floating slab with no footers

[dabob]

Hello,
I am about to build a modest 16x20 garage / shed (biggest i can do in my area)

I have a floating slab 4in that was poured recently, 3/4 crushed washed stone 24inch deep.soil here is clay.
I live in Quebec (same weather as Vermont etc)

Do I need to add footers to this thing? I would build the garage itself in 2x6 and have 12ft walls or would use 2 feet concrete block and 10ft 2x6 (if thats even possible)
thanks!

 

[Stuart in MN]

Is your slab 4" thick everywhere? Typically, floating slabs for garages are thicker at the edges to support the building (just do a google image search on 'thickened edge slab for garage' to see pictures and drawings of what I'm talking about.) If your slab doesn't have this feature I think you will probably need to add footers, but there are real concrete professionals here who can answer with more authority.

 

[dabob]

Is your slab 4" thick everywhere? Typically, floating slabs for garages are thicker at the edges to support the building (just do a google image search on 'thickened edge slab for garage' to see pictures and drawings of what I'm talking about.) If your slab doesn't have this feature I think you will probably need to add footers, but there are real concrete professionals here who can answer with more authority.

Its the same thickness everywhere :S

 

[Shiftless]

Hello,
I am about to build a modest 16x20 garage / shed (biggest i can do in my area)

I have a floating slab 4in that was poured recently, 3/4 crushed washed stone 24inch deep.soil here is clay.
I live in Quebec (same weather as Vermont etc)

Do I need to add footers to this thing? I would build the garage itself in 2x6 and have 12ft walls or would use 2 feet concrete block and 10ft 2x6 (if thats even possible)
thanks!

Am I missing something? How would you go about adding footers to a slab that was already poured? Excavate at the perimeter and build forms, install rebar, and do another pour? Is the slab already 16x20, the maximum allowable size?


.

 

[brownbagg]

yes you need a footing, so dig under the existing slab

 

[Natty Bumppo]

Not sure what's the maximum allowable building you can build on a slab in Canada, but even if a 16x24 is code compliant you would have poured an Alaskan Slab....the edges are thicker. I have very similar soils to you...I think even with an Alaskan Slab there's a good chance you're going to get a lot of movement of that slab each winter, cracking, etc.

Any chance you can change your design and build a pole barn? Cut out square holes in the pad where you want poles to go, have somebody come in and drill down 5' or so, and put your posts in the ground or use Perma-columns? Your building will be below frost line and the pad will move freely within the poles. Might be a less expensive option than adding footings to an existing slab.

 

[ConCretin]

Thickened edges are more typical and generally advisable but if you are sitting on 24" of crushed stone, you are probably fine for small wood framed structure. Driving a car onto the slab will impose more of a load than the building will.

 

[infinkc]

I had an existing slab at one of my properties, they wouldn’t build the building on it cause they needed footings. Landed up going a tad bigger and poured footers around the existing slab.

 

[Bent Handle]

Seems like an odd time to be asking about how to do the concrete.

 

[dabob]

Am I missing something? How would you go about adding footers to a slab that was already poured? Excavate at the perimeter and build forms, install rebar, and do another pour? Is the slab already 16x20, the maximum allowable size?.

Seems like an odd time to be asking about how to do the concrete.

House was bought last month and previous owner poured the concrete in may. Slab is already 16x20

Not sure what's the maximum allowable building you can build on a slab in Canada, but even if a 16x24 is code compliant you would have poured an Alaskan Slab....the edges are thicker. I have very similar soils to you...I think even with an Alaskan Slab there's a good chance you're going to get a lot of movement of that slab each winter, cracking, etc.

Any chance you can change your design and build a pole barn? Cut out square holes in the pad where you want poles to go, have somebody come in and drill down 5' or so, and put your posts in the ground or use Perma-columns? Your building will be below frost line and the pad will move freely within the poles. Might be a less expensive option than adding footings to an existing slab.

Thanks for the idea, I guess that would be doable, Any ideas where i could get more info on this?

Thickened edges are more typical and generally advisable but if you are sitting on 24" of crushed stone, you are probably fine for small wood framed structure. Driving a car onto the slab will impose more of a load than the building will.

I actually wont be driving a car there much, maybe only when i need to work on it, it will mostly be for a small 24hp john deere,storage, motorcycle and a small workshop. You think I could get without using footers or other things?

I had an existing slab at one of my properties, they wouldn’t build the building on it cause they needed footings. Landed up going a tad bigger and poured footers around the existing slab.

That could be done maybe, but I can't extend the garage more than 16x20

 

[The Cobbler]

if you can't go beyond the existing dimensions might be easier & cheaper to rip up the slab & start over with a monolithic pour with thickened edges
or cut the slab back so you can run a footing around the perimeter .

 

[bczygan]

Photos?

Sounds like you have a good base.

You don't know if any reinforcing was used in the slab?

Do you need permits to build the structure?

If not, I would just build it.

If permits are required, check with authority to see if the slab is adequate for what you want to do.

Too late for any other foundation work unless you cut back the slab and either trench or dowel in a thickened edge.

Bill

 

[dabob]

Photos?

Sounds like you have a good base.

You don't know if any reinforcing was used in the slab?

Do you need permits to build the structure?

If not, I would just build it.

If permits are required, check with authority to see if the slab is adequate for what you want to do.

Too late for any other foundation work.

Bill

Ill post photo tomorrow
Steel mesh was used when they poured
I would need a permit for the building

if you can't go beyond the existing dimensions might be easier & cheaper to rip up the slab & start over with a monolithic pour with thickened edges
or cut the slab back so you can run a footing around the perimeter .

ill look into that. thanks for the info

 

[brownbagg]

steel mesh is not reinforcement

 

[ConCretin]

I actually wont be driving a car there much, maybe only when i need to work on it, it will mostly be for a small 24hp john deere,storage, motorcycle and a small workshop. You think I could get without using footers or other things?

I was just using the example of car a car driving onto the slab to put the loading into perspective. People drive cars onto 4" slabs everyday, which puts a higher load on the slab than a wood framed shed.

Even if the slab edges were thickened, they would still rely on the stone beneath for support. If the stone can't support the slab, it's going to crack somewhere unless the whole thing is a foot deep. If the stone can support the slab than a 4" slab is fine.

Obviously, local codes might contain more stringent requirements but from a practical standpoint, I'd just build the shed on what you have.

 

[walrus]

If you need a permit and its going to be inspected not much sense asking us whether you can build on it or not. Its up to the towns building codes.



My question is why would you pour a slab like that without knowing if you should build on it or not?

 

[Natty Bumppo]

Thanks for the idea, I guess that would be doable, Any ideas where i could get more info on this?

You can hire it out, or rent a Stihl cut-off saw to cut holes in your slab. Then hire out a landscape company to come in and drill the holes. I used Perma-columns on my pole barn to keep the posts out of he ground.

https://www.permacolumn.com/products/perma-column

It might be a decent solution if you want to salvage the slab. But as somebody else mentioned, it might be easier to have the slab removed and just start over. I had a slightly smaller Alaskan slab removed when I built my pole barn. The guy had it broken up and in his dump truck in about 30 minutes.

Starting over would also allow you to pour a thicker slab in case you ever want to put a lift in.

 

[Miss the Pontiacs]

Where I live IIRC a floating slab is fine for up to 500sq’. After that you are required to put in a thickened edge that has been engineered by a professional. Expect to pay $500-1000 for permits, engineer plans and documents.
At under 500 sq’ you are likely good to go after getting proper local permits. In my area 3 permit visits were required to have the powers that be to sign off on my garage and have the additional property taxes added to my yearly statement.

 

[CraigStu]

If you need a permit and its going to be inspected not much sense asking us whether you can build on it or not. Its up to the towns building codes....

I agree. I would make an appointment and talk to someone in the code enforcement dept. If they say you need footers and you are absolutely at the max size, I'd cut a foot off a short side and a foot off a long side. No use making 4 cuts. BTW doing 2-3 rows of block above floor level is real common here. That will usually get you your 12ft ceiling height less expensively than using 12 ft 2x6s.

 

[K13]

Where I live IIRC a floating slab is fine for up to 500sq’. After that you are required to put in a thickened edge that has been engineered by a professional. Expect to pay $500-1000 for permits, engineer plans and documents.
At under 500 sq’ you are likely good to go after getting proper local permits. In my area 3 permit visits were required to have the powers that be to sign off on my garage and have the additional property taxes added to my yearly statement.

Pretty much the same in Alberta anything under 592 sqft (55 sq m) doesn't require footers just a 4" slab. Most municipalities have the information online. Probably better to check there than here.

 

[428PI]

Why would you want 12 ft walls on such a small building when not working on a car? Harder to heat in the winter and I think it would look strange.

 

[johnnyradiant]

I had an existing slab at one of my properties, they wouldn’t build the building on it cause they needed footings. Landed up going a tad bigger and poured footers around the existing slab.

That seems like a PHD kinda solution, or maybe too simple for some. I guess if that pushes the actual dimensions beyond the legal limit it could be a problem but then there are things called saws to shrink the pad before applying the solution.

I wonder if anyone would pick up the increased dimension to the point of making anyone undo it as long as it didn't encroach a set-back which are the dimensions frequently checked.

 

[PCustoms]

Why would you want 12 ft walls on such a small building when not working on a car? Harder to heat in the winter and I think it would look strange.

Part of me wonders if the OP realizes the footer goes UNDER the slab.

I feel like this is about stem walls....

 

[like2wheel]

 

[Showkey]

Floating slabs are common in small shed and garages that are not attached to an existing building. Aka stand alone building.
What is common, allowed, required or permitted in other areas is irrelevant to your project.

Even the “floating slab” has multiple meaning with a plain slab floating on the gravel bed vs full footing with floating slab on the center as the floor floating not supported by the footing.

OP ........Google floating slab for Several 100’ Images and variations.

Examples for slab on grade vs footing:
http://www2.minneapolismn.gov/www/groups/public/@regservices/documents/webcontent/convert_264724.pdf

Would agree a built up thicker edge slab on grade might be preferred but a full below frost line footing is over kill for small stand alone garage in most areas.

 

[dabob]

Where I live IIRC a floating slab is fine for up to 500sq’. After that you are required to put in a thickened edge that has been engineered by a professional. Expect to pay $500-1000 for permits, engineer plans and documents.
At under 500 sq’ you are likely good to go after getting proper local permits. In my area 3 permit visits were required to have the powers that be to sign off on my garage and have the additional property taxes added to my yearly statement.

I agree. I would make an appointment and talk to someone in the code enforcement dept. If they say you need footers and you are absolutely at the max size, I'd cut a foot off a short side and a foot off a long side. No use making 4 cuts. BTW doing 2-3 rows of block above floor level is real common here. That will usually get you your 12ft ceiling height less expensively than using 12 ft 2x6s.

Pretty much the same in Alberta anything under 592 sqft (55 sq m) doesn't require footers just a 4" slab. Most municipalities have the information online. Probably better to check there than here.

I finally managed to talk with someone from the city and they told me that under 500sq ft its not needed. So all is good thanks!

My question is why would you pour a slab like that without knowing if you should build on it or not?

As I stated before, It was poured before I bought the house.

Why would you want 12 ft walls on such a small building when not working on a car? Harder to heat in the winter and I think it would look strange.

I said I will only bring the car in when I need to work on it, and you never know what use you can have for it later on,(mezzanine etc?) better put 12 ft now than 8 and regret it later no?

 

[dabob]

Part of me wonders if the OP realizes the footer goes UNDER the slab.

I feel like this is about stem walls....

No, I do know footers are under the slab lol I was just asking for ideas (IE pole barn etc) if the town wouldn't allow it.

 

[dabob]

Floating slabs are common in small shed and garages that are not attached to an existing building. Aka stand alone building.
What is common, allowed, required or permitted in other areas is irrelevant to your project.

Even the “floating slab” has multiple meaning with a plain slab floating on the gravel bed vs full footing with floating slab on the center as the floor floating not supported by the footing.

OP ........Google floating slab for Several 100’ Images and variations.

Examples for slab on grade vs footing:
http://www2.minneapolismn.gov/www/groups/public/@regservices/documents/webcontent/convert_264724.pdf

Would agree a built up thicker edge slab on grade might be preferred but a full below frost line footing is over kill for small stand alone garage in most areas.

Yes thanks for the info, I agree it is overkill but you never know how stringent the town will be