How to pour a concrete slab

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

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Garage and Shed

Comments

Noyade

Hi Tom.

Are those saw horses from Aldi?? 😏

Very nice instructions/advice. 👍

To each their own - but as a home owner I made it my mission to buy a small concrete mixer which I achieved in 2004. I have no problems with 'hand' mixing but purchasing that electric concrete mixer was one of the wisest decisions I ever made.

Old-gal24

Well done, any chance you have a spare few hours I'd love to have one just a bit bigger installed... LOL..

Looks good thanks for sharing..

This old gal

th-DIYER

Thanks @tom_builds very clear steps. I’m considering doing 2 slabs one for a shed and another for a fire pit which will be round. Any suggestions to use as form work? I’m looking for something easy and economical to use. Could 100mm x 15mm rough sawn fence panel work? Or some type of plywood? I was thinking using hot water to shape it? Thanks

MitchellMc

Hi @th-DIYER,

You could use the pailings for the rectangular shed slab, but they would be challenging to bend into a circle. For the circular fire pit slab, I'd advise using 6mm MDF sheet cut into strips and held in place at regular intervals with timber stakes. You or our team can cut strips off a larger board. Our team might only be able to cut 30cm high strips on our machine for you, so speak with a helpful team member in-store. You might like to use 9mm or 12mm MDF for the shed slab.

Please let me know if you have any questions.

Mitchell

Old-gal24

Well done @tom_builds my 1st concrete slab was a shocker as I tried to use fast setting as it was a small job... WRONG... yours looks great.

tom_builds

Hi @th-DIYER ,

Sorry I have been away on holidays so just catching back up on Workshop stuff.

@MitchellMc has pretty much covered what I would have suggested. The thinner plywood will help you shape the formwork for the circular slab, and if you are worried about the formwork holding the weight of the concrete you can double up with an additional layer of the thin plywood to strengthen it.

Best of luck with your concrete slabs!

MitchellMc

That sounds like a terrible pickle to be in @Old-gal24! You've likely become accustomed that labels such as "fast" or "quick" are most often merely buzz words used to sell products. In the case of fast set concrete, it does go off quickly and is only suitable for things like post holes.

Did you ever redo the slab with regular concrete?

Mitchell

th-DIYER

Thanks @tom_builds and @MitchellMc .
Tom I hope you had a good holiday!

could I clarify a few more questions please.
Standard slab thickness from what I read is 100mm/10cm? So if I cut 100mm strips and shape it with the stakes to hold in place I should be good? Then lay some kind of builders plastic? Followed by some reo, then pour away?

Finally, do both slabs need a slight fall for water to run off or will level be ok?

TIA

MitchellMc

I'd be looking at cutting strips slightly higher than what the slab is to be @th-DIYER. That way, a little can be pushed into the ground, and some is proud of the surface of the slab. The last thing you've want is the retained concrete pushing through the formwork. Remember, the concrete exerts a substantial amount of sideways force onto the formwork.

Builders plastic is a good idea before the reo is laid. Make sure you raise the reo up with bar chairs or rocks, so it sits in the middle of the slab and not at the bottom.

Mitchell

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