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A reminder about slabs

In the last few weeks, several building permit applications have been delayed because the applicants weren’t aware of the rules and regulations regarding site-built slabs on grade.

Although these are a common way of constructing small garages (especially in rural areas), building permit structures – this includes a house/cottage or large garage – cannot be built on a slab in the same way as a small accessory structure.

The key is that all building code compliant construction requires a foundation. The foundation is what ultimately carries the load of the structure above and disperses it to the ground below. In general, foundations have to be four feet below grade to avoid the perils of frost heaves, which can cause considerable damage over time. (This includes decks attached to houses: our staff have seen decks on deck blocks rise and fall up to eight inches due to the power of frost heaves, and these forces can rapidly destroy a deck – and anything the deck is attached to. It’s why we constantly rant about not using deck blocks for deck construction if the deck is attached to any building.)

Otherwise, foundations require a frost wall with a minimum depth of 4’, sonotubes supported on either a “Bigfoot” style foundation or a site-built foundation capable of handing the loads of the building, or screw piles.

In order to submit a plan relying on a slab-on-grade foundation, an applicant must usually have a plan approved by a professional engineer licenced to work in the province, although there are exceptions. One exception is for building-code garages of less than 55 m2 (592 square feet), the other is if the slab can be poured directly onto solid rock – which, when you think about it, makes a lot of sense.

In some circumstances, a building inspector is allowed to waive the minimum foundation depths when the soils of the area are suitably course and well-draining. This is a case-by-case analysis, and requires a site visit prior to the permit being issued.

In order to avoid delays, we urge clients to discuss this with a building inspector prior to applying for a permit.