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Slabs – a quick guide

It’s quite common to build garages, sheds and homes on concrete slabs – here are a few things you need to know before choosing this building method.

Most small outbuildings and the like can be built on a normal concrete slab without any special designs, however, some buildings will require a design created by a certified engineer. We call those “engineered slabs.”

Engineered slabs are specifically designed to handle larger loads as an alternative to the standard four-foot frost walls or concrete columns (sonotubes). It costs about $400 to have an engineer design a concrete slab, and engineered slabs are required for certain kinds of construction.

In general, there are a  few more requirements for municipal clients (incorporated areas) than those building in unincorporated (rural) areas.

Here's a quick guide on when an engineered slab is required, and when a contractor or homeowner can build on a self-designed slab.

Project/construction type Normal slab Engineered slab*
Garage, incorporated area <55 m(592 ft2)
Garage, incorporated area >55 m2 (592 ft2)
Rural garage/etc, truss span <32 ft
Rural garage/etc, truss span >32 ft
Minihome
Rural camp/home <604 ft2
Rural camp/home >604 ft2
Dwelling, incorporated area
* or frost wall/sonotubes to 4’ depth
incorporated = inside a town/village.

Tips and best practices:

  • Regardless of what the function of the building, it's wise to place a layer of vapour barrier under the concrete even if not required by Code. This will limit the chance of water pushing up through the concrete to the construction above. (In engineered slabs for residential use, a vapour barrier is mandatory.)
  • In general, Code requires only a 4" depth of concrete, however, it is wise to have a greater thickness, especially in areas that will support walls or other loads.
  • Floors for garages should be reinforced with a welded wire mesh at least. Also, there are requirements for a greater compressive strength concrete (4,000 PSI) in garages and carports.
  • A common practice for detached garages is to have a "sill" of concrete at the perimeter.  This is usually about 5" or so in height.