Concrete Raft Slab Guide - Design, Cost and Construction

Concrete Raft Slab

What is a raft slab and how does it compare to waffle pod slabs? In this post, we examine the design, cost, construction and benefits of a raft foundation.

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    Concrete Raft Slab Guide: Design, Cost and Construction

    If your new home construction requires a raft slab design for its foundation, would you know what was required? How is this concrete slab constructed? And how does this type of slab compare to other concrete slabs? 

    In this post, we are going to look at just that. We will look at raft slab design, construction, costs and other associated benefits.  

    Let’s begin by looking at what a raft slab is. 

    foreman spreading sub base, preparing a site for concrete raft slab foundation

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    Concrete Raft Slab Design

    The basic design of a raft slab is a concrete slab that is built onsite and strengthened by concrete beams spanning both directions. This type of slab is used in new home construction as well as extensions added to your home. 

    Raft slabs constructed in Australia comply with Australian Standard AS 2870. The beams that strengthen these slabs are the perimeter footing beams and the strip footing that run parallel to the slab itself. 

    The slab can vary in thickness from a minimum of 85mm (3.35 inches) to a maximum of 150mm (8 inches thick) depending on structure requirements. The perimeter footings and strip beams will vary in depth according to the particular set of characteristics the soil around the footing comprises. This strip footing design is also done in accordance with AS 2870. 

    That’s how a raft foundation slab is designed. Now, let’s look at how it is constructed. 

    Concrete Raft Slab Construction 

    Once the surveying equipment has determined the exact location of the home, perimeter footing can then be excavated. These footers can vary in size depending on soil conditions, but all footers start out at a minimum of 300mm (16 inches) wide. Then, reinforcing steel rods are added according to specifications and the concrete footings are then poured. 

    Once these footers have cured, a damp-proof membrane, granite or other supporting medium is added to the area where the ‘raft’ will be within the confines of the perimeter footing. Once these support mediums are sufficiently compacted and the steel reinforcements are added, the actual ‘raft’ of concrete can be poured inside the walls of the footers. 

    When needed, additives can be added to the concrete in the ‘raft’ so that it cures at the same rate as the footer. The actual curing time is 23 to 28 days. 

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    Concrete Raft Slab Cost 

    In a nutshell, the choice of slabs for your construction project comes down to a matter of requirements. Certain types of soil and other specifications may prevent you from choosing waffle pod slabs that tend to be a bit cheaper to produce. 

    That being said, raft slabs are chosen by far more contractors as the ‘slab of choice’ as the raft design tends to be more forgiving for contractors and designers alike. 

    The amount of concrete that makes up this slab design is quite a bit more than that required of its waffle pod slab counterpart. 


    Concrete Raft Slab Benefits 

    If the area under and around your home is subject to erosion, waffle pod slabs offer little protection in that event. Raft slabs are more efficient where areas of high erosion are commonplace. 

    Also, the raft slab design is cast against the ground itself whereas a waffle pod slab is cast around the polystyrene formers. If your slabs are overloaded, they are less likely to crack because they are cast against the ground. 

    Raft slabs are also a more favorable option if there is to be any type of gardening or landscaping that comes right up next to the house. 

    These are the basics of raft slab design and some benefits that are only found in that design. 


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