How to pour a concrete slab

Bringing the indoors outside often involves projects like installing a garden shed, creating a patio area or setting up a gazebo. It can also mean investing in some outdoor equipment such as a garden dining set, a barbecue or even a plancha grill. But in order to ensure that your outdoor space stands the test of time you need a solid surface. Rising moisture from the soil tends to damage garden sheds and if you want a tiled patio, you have little choice but to pour a concrete slab. It is absolutely possible to pour a concrete slab yourself (with the help of a friend!) but you do need to take great care to plan the job properly, ensure you have the have the right tools and follow all the tips provided in this guide.

Steps

1. Lay and compact your sub base
2. Build your formwork
3. Add dampproof membrane and wire mesh
4. Mix and pour the concrete
5. Put expansion joints in place

If you want to pour a concrete slab, you'll have to start by digging out a hole for your sub base; this should measure around 150 mm in depth. For any area measuring less than 10 m², you'll need a wheelbarrow and shovel to dig out the soil. Any larger surfaces can be tackled with a mini digger; these machines can usually be hired by the day.

• Mark out your slab measurement on the ground using a straight edge and place pegs roughly where each corner will go before stretching string between each peg.
• Then, use the 3-4-5 method to check your corners are square. Measure three inches from one corner along one string and four inches along the other string. If the distance between the two marks is 5 inches, your corner is a 90-degree angle. If not, adjust the pegs until your corner is perfectly square. Depending on how you want to work, your formwork can go on the inside or outside of your string. If your formwork is going on the inside of the string, you need to account for the width of your planks.
• Start digging out your hole using a shovel or mini digger.
• Once you've dug out the hole, go over the surface with a rake to remove any large debris and ensure it is relatively level.
• Now is time to lay your sub base. It is possible to find pre-mixed sub bases. Alternatively, you may want to make your own by adding a 100-120 mm layer of crushed stone or gravel followed by 30-50 mm of sand; this type of sub base allows for better drainage.
• Your sub base should be at least 100 mm thick but can measure up to 150 mm, depending on how deep your hole is.
• Compact your sub base using a tamper or a plate compactor if the size of the slab warrants it.
• Check that the sub base is relatively level and adjust as required.

The depth of the hole needs to be greater than the thickness of the slab. If you plan to pour a 100 mm concrete slab to install a garden shed, for example, you need to dig a 220 mm deep hole. This leaves enough room for 150 mm of sub base and 70 mm of concrete slab (leaving 30 mm of slab above ground).

• Use 25 mm timber planks to build your formwork.
• Place your planks inside or outside your string lines, depending on how you've taken your measurements. Ensure you do the same all the way around as you don't want one side to be longer than the others.
• Hold your planks in place using timber pegs.
• Be sure to check that your planks are perfectly level and that your formwork is the correct height, width and depth. You can now lay a perimeter rebar if required. Your planks can be nailed together but do not hammer in the nails completely to ensure you will be able to remove them easily enough later on.
• Use a chalk line and a straight edge to mark out the finished level of your concrete slab on your timber planks.

• Cover your sub base with a dampproof membrane (DPM) to prevent moisture seeping up from the ground. If you have to use several sections of membrane, ensure that each strip overlaps by 10 to 15 cm. Your DPM must cover the entire surface beneath the slab. 
• If you are using a ready-mixed fibre reinforced concrete you won't have to reinforce the concrete yourself. Otherwise, you should reinforce the slab using wire mesh or rebar. Ideally, you should sit the wire mesh on stones in each corner so that it gets sandwiched by the concrete. Cut your mesh as required using bolt cutters and place it around 5 cm from the edges of the slab. If you are using several layers of wire mesh, they should be tied together and held about 10-15 cm apart. 

The amount of concrete you need depends on the size (or volume) of your slab. The thickness of the slab depends on what kind of loads it will be expected to withstand:

• 8 to 10 cm for a patio or a garden shed base;
• 15 cm for a deck base (using reinforced concrete);
• 15 to 20 cm for a concrete garage base (using reinforced concrete).

Concrete mixes

A concrete slab measuring 1 m² with a thickness of 10 cm (7 cm buried and 3 cm above ground) corresponds to 0.1 m3 (1 x 1 x 0.1) of concrete or 1 m3 of concrete for 10 m².

Concrete mixes per 1 m3

Concrete mix for 1 m² concrete slab (non-reinforced)

1 m3 of non-reinforced concrete requires 300 kg of cement, 830 kg of coarse sand, 1100 kg of aggregate and 155 litres of water. For 1 m² of concrete (10 cm thick) you need:

• 30 kg of cement;
• 83 kg of coarse sand;
• 110 kg of aggregate;
• 15.5 litres of water.

Concrete mix for 1 m² reinforced concrete slab

1 m3 of reinforced concrete requires 400 kg of cement, 720 kg of coarse sand, 980 kg of aggregate and 195 litres of water. For 1 m² of reinforced concrete (10 cm thick) you need:

• 40 kg of cement;
• 72 kg of coarse sand;
• 98 kg of aggregate;
• 19.5 litres of water.

It's worth noting that getting your concrete delivered by a truck will save you a lot of time. However, you need to ensure you require enough concrete to make it worth your while.

• Starting in one corner, pour the concrete evenly into the framework walking backwards as you go.
• Once you've reached the end of your slab, use a concrete finishing tool, such as concrete rake or placer, to remove air bubbles from the concrete and level out the surface.
• Make sure you overlap your passes to be sure to cover the whole surface.

Cleaning tools 

Once you've finished, clean your tools under running water. This includes your:

• concrete placer;
• cement mixer;
• bucket;
• boots.

After 36 to 48 hours, your concrete slab should be dry. 

• Check that everything is level then grab an angle grinder so you can fit your expansion joints. Also known as contraction or control joints, expansion joints allow the concrete to expand without cracking. You will need one every 15 m² or so.
• Mark out where you intend to make cuts and fit your angle grinder with a diamond cutting disc. Saw into the concrete slab to a depth of about 50 mm and insert your expansion joints. 
• Remove the timber formwork and check the side of the slab for air bubbles in the concrete.

Required skills

It's essential to be able to use a shovel, rake and straight edge. You must be able to handle timber planks (to build your formwork) and know how to prepare and pour the correct amount of concrete. To do so, you'll need to be in fairly good shape and be able to work bent over. Otherwise, just remember that you only get one shot at pouring so try your best if you want a good result!

Time required

8 hours approx., depending on the size of the slab

Number of people required

2 people

Tools and equipment

• Chalk line
• Straight edge and measuring square
• Spirit level or laser level
• Wood saw, hammer and bolt cutters
• Builders bucket, wheelbarrow and cement mixer
• Tamper or plate compactor
• Sub base (crushed stone + sand) and timber planks
• Dampproof membrane
• Steel reinforcement (optional)
• Cement + gravel + sand or concrete mix
• Concrete finishing tool and 230 mm angle grinder with diamond cutting disc

Safety and workwear

• Hard hat
• Safety shoes or wellies
• Work clothes
• Eye protection
• Protective gloves
• Hearing protection

Please note: this is a non-exhaustive list; be sure to match your personal protective equipment to the job at hand.