Types of slab foundations

[TigerfaninAR]

Here in central Arkansas I'd bet close to 90% of all homes are built on a slab foundation, at least here in the central and southern part of the state. As someone who doesn't know much at all about construction I thought a slab was a slab but boy was I wrong.

We closed on our contruction loan and are ready to begin. The only negative is our builder, who we trust very much and have seen many of his homes, builds all of his home about 30 miles away. We were talking the other day about the process as I questioned him on many things and he plans on using a T-shaped slab, think thats the term (pour footings, pour slab later). Talked to my neighbor and he has a monolithic slab. Has me slightly concerned or maybe I shouldn't be. Any big difference as far as how they work in different soils? The guy who did most of our dirt work, dug our pond etc, said he'd use a monolithic slab but I didn't know enough at the time to even know what he was talking about. Meeting with the builder today and of course all these questions will be asked but would be nice to have ya'lls opinion if you can.

fwiw, had a geo engineer come out about a year ago to take a look around and he said no expansive clay which he said was a very good thing.

 

[/pine]

a "mono" slab is where the footings are poured at the same time as the slab...sometimes referred to as a "bell footing" the ditches (and pads) are dug around the parameter and under other bearing points...reinforcing steel is placed and it is all poured at one time...inside a form...

a "stem wall" foundation is where the footings and or pads are poured then (usually) cocrete blocks are laid up forming a wall to the slab grade...then backfill is compacted inside the stem wall and the slab is poured on top...the stem wall acts as the form as the top course is usually finished with foundation blocks that create the form...


another option is a floating slab but I doubt that is what the contractor is intending...

 

[/pine]

I should have stated...the major contributing factor is how much higher than the existing grade the slab will be ...if the floor slab is going to be much higher than the existing grade monolitiic slabs are not feasible...

A stem wall allows the footings to be placed on undisturbed earth...and the only fill that is required is inside the wall....to use a mono pour the entire area would need to be filled and compacted to slab grade and the footings would be on compacted soil...

 

[TigerfaninAR]

Builder talking about a stem wall. Floor slab and grade to be same or close to it.

 

[/pine]

Builder talking about a stem wall. Floor slab and grade to be same or close to it.

If the home site does not need building up the only drawback to a monolithic pour is the form material required and the labor to set it up...the form material is reusable and should not be billed to the client...


usually the only form work required on a stem wall foundation is at the door openings...mono slabs are generally cheaper because there is less labor and material as opposed to the masonry units and labor erecting the stem walls...

 

[tommu56]

Another term is haunched slab were the load points (around the outside walls and columns inside) the slab will be thickened at these points 2 x some times 3 x the floor thickness but it is poured all at once

primer on foundations

http://faculty.delhi.edu/hultendc/A220-Week1-Footings and Foundations.pdf

another picture
http://www.wrmeadows.com/details/Waterproofing/PDF/MWP-17-HS.pdf

how to do it
How To Build A Slab Foundation

Make sure he puts vapor down with all holes sealed.

tom

 

[inveresk]

I'm with your builder. Unless there are special circumstances, I prefer to pour the foundations and kerb wall then the slab later. Structurally there's not a great deal of difference although I question why one would try to cut foundations on the slope which is often done with locally thickened slab edges (monolithic pour). Much easier to cut the foundations square. It's also difficult to compact soil that's on a slope.

Benefits of a two stage operation include not having to lay your underground drainage and other service installations prior to pouring your foundations. Sleeve the kerb wall and the drainage, power, data and water can be installed any time prior to the slab being poured. This just gives more time for these operations and makes it simpler to accurately set out where the drains pop through the floor because you have some kerb walls to measure from.

 

[TigerfaninAR]

Thanks for the replys. Talked at length with the builder onsite and we are going to do the stem walls. I misunderstood but he has built a few homes in this area in this soil and he has taken extra precautions with widened footings, extending the footings past where the walls will be (the way I understood it), and add fiber to the concrete. He's followed up with the houses he's done this way in areas that have been troublesome for foundations and all have reported back A ok.

Also comforting was that the guy who digs the footings has quit several jobs if the ground doesn't look good because he doesn't want his name associated with a job he knows going in won't hold for the long haul.