How to lay a concrete slab

You can lay a concrete slab yourself with a few basic tools.

We’ll tell you how to lay a concrete slab in this post.

Before you start

Decide if the concrete slab you want is small enough to be completed by one person or if you require a professional to get the job done.

Generally slabs up to 3m2 can be done by one person, anything bigger will get away from you and requires more hands to lay the slab properly. Also if the concrete slab requires a specific grade of concrete, a professional is required.

You can do garden shed slabs and small paths easily with no help.

Once you have decided where you want your path or slab, measure the job and calculate how much concrete you will need. As an example if you are going to lay a 3m2 slab, you multiply 3m x 3m x 100mm, which equals 0.9m3.

There are many sites that have concrete calculators you can use to estimate your requirements.

A general guide for deciding which method of mixing concrete is right for you is. Concrete up to 0.1m3 can easily be mixed with bags of premixed concrete you mix yourself. For concrete slabs between 0.1m3 and 0.5m3 consider mixing the materials yourself, using 1 part cement, 2 parts sand and 3 parts stone. Slabs over 0.5m3 should consider getting pre-mixed concrete delivered by truck.

Tools required

Metal or wood screed
Wooden float
Magnesium float
Steel trowel
Edging trowel
Jointing trowel (groover)
Spirit level
Tape measure
Shovel
Turf cutter (if required)
Mattock
Measuring jug
Hose
Broom
Bucket
Hammer
Timber saw
Carpenters’ square
Rake
Wheelbarrow or concrete mixer
Tamper or plate compactor for compacting the base material

Materials you will need:

Bags of Concrete Mix 20kg or Bags of cement, sand and stone
Lengths of timber for boxing/formwork
Timber stakes
Clean water
Reinforcing mesh (& bar chairs if required)
Double headed or clout nails for easy removal
Road base or crusher dust to bed slab

Prepare your site

Peg out the area you want to concrete and remove any grass from the area. Dig out the area to the required depth of concrete wanted. 100mm is the usual depth of a concrete slab. As most sites are not level, you will need to work out areas that need building up and areas that need to be dug out. Roughly dig out the area and check your levels.

The easiest way to work out your levels for your slab is to build the boxing for your slab and use this to help find your levels. Boxing for concrete slabs is usually 30mm x 100mm wet pine. Once you have dug out your area, place the boxing down and put your level on it. Raise the boxing in low areas and fill and dig out more if the boxing is to high.

When your boxing is level, secure the boxing onto the ground by driving wooden stakes into the ground next to the boxing and nailing it together. Fill in any low areas in the slab area with sand or crusher dust and tamper down to ensure a stable base for your concrete slab. You can line the slab area with black plastic but it isn’t necessary for paths.

Place your reinforcing wire on plastic chairs so that it is in the middle of the slab thickness and not resting on the ground. Reinforcing wire helps stop concrete from cracking structurally.

Mix your concrete

If your using premix bags of concrete, grab your wheelbarrow and empty 2 or 3 bags into it. Dig a well at the front of the wheelbarrow in the materials and tip your water into this well. Then mix in the dry ingredients into the water, continue until all the dry material is wet. Never use to much water as this will weaken your concrete mix. Your better off adding small amounts of water until you get your concrete mix to the right consistency instead of to much from the start.

You can use a shovel, rake or hoe to mix your concrete.

When mixing cement, sand and stone, use a shovel and measure out the number of shovel fulls to get the right ratio. For example, 2 shovels of cement plus 4 shovels of sand and 6 shovels of stone. Another way is to buy the sand and stone already mixed and adding it to your cement.

Before placing your concrete wet the area so that the ground doesn’t suck your concrete dry and gives you more time to finish your slab properly.

Lay your concrete slab

When you have mixed your concrete and are happy with the consistency empty your concrete mix into your boxed out area. As you place your concrete, screed it level to the top of your boxing, filling up any low areas and re-screeding to get it level. Continue filling your boxed area until it is completely filled and screeded level.

You now have to wait for any bleed water to evaporate from the concrete before finishing the slab off. Once the bleed water has gone you need to float the slab with your wooden float to smooth out any ridges from the screeding process. A metal trowel can be used if you want a smooth finish, however this will be very slippery when wet. To provide a textured finish use a broom to give your slab some rough surface.

Use a edging trowel around the whole edge of your slab to compact the edge and give a finished look.

A jointing tool is used on large slabs to control cracking. Jointing is usually at 1800mm intervals and generally 20mm deep.

Allow your concrete to dry for a day or two before removing your boxing. Keep your slab moist for 7 days by misting water over it daily. Concrete takes 28 days to fully cure. Don’t walk on your slab for 24 hours but avoid heavy loads on it for 7 days.

Got an old path or slab that needs renovating, checkout this post how to renovate your concrete