What is a waffle slab?
A guide for homeowners. What a waffle slab is, how it is built, and what to know before and after your house goes on one.
A waffle slab is a stiffened raft slab. Like every stiffened raft, it is a concrete floor slab made rigid by a grid of reinforced concrete beams underneath, so the house moves as one unit when the ground beneath it swells and shrinks.
The difference is how the beams are formed. In a conventional stiffened raft, the beams are dug as trenches into the ground. In a waffle slab, most of the beams are formed on top of a level pad by laying polystyrene blocks, called pods, in a grid and pouring concrete between and over them. From underneath, the grid of beams looks like a waffle, which is where the name comes from. In the Australian Standard it is called a waffle raft.
A waffle slab can be designed for most residential sites, including reactive clay.
How a waffle slab is built
- Level pad. The building area is cut or filled to a level, compacted pad.
- Edge beams. On some sites the beams around the edge of the slab are dug down so they are founded in natural soil or controlled fill. Your engineer specifies where this is needed.
- Formwork and membrane. Edge boards are set to the outline of the house and a plastic vapour barrier is laid over the pad.
- Pods. Polystyrene pods are set out in a grid, with spacers that fix the width of the beams between them.
- Steel. Reinforcing bars go into the beams between the pods and around the edge, and steel mesh goes over the top.
- Concrete. The beams and the floor slab are poured together in one pour.
Why waffle slabs are so common
- Less excavation. Most of the slab sits on the pad, so there is far less digging than a slab with trenched beams. Some digging can still be needed, mainly for edge beams that must reach natural soil or controlled fill.
- Faster to build. With few or no open trenches, there is less to clean out and less delay after rain.
- Cost. Less digging and a quicker build usually make it an economical choice.
- Proven on reactive clay. The slab is made stiffer for more reactive ground by using deeper pods and more steel.
What homeowners should know
- Get the soil test early. The site classification sets the slab design and its cost. A quote given before the soil test is an estimate.
- Ask what is included. Check whether the price allows for deeper edge beams, piers, extra steel or a deeper slab if the soil report and engineering call for them.
- Look after drainage. A waffle slab relies on the ground around the house staying evenly moist. Keep surface water falling away from the house, connect downpipes to stormwater, and fix leaking pipes and taps quickly. Our article on foundation maintenance covers this in more detail.
- Be careful with gardens and trees. Keep garden beds and heavy watering away from the slab edge, and follow the planting distances in your soil report and the recommendations on your engineering drawings.
- Expect fine cracks, watch for larger ones. Hairline cracks in concrete and plaster are normal. Cracks that widen, doors that stick and floors that slope should be checked by an engineer.
How ENGPRO documents a waffle slab
Most of the avoidable cost on a slab comes from questions and rework on site. Our slab drawings are prepared so your builder and their trades have what they need before the pour.
- Service penetrations as standard. Details for plumbing and other services passing through beams and the slab are included on the drawings, so they are not left to be resolved on site.
- Typical rectification details as standard. Details for the common issues that come up during construction are included, so they can be fixed straight away without waiting for a new design.
- Clear documentation. Pod layout, beam and edge beam sizes, steel and any piers are set out clearly for your lot, which means fewer site queries, fewer delays and a lower build cost.
Common questions
Is a waffle slab the same as a raft slab?
Yes, it is one type of stiffened raft slab. The difference is that most of its beams are formed on top of the pad with pods, where a conventional stiffened raft has its beams dug into the ground.
Does a waffle slab need any digging?
Sometimes. The pad has to be levelled, and on some sites the edge beams are dug down so they are founded in natural soil or controlled fill. The engineering drawings show where.
Is a waffle slab cheaper?
Often, because it needs less excavation and is quicker to build. Where deeper edge beams or piers are required, the saving is smaller.
Is a waffle slab strong enough for reactive clay?
Yes, when it is designed for the site. The engineer sets the slab depth and reinforcement from the soil classification. Good drainage around the house matters as much as the design.
Can a two storey house go on a waffle slab?
Yes. The engineer designs the beams and adds thickenings or pads where the loads from the upper floor come down.
Who decides which slab my house gets?
The structural engineer, using the soil report, the site levels and the house plans. Your builder may have a preferred system, and the engineer confirms whether it suits your site.
ENGPRO designs waffle slabs and other stiffened raft slabs for single homes, townhouses and volume housing. See our structural engineering services.
Need a slab designed for your site?
Send us your drawings or project brief and one of our engineers will respond with a scope and fee proposal.