Offsets for a concrete slab

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Matthew Perry
Matthew Perry's picture
Offsets for a concrete slab

Hi Bill,

Could you please explain to me what the offsets should be for a slab at the bottom of a 4 foot crawl space wall?

I went to floor system and draw - slab. and then put my slab in.

Now, from what I understand, I have to edit the slab and give it an offset. I'm not sure what it should be.

The top of the foundation (C.M.U.) has an elevation of 0'-0". Grade is 12" below that.

The top of the concrete footing which is also the bottom of the 3" slab sits at elevation (negative)-4'-0"

So, would my offset be -3'-9" ? (top of slab to top of foundation wall)  Or is it from grade?

Any help explaining offsets in this situation would be greatly appreciated...

 

Thanks.

 

Bill Wimberley
Bill Wimberley's picture
Offsets for a concrete slab

Slab offsets are measured from the bottom of the concrete, excluding any footings that may be present. So for instance if you have 3" of concrete and you want the top of the concrete to be at 0" you would give the slab a 3" offset down.

Assuming a 3" concrete slab and you want your slab to sit on top of a footing whose top is 4' down you would give the slab an offset of 4' down. This would put the bottom of the slab right on top of the footing which is 4' down.

Bill is the owner and maintainer of SoftPlanTuts.com

Chuck Griffith
Chuck Griffith's picture
Offsets for a concrete slab

Matt

the 0.0 offset is from the bottom of the walls, ie if all you have placed on the drawing

is walls then the bottom of the wall will be 0.0, but when you add a floor system, then

the offset for the wall will automatically go up to the height of your floor assembly.

foundation walls work pretty much the same, the 0.0 mark is at the connection of the

wall to the footing. So if you want your mud slab to sit right on the footing, set it's off

set at 0.0, if all the walls are the same height and elevation.  But if you have different heights,

such as a 8' basement and a 4' crawlspace then both the shortwall and the mudslab should

be set at 48" up to align the tops of the foundation.  Be sure to set a reference point on

your initial drawing before advancing to another level (main floor saved as foundation)