What is a Waffle Slab?
Waffle slabs are a reinforced concrete footing and slab system constructed on ground. They consist of a perimeter footing (edge beam) and a series of narrow internal beams (strip footings) at one metre nominal centres running each way. The whole footing and slab system is constructed on top of the ground.
Edge formwork makes the sides of the slab and polystyrene ‘pods’ create the formed voids between the strip footings. When viewed from underneath, the system of internal strip footings looks like a waffle – hence the name.
Upon completion of the waffle slab house, the ground around the slab is built up by the builder to reduce the height of the slab above the surrounding ground.
Waffle slabs achieve their strength by varying their height above ground. The higher the slab above ground – the deeper the beams. The deeper the beams – the more stiffness the system has.
Benefits
-Flexible
-Relatively light, therefore less foundation costs and longer spans are economic
-Speed of construction
-Fairly slim floor depths
-Robustness
-Excellent vibration control
-Thermal mass
-Good for services integration
-Durable finishes
-Fire resistance
A Place for Waffle Slabs
There is definitely a place for waffle slabs in the construction world. Waffle slabs work really well on sites that are almost flat, natural soils or controlled fill, that have good surface strength and where the natural ground surface falls away from the outsides of the building in all directions. They work well on non-reactive sites, slightly reactive clay sites and some moderately reactive clay sites.
Waffle slabs are not recommended on highly reactive clay sites (Class H1 and H2) because the requirements for good drainage are almost impossible to achieve.
Are Waffle Slabs a Great Idea?
A definite maybe. These are the sites where waffle slabs won’t work so well:
-Soft ground conditions. Extra bored piers or screw piers are required so that the system is supported on strong ground.
-Sloping sites. Waffle slabs are built on flat sites. On sloping blocks, the ground has to be made level first by digging some of it out or filling some of it in. Problems arise when some of the dirt dug out is used as uncontrolled fill on the low side of the block. All houses, even waffle slabs, need firm, even support to all parts of the slab.
-Highly reactive and extremely reactive clay sites. These sites need stiff footing systems to span over the swelling and shrinking soils. Concrete beams get stronger and stiffer when the depth of the concrete beams increases but waffle pod void formers tend to max out at 375mm deep (so providing 475mm deep beams and ribs with a 100mm slab). Some designers try to achieve extra slab stiffness by adding more steel reinforcement. This works but the design process becomes more complicated.
Waffle slabs are a reinforced concrete footing and slab system constructed on ground. They consist of a perimeter footing (edge beam) and a series of narrow internal beams (strip footings) at one metre nominal centres running each way. The whole footing and slab system is constructed on top of the ground.
Edge formwork makes the sides of the slab and polystyrene ‘pods’ create the formed voids between the strip footings. When viewed from underneath, the system of internal strip footings looks like a waffle – hence the name.
Upon completion of the waffle slab house, the ground around the slab is built up by the builder to reduce the height of the slab above the surrounding ground.
Waffle slabs achieve their strength by varying their height above ground. The higher the slab above ground – the deeper the beams. The deeper the beams – the more stiffness the system has.
Benefits
-Flexible
-Relatively light, therefore less foundation costs and longer spans are economic
-Speed of construction
-Fairly slim floor depths
-Robustness
-Excellent vibration control
-Thermal mass
-Good for services integration
-Durable finishes
-Fire resistance
A Place for Waffle Slabs
There is definitely a place for waffle slabs in the construction world. Waffle slabs work really well on sites that are almost flat, natural soils or controlled fill, that have good surface strength and where the natural ground surface falls away from the outsides of the building in all directions. They work well on non-reactive sites, slightly reactive clay sites and some moderately reactive clay sites.
Waffle slabs are not recommended on highly reactive clay sites (Class H1 and H2) because the requirements for good drainage are almost impossible to achieve.
Are Waffle Slabs a Great Idea?
A definite maybe. These are the sites where waffle slabs won’t work so well:
-Soft ground conditions. Extra bored piers or screw piers are required so that the system is supported on strong ground.
-Sloping sites. Waffle slabs are built on flat sites. On sloping blocks, the ground has to be made level first by digging some of it out or filling some of it in. Problems arise when some of the dirt dug out is used as uncontrolled fill on the low side of the block. All houses, even waffle slabs, need firm, even support to all parts of the slab.
-Highly reactive and extremely reactive clay sites. These sites need stiff footing systems to span over the swelling and shrinking soils. Concrete beams get stronger and stiffer when the depth of the concrete beams increases but waffle pod void formers tend to max out at 375mm deep (so providing 475mm deep beams and ribs with a 100mm slab). Some designers try to achieve extra slab stiffness by adding more steel reinforcement. This works but the design process becomes more complicated.
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