Slab foundation for Ontario Canada

[WeekendWarrior83]

I understand that you can have a 'floating slab' with thickened edges for a foundation up here in Canada. Here's what I was able to get from the city website:

Buildings of less than 55m² (592 ft²) in floor area and not
more than one storey in height may be supported on
wood mud sills or concrete floating slab rather than a 4'-0”
deep foundation, provided that the building is not of
masonry or masonry veneer construction.

Concrete floating slabs greater than 55 m² (592 ft²) are
allowed, but must be designed by a Professional Engineer.

Does anyone know what the Ontario building code requires for the concrete slab? For instance the minimum thickness, type of rebar or reinforcing wire mesh, etc. Has anyone done a slab foundation here in Southern Ontario? Approximate cost?

Thanks.

 

[Frost]

For a small frame garage, you can use the standard details published by the municipalities. Here's the standard set of garage plans from Clarington's website (the first one I found on Google): https://www.clarington.net/en/live-...ting/standard-garage-construction-drawing.pdf

The slab detail is on page 3. The slab is 75 mm/3", and reinforcement is optional.

 

[73RR]

Have you sat down with the permitting agency folks to discuss the building that you want to have?
Unless they are dicks someone should be able to give you a checklist of requirements for area vs thickness, vs reinforcement vs stem wall vs FPSF and on and on.

They are the folks that will be handing out the permits so you might as well just start with their requirements.

 

[martyre]

thanks for info

 

[p_mori7]

I have a floating slab, and I am in Montreal.

I started by removing all organic material and then scraped out 18" to undisturbed soil.

I put down 18" of compacted 0~3/4 gravel.

The edge is thickened to 12" all the way around. The center portion is 5".

I have rebar on 3' squares, and doubled rebar around the perimeter. I also have 4" mesh.

The slab has not moved in 5 years. It does have couple of minor cracks, but no separation vertically or horizontally.

Pics in my build thread.

 

[MikeF2316]

My garage is on a 4" thick floating slab. I have no idea what's under it, it was done by the previous owner. There is that 6" mesh reinforcing it. There are a couple of fine cracks. It's been "floating" there since 1985. The slab is not thickened around the perimeter.

 

[dhumac]

Not in Ontario (anymore) moved back to NL ... but I am in the process of completing my garage here - currently in the insulation & wiring phase.

But, I am on a solid rock ledge and couldn't afford an actual formed foundation, so I went with the engineered slab. I designed it myself, completed based on the following document: https://www.cmhc-schl.gc.ca/publications/en/rh-pr/tech/2000-127E.html and then thrown at some engineers buddies of mine.

My only differences were 5" rather than 6" for slab floor, but went with 30mpa rather than 25mpa; fiber mesh rather than steel mesh, and I have 1' deep by 2' wide channel running through the middle area with rebar for extra reinforcement for my lift.

That document is a good reference.

 

[Peter Mc Mahon]

Where in southern Ontario are you building?

 

[WeekendWarrior83]

I contacted the building department and they were able to provide me with some additional information. I should be fine with a slab of minimum 3" thickness in the centre and 12" on the edge is more than sufficient - similar to what you posted Frost. I think if I go ahead with the rebuild of my garage I will go with 12" outside and 4" (maybe 5") in the middle.

@p_mori7 - Great pics! Question, when they say remove all organic matter... does that mean roots?

@Peter Mc Mahon - I'm in the Niagara region

 

[WeekendWarrior83]

Here is one of the suggested plans from the city.

If I'm seeing this correctly it's saying 6" above ground and a total of 12" along the edge.

 

[Frost]

If I'm seeing this correctly it's saying 6" above ground and a total of 12" along the edge.

No, that's 18" (12 inches below grade, 6 above). The 12 inches is the assumed depth of of the topsoil which needs to be removed to reach undisturbed soil.

In the newer standard detail that I posted, there is no dimension given because the depth varies. A foot below grade would almost definitely be acceptable for the municipality, but less may be fine too.

 

[WeekendWarrior83]

In the newer standard detail that I posted, there is no dimension given because the depth varies. A foot below grade would almost definitely be acceptable for the municipality, but less may be fine too.

You're right. I called them back and he said they prefer to see 12" below grade, but less is OK if the edge is wider (ie. 12" wide vs 8"). I guess there is some flexibility there.

 

[WeekendWarrior83]

One other concern I have is the fact that I have to take down the current garage and remove its footings. For infilling where the old footings used to be what's the best way to go, fill with gravel and compact it all? They seem to require the thickened slab edge to sit on "firm, undisturbed soil". Will compacted gravel be equivalent to "firm, undisturbed soil"?

 

[p_mori7]

Mine is on compacted gravel. The gravel was laid down 1 year before the build.

As to your question on the roots, I had only grass in the area I built the garage. There were some smaller roots from nearby trees, the excavator just tore right through them. There may be bigger roots down below where I dug, but I didn't worry about those.

 

[The Cobbler]

I don't think they permit compacted gravel as " undisturbed soil" .
how much deeper would you have to go with concrete to get to undisturbed soil?

 

[LX-Markham]

They seem to require the thickened slab edge to sit on "firm, undisturbed soil". Will compacted gravel be equivalent to "firm, undisturbed soil"?

Since there are existing footings there, undisturbed soil will likely be at the underside of the existing footings, i.e. -4'-0"

 

[67ghiaTIV]

I am just finishing up a 18 x 35 2 storey garage properly permitted just outside Toronto.
I use an Engineered Monolithic Slab because it is over the 55m max. I simply paid $300 for engineered drawings.
The center is 6in thick, the perimeter is 18in deep with a 12in foot print and then tapers up to the 6 in center.
I chose the fiberglass reinforced concrete, rebar and wire mesh, it also has 24in stryrofoam wings 16" down to keep the frost away.
The concrete was the only part of the garage I did not do myself. It didn't make financial sense to do it myself. I've only done shed pads and post holes anyways.

 

[WeekendWarrior83]

Since there are existing footings there, undisturbed soil will likely be at the underside of the existing footings, i.e. -4'-0"

Then I might as well do footings and not a floating slab... but if that's the case I probably won't even bother with a complete demo and rebuild. It won't be worth it for the relatively small increase in size I would get. I'll have to follow up with the city on this.

@67ghiaTIV - Thanks for the info!

 

[LX-Markham]

If you excavate and remove the existing footings, you can backfill with engineered fill (compacted granular), or depending on the volume you could also use u-fill (lean concrete). It doesn't require compaction. Both methods would then be capable of accepting a floating raft slab.

But once you've done all that, you might of just as well have constructed proper foundation walls.

 

[WeekendWarrior83]

Thanks for the info LX-Markham!

 

[WeekendWarrior83]

So I had a contractor come out and what he recommends is excavating and only removing the top of the old foundation (stem wall) to the required depth for the new slab. The new floating pad would then sit on undisturbed ground and/or whatever old concrete remained. I questioned this at first, but I guess it's not much different than excavating and laying the concrete on a mix of dirt and rock if that happened to be the composition of the ground. Thoughts?

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[MikeF2316]

So I had a contractor come out and what he recommends is excavating and only removing the top of the old foundation (stem wall) to the required depth for the new slab. The new floating pad would then sit on undisturbed ground and/or whatever old concrete remained. I questioned this at first, but I guess it's not much different than excavating and laying the concrete on a mix of dirt and rock if that happened to be the composition of the ground. Thoughts?

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I'm not basing this on anything but gut feeling. The existing slab will provide better support than the rest of the area will. So I'll bet there will be a crack in the new slab at the perimeter of the old one. Whether that crack will be minor or major will depend on how good the rest of the preparation is done.

 

[Radix2]

Why not use the old foundation stem wall for the whole thing ( if you keep the size) or for some if it if you add area - build your walls on that and place your new stab inside as is normally done? Then you have a nice full foundation building but save some of the cost.

Digging out old and settled stuff to replace with a surface slab seems wrong unless you really want to move the footprint.

 

[John Kay]

Then I might as well do footings and not a floating slab... but if that's the case I probably won't even bother with a complete demo and rebuild. It won't be worth it for the relatively small increase in size I would get. I'll have to follow up with the city on this.

@67ghiaTIV - Thanks for the info!

im in Thunder bay . Ont I have a rough 2by4 framed . . 20 by 39 foot garage . Its sitting on concrete pilings . The rafters are sagging ..The walls have dropped and bottoms have spread out at least 1.5 inches over 9 feet. Im stressed right out cuz it needs shingles yesterday .

I was thinking ripp of the roof . Underpin the foundation .