Plenty of new houses in Australia sit on a concrete slab. It acts as the floor, part of the structure and part of the home’s termite defence all at once. Because it is poured once and then covered, mistakes are hard to fix later. A little knowledge before the pour helps you ask better questions of your builder and engineer.

This guide covers the main slab types and what helps a slab perform well. It also looks at insulation, thermal mass and lower carbon concrete mixes. The facts here come from the Australian Government’s Your Home guide to concrete slab floors. Your own design will depend on your site, so treat this as a starting point.

The main types of concrete slab

Slab on ground is the most common type of slab, according to Your Home. It is poured straight onto the prepared site. Suspended slabs are also poured on site, using formwork that is either removed or left in place. Precast slabs are made off site and craned into position.

Precast slabs can be made from conventional concrete, post tensioned concrete or autoclaved aerated concrete. These systems are normally designed by an engineer. They are then installed by builders or specialist subcontractors.

Waffle pod slabs

A waffle pod slab is a slab on ground built over a grid of foam pods. The pods are interconnected blocks of dense closed cell foam. Your Home describes this type of slab as cost effective. It prefers pods made from XPS or PIR foam. Expanded polystyrene breaks up easily on site and causes pollution, so it is not recommended.

Your site decides a lot

Ground conditions shape the slab design more than many people expect. Sites with reactive soils can be difficult to build on. Steep sites may make a slab on ground impractical, so another floor system may suit better. In cyclonic areas, a slab on ground also helps anchor the whole building.

Earthworks come first, and they set the base for everything above. If your block needs cutting or filling, read our guide to earthmoving and site preparation before work starts. Any digging brings safety risks too, so it pays to know the basics of excavation safety as well. Proper preparation of the foundations is one of the main ways to control cracking later.

Getting the pour and curing right

Concrete gains strength slowly and needs care in the first few weeks. Your Home lists several things that help a slab perform. The water content needs to be right, because excess water causes cracking and weakens the slab. The concrete is usually compacted by vibrating it as it is placed.

Curing matters just as much as the pour. A curing membrane is used in the first 3 to 7 days. Before heavy loads go on, Your Home suggests allowing 28 days for the concrete to reach design strength, or the period your structural engineer specifies. Rushing this stage to save time can leave you with a weaker floor.

A structural engineer should always specify the final mix. That is not a step to skip on a home build. With reinforcement correctly designed and placed and the concrete placed and compacted well, a slab can have an almost unlimited lifespan. That is a strong reason to get the details right the first time.

Termites and the slab edge

A slab is part of a home’s termite defence, but only if it is built well. Care is needed during construction to prevent cracking and termite entry. For minimum termite risk, slabs should be designed and built to Australian Standards. Where a brick cavity extends below ground, a sheet barrier such as a polyethylene vapour barrier impregnated with termiticide, a damp course or both can be used.

The slab edge is the easy part to check. Leaving at least 100 mm of slab edge exposed above the ground or paving is a simple form of edge treatment. Garden beds, paving and soil built up against the edge later can cover it. Keep this in mind when you plan the landscape design for your home, so new beds and paths stay below that line.

Insulation and thermal mass

Concrete slabs offer high thermal mass. That means they take up heat and release it slowly, which can steady indoor temperatures. Thermal mass is useful in most climates, particularly cool ones. It only works well when combined with other passive design principles.

Insulation helps the slab do its job. Your Home says slabs should be insulated underneath in cold climates and can be insulated on the edge in all climates. Edge insulation stops warmth escaping through the sides of the slab. In colder areas, insulation underneath separates the slab from the cold ground.

Floor finishes make a difference too. Polished concrete and tiles leave the thermal mass exposed. Carpet insulates the slab and reduces the benefit. If you tile, avoid rubber based adhesives, as they have an insulating effect.

Slabs also suit long cycle slab heating, provided the system is efficient. In hydronic systems, warm water runs through pipes embedded near the top of the concrete. Electric heating coils are attached to the reinforcement before the slab is poured. This has to be decided early, because nothing can be added once the concrete is down.

Lower carbon concrete

Conventional concrete has high levels of embodied energy. One way to reduce its impact is to replace part of the cement with waste products called extenders. These include fly ash, ground blast furnace slag and silica fume. Some blended cements contain up to 85 per cent extenders.

Recycled aggregate is another option. A common approach is to use 30 per cent recycled aggregate in typical structural concrete. Your Home notes no noticeable difference in workability or strength at that level. Ask your builder and engineer what their suppliers can offer.

Slabs in commercial buildings

The same basics apply to warehouses, workshops and other commercial floors. A slab that cracks or settles causes trouble for racking, forklifts and floor markings. Planning the layout before the pour helps place drains and services where they are needed. Once the floor is down, good warehouse line marking helps keep people and vehicles apart.

Frequently asked questions

How long does a concrete slab take to cure?

A curing membrane is used in the first 3 to 7 days after the pour. Your Home suggests allowing 28 days for the concrete to reach design strength before significant loads go on. Your structural engineer may specify a different time for your slab. Follow their advice before heavy work goes ahead.

Is a waffle pod slab a good choice?

Waffle pod slabs are a cost effective type of slab on ground. Whether one suits your block depends on the soil and your engineer’s design. Your Home recommends XPS or PIR foam pods over expanded polystyrene. Expanded polystyrene breaks up easily on site and causes pollution.

Do concrete slabs crack?

Some risk of cracking comes with any concrete. Too much water in the mix causes cracking and weakens the slab. Good foundation preparation, compaction and curing all help control it. Cracks also matter for termites, so care during construction is important.

Will a slab floor be noisy?

Airborne noise is not transmitted through a slab. Impact noise is a different story. Footsteps and high heels on hard floors can carry to the room below. Keep this in mind when choosing finishes for rooms on upper levels.

A slab is one of the few parts of a house you only get one chance at. Talk to your builder and engineer about soil, insulation, termite protection and the mix before the pour. Those conversations cost little and can save a great deal later.