Raft slabs are a traditional construction method less common today due to the superior expertise required to execute their construction as opposed to the simpler “Kit” method of waffle pods.
We firmly believe Raft slabs are vastly superior to Waffle Pods due to the inherit strength and anchorage into the ground especially in highly reactive clays, Raft slabs rely on concrete beams that are deeply embedded into the ground where Waffle pods rely on a grid of supporting polystyrene pods and concrete to distribute the weight sitting on top of ground surface.
A major weakness of Waffle pod slabs are they are heavily dependent on the proper preparation of the surface on which they sit and any deficiency in the preparation becomes problematic, and results in subsidence of the supporting foundation which in turn is susceptible to undermining by surface and subsurface water movements over time
- Each slab is individually designed by our engineer’s providing flexibility and multiple solutions to adapt to individual siting requirements.
- Reduced site costs due to less disturbance of the site and need for removal of excavated materials, retaining walls and perimeter paving.
- Siting flexibility to take advantage of views, site slope and driveway access.
- Less environmental impact as Waffle pods are a petroleum-based product which is difficult to transport and store , off-cut management, wastage, particle escapement, and end of life recycling.
- Waffles pod slabs can be susceptible over time to surface subsidence and undermining providing rodent access and room bounce
- Well documented Waffle pod slabs failures because of lack of engineering options and poor foundation preparation
- Our clients can enjoy peace of mind having chosen a superior and reliable construction method proven over many decades .
McMaster Designer Homes chooses to use raft slabs, despite their higher cost, because they offer a superior outcome for clients.
Key Advantages of a Stiffened Raft Slab versus a Waffle Raft Slab
- Environmental impact: Waffle pods are a petroleum-based product, which can result in environmental issues like pod wastage and particles escaping into the environment. In addition, recycling or reusing waffle pods during demolition is difficult. Stiffened raft slabs do not have these issues.
- Strength and stiffness: Code-compliant designs for stiffened raft slabs are generally stiffer and stronger than those for waffle raft slabs, particularly on highly reactive sites (H1 and H2 class sites). As a result, stiffened raft slabs flex less in response to changes in soil moisture, providing consumers with added protection from future soil moisture changes.
- Minimum requirements: Waffle raft slabs are generally only designed to meet code-compliant requirements, whereas stiffened raft slabs often exceed the minimum standards due to the arrangement of the beams, which can be less than the maximum allowed due to house geometry.
- Digging-in: The edge and internal beams of a stiffened raft slab are dug in trenches into the ground, while a waffle raft slab sits on the prepared site cut. This provides advantages like founding on stronger soils, founding on soils less susceptible to moisture change, and allowing for better inspection of foundation soils to ensure the house is founded on good ground.
- Less Earthworks: Stiffened raft slabs generally require less disturbance to the overall site, and the slab is not typically constructed in as deep of a site cut as a waffle raft slab.
- Adaptability: Stiffened raft slabs are more easily adaptable to moderately sloping sites requiring deep edge rebates and the like.
- Soil Reactivity: Stiffened raft slabs can be used on extremely reactive sites, while waffle raft slabs generally aren’t suitable.
- Construction Risks: Waffle slab construction can easily go wrong without proper inputs and care from the soil investigation, structural engineer, builder, and concreting team. Stiffened raft slabs avoid these risks.
- Hidden Costs: Extras for waffle raft slabs are generally more readily identifiable and visible to consumers, resulting in added expenses. In contrast, the extras for stiffened raft slabs are often not as obvious and can be “soaked up” by the builder, resulting in lower costs for the consumer. However, not all builders who do stiffened raft slab construction may follow this practice, and some may lose out on additional costs.