Frame a basement wall over a floating slab?

[Tom Sestito]

A friend and I want to put up a few walls in his unfinished basement. Here in Wpg we build houses with floating slabs, and I know there are a specialized framing details that go along with this.

I get the bottom plate spacing etc but I have a few specific questions:

- Is there an easy way to tell if the slab is floating vs. fixed? I suppose there might be some history on the walls but other than that I'm not sure.

- How much spacing between the bottom plates? I saw a google image showing 3" but that seems like a lot.

- Any extra structure required above the wall at the main floor joists? Seems to me that there's a lot of weight now in lumber, drywall etc hanging from the joists. Mind you this house is only 10 yrs old and the basement already has roughed in plumbing so the builder likely planned on someone finishing the basement.

 

[The Cobbler]

the house has a footing below frost level, the basement floor is poured within the foundation walls , correct?

I don't see the basement floor moving as the temp will be above freezing and fairly constant .

 

[rayra]

If his 'floating' slab is actually floating, you've got bigger trouble than how to frame some walls.
You frame them normally, far as I know. Slab shouldn't be going anywhere. And ought to be well below the frost heave line.
Hell I've got more chance of my 'floating' slabs floating here in earthquake country, then you should have there.

 

[Tom Sestito]

https://goo.gl/images/MRRYDh

This is what I mean. This is how we build new basement walls in Wpg.

 

[Engineer61]

It isn't frost heaving they are trying to avoid, it's expansive soils. Certain types of clay that expands when it gets some moisture in it (typically spring time snow melt), then shrinks when it dries out (the rest of the year). Moves MUCH more than simple frost heaves, and floating slabs that aren't tied to the foundation walls along with that type of framing is how it is handled here in Colorado; along with "piers" sunk to bedrock tied into the foundation walls.

 

[ItsNemo]

I'm up here in the north too and have never see anything but framing directly on the floating slab. Yeah the slab is poured separately (hence floating) but it's inside the foundation and below frost level so it shouldn't be moving. There should also be plenty of drainage around the foundation so moisture under the slab should not be a problem.

As such, frame on top like a normal wall but use sill gasket or vapor barrier underneath the bottom plate so you don't have wood in contact with concrete (and don't use pressure treated indoors). I have never seen any house in Canada with anything different in the basement short of some with dricore or similar subflooring that the framing is done on top of.

 

[CraigStu]

The house we are having built is a floating slab I guess. Poured concrete foundation, gravel,PVC drain pipes, and more gravel then floor poured. Very standard wall construction. This wall is 2x6 because it is specifically a load bearing wall.
looking towards left rear by craig stuard, on Flickr

 

[The Cobbler]

https://goo.gl/images/MRRYDh

This is what I mean. This is how we build new basement walls in Wpg.

what is the reason this is done?

 

[CraigStu]

...(and don't use pressure treated indoors)....

I am wondering why you say that. Admittedly they are both garages but my friend's and mine have block foundation walls. Code requires the 2x that sits on top of the block to have plastic between it and the block, and to be pressure treated.

 

[ItsNemo]

I am wondering why you say that. Admittedly they are both garages but my friend's and mine have block foundation walls. Code requires the 2x that sits on top of the block to have plastic between it and the block, and to be pressure treated.

Generally just because it's full of chemicals...not as bad as days of old but still not ideal and not necessary when you use vapor barrier under regular SPF lumber.

 

[The Cobbler]

I used strips of 1" xps under my bottom plate

 

[nharris300]

We have floating in colorado below is how I did mine.

Sent from my SM-G960U using Tapatalk

 

[Stoshu]

1 1/2" gap between the plates will be plenty of space unless he is in Tuxedo. I have seen slabs move 5 or 6 inches over there. I have been framing in Winnipeg for 27 years. Check the gap under the frost walls, if it's still there the slab is not moving.

 

[DPG]

It isn't frost heaving they are trying to avoid, it's expansive soils. Certain types of clay that expands when it gets some moisture in it (typically spring time snow melt), then shrinks when it dries out (the rest of the year). Moves MUCH more than simple frost heaves, and floating slabs that aren't tied to the foundation walls along with that type of framing is how it is handled here in Colorado; along with "piers" sunk to bedrock tied into the foundation walls.

This.

 

[Tom Sestito]

1 1/2" gap between the plates will be plenty of space unless he is in Tuxedo. I have seen slabs move 5 or 6 inches over there. I have been framing in Winnipeg for 27 years. Check the gap under the frost walls, if it's still there the slab is not moving.

Thanks. He’s in the south, past the university close to Pembina and the Perimter.
Appreciate the advice, this aligns to what other have been telling me over the last few days.