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Discussion Starter · #1 · (Edited)

I am getting ready to retrofit my garage extension into an in-law suite. The garage is a wooden addition to a cinder block workshop. The workshop is a solid structure with a concrete slab. The garage is attached to the cinderblock but has no slab; resting on a gravel surface.

The original grade was far from being level so I ordered enough crushed stone (0-3/4”) to create a level surface. I was told that a slab could be poured inside as long as it was independent of the current structure. So I framed the inside perimeter with 2x10 and then added my crushed stone, levelled it, and then compressed it. I then had the plumbing team come in and place the required sewer and water pipes. As a final prep for the concrete pour, I stapled plastic sheeting on the 2x10 concrete stop and extended onto the gravel so it would place a barrier between the concrete and the stop, and avoid the concrete from seeping under the stop. Everything made sense and was going well until I discovered that the garage had no concrete reinforcements (footings) under the exterior walls!

1. What, if anything, should I do about the lack of footings under the exterior walls. (I live in Atlantic Canada and we get 4-5’ underground frost line, making the ground shift in spring). We had planned to build on to the exterior wall studs with a 2x2 to run flush with our 2x10 concrete stop. Now we are worried that the lack of footings will create a difference of support where the interior partition walls meet the outer walls and cause the drywall to crack at the corners.

2. We have noticed that the front wall has a 4” arc. This probably happened because this wall has a garage door opening allowing the bottom corners of the garage door opening to move with each spring thaw. Our first thought was to dig beneath the length of the wall and rectify it. But doing this would change my shower drain position relative to the wall. It’s a $2k shower so this solution won’t do.

We have thought of 2 solutions that would solve both problems.

We could dig under the structural points of the perimeter and somehow place precast footings like those made for patio posts, if that’s even possible? Using this solution, we would also cut the arc wall at the corners and the reattach the corners with a 4” extension, thereby turning the arc into a straight line and keeping my shower drain in position.

The other solution is to leave the exterior walls without footings and build new perimeter walls on top of the slab we pour. With this we would need to tear down the front wall and rebuild it approximately 4” further from the arc point wher the shower drain was measured (so approximately 8” from the corners) so that when we build the interior perimeter walls on top of our slab, the shower drain would remain at the proper distance. This solution would create the most independence between the slab (and the new construction) and the outside unsupported walls.
It would also be more costly and time consuming as well as loosing 8” in width.

Looking for any advice on this situation or future problems we might encounter with this type of retrofit.



· Naildriver
25,058 Posts
I then had the plumbing team come in
I'll be they loved you for that :eek: They should have been first.

then compressed it.
Gravel is compressed once it is leveled.

I stapled plastic sheeting on the 2x10 concrete stop and extended onto the gravel
Plastic should have been on grade before gravel.

our 2x10 concrete stop.
Hopefully it is ground contact pressure treated lumber. It looks like SPF

You may be required to dig perimeter footings and lay in rebar. Check with your building inspection department, locally, to see what they will require. I am sure the job is permitted.
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