How thick should a concrete slab be?

Concrete driveway costs-concrete driveway on pouring day

The usual thickness of a concrete slab is around 10 cm; however, any concrete slab should be a minimum of 5 cm (a 4-inch slab) in thickness no matter what the application.

The thickness of a concrete slab will depend on the type of the slab and its purpose, as the thickness directly influences the amount of cement and concrete that needs to be poured.

How Deep Does a Concrete Slab Need to Be?

Depending on if a gravel base will be used for the concrete slab, a hole needs to be dug deep enough for the concrete to be level with the ground or slightly above the ground depending on what it is being used for.

Before pouring the concrete, the ground needs to be dug to a proper depth. To determine the proper depth the height of the concrete slab must be established. An extra 2 inches need to be added to the depth for the gravel base if the soil is normally moist or has a high concentration of clay. Most concrete slabs will sit one to two inches higher than the dirt and two to three inches below the surface of the dirt, making the needed depth of the hole 3 to 5 inches.

The soil conditions must be checked as all organic material and large hard objects such as stones, rocks and tree roots to a depth of 4 inches thick must be removed. Once removed before the concrete floor can be placed the soil and gravel underneath must be compacted using a vibratory plate or similar machines. The compacted soil and compacted gravel will ensure a level surface to ensure an efficient design process.

Concrete being spread in formwork

Concrete slabs for sheds

A concrete slab thickness for a typical shed needs to be between 7.5cm to 10cm. The existing concrete floor must be removed from the area in which the new concrete slab will be placed. Council regulations and local building codes must be checked as placements of any typical shed may need building approval.

Concrete slabs for garages

Garage slabs need to be able to hold the weight of a car, as well as the tons of other car-related items that you store in a garage. This means that a thicker slab must be made due to the significant weight of the items which will stand on it, the concrete floor foundations for a garage must be more than 15 cm in thickness.

If the garage is to park larger or multiple heavy vehicles for example a truck or a double garage for two smaller cars or heavy equipment the minimum thickness that needs to be installed for the concrete slab is 20 cm. If the weight is sufficient steel reinforcement may need to be added to the concrete to protect it from damage and prevent cracking.

The soil found in the surrounding ground needs to be taken into consideration as the concrete garage foundations can be put directly onto soil if it consists of sandy soil that drains well. However, if the professional concrete finds soil filled with clay, there needs to be a minimum of 15 cm - 20 cm of gravel placed on the surface of the soil for a base.

Concrete slabs for patios

It all matters of the size of the concrete slab, the soil which is found under the new patio and the weight which is planned to be placed on it. Depending on the weight of what will be placed on the patio the thickness of the concrete slab can differ.

  • The average thickness should be 15 cm
  • For a small amount of weight 10 cm in thickness can be used
  • For heavyweights of outdoor kitchens and BBQs, the minimum thickness should be 15 cm

It is important to check the composition of the soil found as if the soil does not drain well a base is needed. This means digging a hole, placing formwork around it and filling it up with gravel before pouring concrete.

Outdoor patio area

Concrete slabs for carports

Carports are very similar to garage concrete slabs, as they both require the strength to hold heavy equipment, heavy vehicles or two smaller cars. The good thickness to pour concrete for a carport is 15 - 20 cm and if weight is sufficient steel reinforcement may need to be added to the concrete to protect it from damage and prevent cracking.

Concrete slabs for driveways

Depending on the weight of the vehicles found on the driveway there needs to be a difference in thickness.

  • The average thickness should be around 10 cm thick
  • To be able to hold heavy vehicles on the concrete slab the general rule is that it must be at least 12 cm thick to prevent cracks

Beneath the concrete slab, a gravel base does not have to be used but materials in the soil underneath need to be prepared. This means rocks and stones must be removed and the soil can be wet and compacted flat.

Get-Rates-2

Pouring process

The concrete mix must meet compressive strength requirements (usually 3000 pounds per square inch), without measures that cause excessive shrinkage. Water is known to increase shrinkage and cracking due to the moisture of concrete. To ensure this does not occur a gravel base should be used on the surrounding ground, the compacted gravel will ensure no water or moisture affects the concrete slab.

The concrete must be distributed evenly around the concrete slab area to be as close to its final position as possible, in terms of width, lengths and depth. Consolidate low-slump mixes with a handheld vibrating plate and finish with the minimum force and strokes of the float necessary to achieve a smooth surface and good thickness.

Create control joints no farther apart than 24 to 30 times this slab thickness and no time greater than 15 feet along the width and length of the concrete slab and press it 1 inch deep into the slab. For slabs that require long joint spacing or no joints, steel reinforcement is recommended. This could increase the potential for the concrete slab to lift or for cracks to occur; however, it will hold cracks tightly to ensure good structure performance.

Talk to a professional

Before starting your project always remember to check any local building codes.

Use our Get a Quote Tool today to find the best experienced concreters to suit your project needs. Have more questions? Email our project team at projects@iseekplant.com.au.

Subscribe to receive news on projects as it happens

Get the latest updates in Major Projects and Industry News straight to your inbox