Waffle Slab Foundation vs. Reinforced Slab Foundation

Waffle slab are a re-invention of an old idea in the modern world. Slab foundations are a lot more common we see them when we: walk on the sidewalk, drive on the roads, or go down in our basements. So, how does one stack up against the other, and which one is better? There are pros and cons of each which you should consider when choosing your next house or garage foundation. 

Waffle slab foundation is type of foundation which is laid on grade and poured over interconnected boxes, which once done give the impression of a waffle pattern as can be seen in Figure 1. The benefits of such a system is its very effective when places on expansive soils. Expansive soils such as smectite clays¹ are soils which swell when they absorb water. This causes an uplift force and can lead uneven floors and cracks as the floor starts to shift under a regular slab. With a waffle slab though the soil fills in the crevices between the vertical reinforcement sections. This means that the house's foundation is capable of absorbing volume. It also saves on time since no excavation is required and no engineering fill is required. Some cons of a waffle slab foundation is it's not very useful on soft ground, since it sinks in and would require extra bored holes and piers for support². Waffle slabs use a special form which are placed and have concrete poured over them. This though limits their height to 375mm max². Pouring footings for slabs can be any depth up to 475mm and if you choose to place a grade beam then the possibilities are almost endless. Another issue is since the waffle slab lies on the ground houses constructed in this manner don't do well in high wind, or cyclonic areas. When high winds blow along a house's roof, it creates uplift pressure which could cause deformation of the slab and damage to it.

Figure 1: Waffle slab foundation

Most foundations that are built though, especially in suburbs are standard slab on grade, or reinforced slab foundations. Slab on grade meaning a concrete slab placed directly on the ground is very common and if you live in a single family house you most likely have one. A reinforced slab is very similar idea except it has steel reinforcements, otherwise known as rebar running underneath it. The slab itself is thicker which costs more materials, but this also makes it very  rigid. Some of the cons of reinforced slab foundations is expansive soils can lead to problems. Once the soil expands it causes the slab to bend and deform³. This repeated deformation causes cracks, failure as well as can push the house out of level. Other costs would include backfill costs, and purchase of non-expansive soils³. The convenience of a reinforced slab are that they can be build almost anywhere. You could build your house on the side of a slope while with a waffle slab, it must be built on level ground². SIBWEST Inc. recently build exactly that, a house on a steep slope, where we incorporated reinforce concrete slabs and slabs on grade. You can check out lots of our pictures taken directly from our previous projects on our site www.sibwest.ca.

In conclusion, waffle slab foundations do have their special niche where they can be cheaper, easier and outperform traditional reinforced slab. But reinforced slabs are a much older history meaning they have been tried and tested, as well as have been widely used. SIBWESET Inc. has been doing all types of concrete work for over 10 years. We have been “Building better and & faster than the competition”.

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Citation
^1. Expansive Soil and Expansive Clay. Geology.com. [accessed 2017 Jun 1]. http://geology.com/articles/expansive-soil.shtml
^2. Cornell M. BEWARE WAFFLE SLABS. Cornell Engineers. 2017 May 7 [accessed 2017 Jun 1]. https://www.cornellengineers.com.au/beware-waffle-slabs/
^3. Ribbed Slab Foundation. Wafflemat USA. [accessed 2017 Jun 1]. http://wafflematusa.com/hybrid-systems/ribbed-slab-foundation/