the second long edge and then finally the second short edge. Measure each diagonal to check that the frame is square.

2.1.png 2.2.png 2.3.png 2.4.png 2.5.png

Step 3

Clear the site and prepare the ground for the slab. Excavate an area that will fit the formwork within it by first laying the formwork on the ground and marking around the edge with the blade of your shovel. Check that it is level.

3.1 - Lay frame on site to mark area to be excavated (or if above ground slab, lay on site to check for level).png 3.2 - Check that the frame is level with your 1200mm spirit level.png 3.3 - Check that the frame is level in all directions.png 3.4 - Confirm that the formwork is square by measuring the diagonals. If the lengths are the same the frame is square..png 3.5 - Check that the depth of the slab is consistent by using a tape measure to measure from the ground to your spirit level..png

Step 4

Place a layer of heavy-duty builder’s plastic at the bottom of the formwork to stop the soil from drawing moisture out of the concrete too quickly, which can cause cracks to appear. Cut it slightly longer and wider than the slab itself so you can tuck the overlap into the sides of the frame.

4.1 - Lay the heavy-duty builder's plastic alongside the formwork to measure the right length..png 4.2 - Roll the reinforcing mesh alongside the formwork to mesure the right length.png

Step 5

Use a hacksaw to cut the reinforcing mesh to size so that there is mesh over the slab’s entire surface area. Prop the mesh up on bar chairs so that the reinforcement sits in the middle of the slab.

5.1 - Cut the reinforcing mesh to size with a hacksaw.png 5.2 - Lay the plastic sheet in th bottom of the formwork, allowing the edges to come slightly up the side of the frame.png

Step 6

With your wheelbarrow set up as close to the formwork as is practical, load 3-4 bags of concrete mix, and add the recommended amount of water found on the package.

Mix the concrete by digging your shovel into the dry mix and turning it over. Add more water until the mixture turns slushy, but not watery. The mix should flow slowly when the wheelbarrow is tilted. You will need three wheelbarrow loads of concrete for this project.

6.2 - Add water as per manufacturer's instructions.png 6.3 - Beginning mixing the concrete by turning over shovels full of mix.png 6.4 - Ensure you mix it thoroughly by getting the shovel blade right to the bottom and sides of the wheelbarrow.png 6.5 - Only add as much water as is required to mix the concrete, you don't want it to be watery.png

Step 7

Tip the concrete inside the form, taking care not to spill any outside the frame. Spread it evenly once all the concrete from this wheelbarrow has been added. Mix and pour your other loads of concrete, keeping one bag spare.

Once all the concrete has been added, use the blade of your shovel to push the concrete down and pack it in at the edges and in the corners of the formwork so there are no gaps.

7.1 - Pour the concrete into the middle of the formwork,.png 7.2 - Once all the concrete has been added roughly level it with your shovel.png

Step 8

Use an 800mm length of timber to level the slab off by running it from one side of the slab to the other. Do this with each side of the timber resting on either side of the formwork, and gently move it back and forth so that sections of the slab that are too high are pushed along by the timber to fill in sections that are too low.

If there are gaps, mix small amounts from the spare bag of concrete until you have an even surface. Continue to screed the concrete back and forth until it is level and wipe off any excess concrete from the formwork.

8.1 - Screed the concrete by running an 800mm length of timber from side to side across the form, Move from left to right and back again until the slab is level.png 8.2 - Slab levelled after screeding.png

Step 9

Use a trowel to achieve a smooth finish. Let the slab set for half an hour so it’s not too wet and then smooth out the surface with large sweeping motions. Repeat with the trowel at a slightly higher angle until the desired smoothness is achieved.

For other finish options, check out How to finish a concrete slab.

9.1 - If trowelling for a smooth finish, use a sweeping arc motion across the surface..png

Step 10

Spray the slab down with a hose a few times a day for three days to avoid it drying too quickly. The concrete can crack if some areas of the slab dry out quicker than others.

Leave the formwork in place for at least 48 hours. Then to remove, unscrew one side of the box frame and then slide the rest of the frame off. You can remove any attached concrete with a bolster and use the timber in another project.

Now you’re ready to install your shed.

10.1 - Unscrew one side of the formwork and then slide the rest of the frame off..png 10.2 - Formwork removed.png

Materials

3 pieces of 90 x 35mm structural treated Pine
90mm Pine offcuts for clamping
800mm scrap piece of timber for levelling
10 bags of 20kg ready-mix concrete
8 x 75mm 10g screws
5m reinforcing mesh
Heavy-duty builder’s plastic
20 x 40mm reinforcing bar chairs.

Tools

Wheelbarrow
Square-mouthed shovel
Impact driver
Hand saw
Hack saw with steel blade
Tape measure
Marker
Combination square
1200mm spirit level
2 quick-grip clamps
Concrete trowel
Gloves, safety glasses and dust mask.

Images

1.1 - Measure timber with your tape measure.png

1.2 - Mark a dot at the required length.png

1.3 - Use your set sqaure to mark a line across the length of timber.png

1.4.png

2.1.png

2.2.png

2.3.png

2.4.png

2.5.png

3.1 - Lay frame on site to mark area to be excavated (or if above ground slab, lay on site to check for level).png

3.2 - Check that the frame is level with your 1200mm spirit level.png

3.3 - Check that the frame is level in all directions.png

3.4 - Confirm that the formwork is square by measuring the diagonals. If the lengths are the same the frame is square..png

3.5 - Check that the depth of the slab is consistent by using a tape measure to measure from the ground to your spirit level..png

4.1 - Lay the heavy-duty builder's plastic alongside the formwork to measure the right length..png

4.2 - Roll the reinforcing mesh alongside the formwork to mesure the right length.png

5.1 - Cut the reinforcing mesh to size with a hacksaw.png

5.2 - Lay the plastic sheet in th bottom of the formwork, allowing the edges to come slightly up the side of the frame.png

6.2 - Add water as per manufacturer's instructions.png

6.3 - Beginning mixing the concrete by turning over shovels full of mix.png

6.4 - Ensure you mix it thoroughly by getting the shovel blade right to the bottom and sides of the wheelbarrow.png

6.5 - Only add as much water as is required to mix the concrete, you don't want it to be watery.png

7.1 - Pour the concrete into the middle of the formwork,.png

7.2 - Once all the concrete has been added roughly level it with your shovel.png

8.1 - Screed the concrete by running an 800mm length of timber from side to side across the form, Move from left to right and back again until the slab is level.png

8.2 - Slab levelled after screeding.png

9.1 - If trowelling for a smooth finish, use a sweeping arc motion across the surface..png

10.1 - Unscrew one side of the formwork and then slide the rest of the frame off..png

10.2 - Formwork removed.png

How to pour a concrete slab

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