What is the best way to build a Slab on grade on bedrock

peter owen Published: Oct. 31, 2019, 3:33 p.m.Last updated: Nov. 6, 2019, 12:34 p.m.

I watched your videos with much interest but wasn't sure about the "trench" type perimeter "grade beam" in terms of supporting the house structure, and the fact that I have bedrock (Lake Huron / limestone) 18" below the surface. Wouldn't a poured perimeter wall down to bedrock be a better approach - before pouring the slab?

Responses (1)

Emmanuel Cosgrove
Emmanuel Cosgrove Nov. 6, 2019, 9:39 a.m. Reply

Hi Peter

Building a slab on grade on bedrock is nice and solid, so as long as any water around the perimeter and  below can drain to daylight you’re probably going to be in great shape. You’d need to have it designed by an engineer of course so you’d need to go with whatever they recommend, but there are a couple of ways you could go about it. 

I think what you may be referring to in our video is the interior mound of compacted fill we built to allow for a grade beam / thickened edge exterior wall footing. A monolithic slab on grade can have concrete poured all at one time rather than have a second costly trip by a pump truck and foundation team. 

You will need to do something like that anyway, but given that you are that close to bedrock I don’t think I’d be considering a concrete all on top, I would sooner just do the slab. You could also pour a raft slab frost protected shallow foundation as seen in this video, all you would need to do for that would be strip the 6 inches of organic matter, lay a 6 inch bed or clean stone and then your slab on grade form kit.