DIY Concrete Slab

How to make or pour a Concrete Slab

Concrete slabs are common practise for floors of patios, garden sheds, carports, and the like. Setting out and pouring a concrete slab in a fairly straight forward exercise if you follow some basic steps. So whether you are mixing the concrete yourself or using ready mix concrete the principles are the same for pouring a concrete slab.

How to make a Concrete Slab step outline

• What type of concrete to use?
• Estimating how much Concrete you need
• Setting out and building the formwork
• Placing in the Reinforcing
• Pouring the Concrete
• Finishing the Concrete
• Curing the Concrete

Concrete Type and Concrete Mixing

Whether you have a large of small concrete slab to do it is possible to do it yourself with great results. For concreting mix ratio see our concrete mix ratio page. If your slab is small it is possible to mix the concrete by hand or with a portable mixer. However, if your slab is over 3 metres by 3 metres, it is best to use ready mix concrete that is delivered ready to pour by truck.

It is important to match your concrete to the load you are placing on it. 20 MPA is the general strength used for driveways however if you have high loading like trucks and heavy loads you would use 35 MPA. It is also possible to use 10M MPA; however this is only used for light loads. 10 MPA has a higher water to cement ratio than 20 MPA which makes it less durable and weaker.

Estimating - DIY Concrete Slab

To estimate the Concrete slab you will first need to calculate the area in square metres. Area in square metres = length x width (metres)

Calculate the Cubic metres of concrete needed by multiplying the area in metres squared by the desired thickness in metres. For Example: If you wanted to work out how much concrete you need for a DIY concrete slab that is 3 by 4 metres by 100mm thick for a garden shed concrete slab. The calculations are as follows:

• 4 x 3 = 12 metres squared (area)
• 12 x 0.1 = 1.2 cubic metres of Concrete is required
• Plus 10% for errors in the formwork and ground preparation.
• Total = 1.32 = Order 1.3 cubic metres of concrete

Note: If the thickness is in millimetres you will need to change it to metres. For example

• 100mm becomes 0.1metres or 75mm becomes 0.075metres etc.
• DIY Concrete Slab: Set out and formwork
• You will need the following:
• Timber the same height as slab thickness: For a 100mm thick slab = 100mm x 50mm timber
• Timber pegs, level, hammer, digging tools, measuring tape, carpenters square

For a 3 x 4 metre slab cut two lengths of timber exactly to 3 metres. The other two lengths of the timber should by 4.2 or 4.8 metres long. Mark out and nail the lengths together so that the internal measurement is 3 metres by 4 metres. Place into position and mark the ground area and remove ready to dig out. Dig out the marked area so that top of the slab is at the height that you want. Place the timber work back into position and check that the timber form work frame is level. Hammer in your timber pegs every metre around the frame checking that the frame is square as you go.

Tip: make sure you saw the pegs off level so you can screed across the whole surface.

DIY concrete reinforcing

Reinforcing mesh is purchased in large 6 x 4 metre sheets. If you need to use this (you do not need it for a garden shed concrete slab) it is cut to fit inside the form work but should be completely surrounded by concrete. Small plastic chairs are used to hold the form work is place while the concrete is being poured in.

Note: If you are using multiple reinforcing mesh sheets they must overlap by 225mm and be wired together.

Pouring your DIY Concrete Slab

The best way of pouring the concrete is to make sure everything is ready before you mix or order for concrete.

Tools needed:

Shovel straight long screen or a straight piece of timber, concrete finishing towel, and Edging tool. Your formwork should be complete and ready and your reinforcing mesh cut wirered together, and placed of plastic mesh chairs. Work from one end pouring the concrete in and levelling it roughly with a shovel as you go. When you have filled formwork with concrete take you straight screed and with the aid of another person work it side to side and pull it from one end of the form work to the other. You will find that it is necessary to do this a couple of time to get it right. You should have a level concrete slab that is flat but will look a bit rough and watery.

Finishing you DIY Concrete Slab

Once the surface water has been taken into the slab and it is starting to set it is now time to finish your concrete slab. Using a concreter’s towel smooth over the surface getting it as flat and smooth as possible. Your slab can be left to cure like this or you could use a yard broom to roughen the surface to give it grip. You can also use a wooden float to slightly roughen the surface. The choice of finish is up to you; however a very smooth metal towelled surface can be slippery when wet.

Concrete Slab: Curing

If is important to cure the slab before using it. This is done by placing plastic over the slab after it is set. This will hold in the moisture helping the concrete to cure. If hot weather is expected wet the surface of the slab down before placing the plastic cover over it. The most important thing is to not let the concrete slab dry out before it set properly as this will weaken the concrete. It is also best not to place loads on the concrete for a week minimum. However if the concrete slab is for a house of structure you should cure for 28 days before loading the concrete slab.