pouring basement floor before pouring walls.

[mneuner]

Hopefully someone here can help me out. I'm currently in the process of building a new home. As of right now the footings are in place, the contractor I'm dealing with, has hired the concrete guys. Apparently they pour footings, NEXT they pour the floor, LAST they pour the walls. The walls are set on top of the floor after it is poured. I will have lots of other questions but my big question is... Has ANYONE ever heard of this? According to my contractor these concrete guys do a lot of homes this way. Everyone I've talked to seem to look at me like I'm speaking another language when I tell them this...any info or help here would be greatly appreciated.
Thanks
Matt

[Rich]

If they are going to do it wrong then at a minimum make sure they have a keyway in the floor and some type of waterstop in it - bentonite tube or similar.
Now my personal opinion - tell him they're high and they need to do standard practice on placing. They're taking the easy route.. and if it was easy everyone would do it.

[mneuner]

Rich, that isn't exactly what I wanted to hear. My contractor has tried to explain this to me. From what he says and also what I see at the home site there a 2x4's laying on top of the footings, the floor will be poured to the top of the 2x4's level, then the 2x4's are removed and when the walls are poured it creates a type of keyway (I don't know much about construction and keyway is a new term to me.) Currently there is NO steel attached to the footings, (like there should be if they were pouring walls first) I thought this was odd, but I thought since they aren't pouring this the conventional way that they must know what they are doing. Basically is this going to be a disaster and something I can still correct or is this something that can be worked around???
I noticed you said if it was easy everyone would do it...Are you saying it's the easiest way but the wrong way???
Also why would they want to do it this way??? What is the advantage to them???

[Rich]

The advantages that I can see would be having a floor to work off from. It makes it very easy to form up and square the forms.. not that it's hard to square it up without the slab on. The one benefit I can see from it is that you get a true 8' wall without having to splice forms on top - but that's not difficult either.
A keyway is something like this (I'll try my hand at ascii art here )
_ _
\__/

So there is no rebar from the footing into the stemwall? The keyway makes it more difficult for water to pass through.. and along with rebar gives the wall resistance to the pressure of the backfill. Another benefit of having the slab on the interior of the walls is it locks the bottoms of the walls from exterior soil pressure as well as putting your thermal break in the wall below the slab.. and a possible area for water infiltration below the slab.
I think they are trying to cut corners at your cost. Pouring a basement is not all the difficult.. and it's not much more difficult to do it correctly - giving you the non-wet, structurally sound, basement you want. What they have proposed will work in most cases.. but what if yours is the one time that it doesn't?

[mneuner]

Rich, I appreciate your insight and attention to my questions. I probably sound a little stupid, but this is a new process to me and I'm learning on the fly. I learn little by little all the time. Here's what I learned today from my contractor. The floor, as mentioned before, is poured to the 2x4's, when the floor has set-up the 2x4's are removed and set so that the outside of the form is placed on top of them, the outside of the floor is then center in the forms and the inside of the form is placed onto the floor, I hope you can follow that. This creates the keyway and apparently it is a common keyway just acheived in a different manner.

MY KEYWAY

| form|
| |
| ___|_________________
| | floor
______|_____
| footing|________________
| |
| |


The norm



| form |
| |
| |_________________
| |
___|______|______floor
| |_______________
| footing |
| |

the keyway in my circumstance is created where this shape is made:
------------
_____|

or where footing, wall and floor all come together, basically the same as on the norm, just a different way of getting there.

I asked why these guys do this, and apparently a few of the guys on this crew worked with a large construction outfit in St. Louis and the ramrod of that came up with this, and it is used widely in the St. Louis area and making it's way to my part of Mid-MO. It at least makes sense to me now and I'm feeling relieved. Also, when the floor is poured they drill and put in re-rod to attach footing, floor, and wall together. I'm convinced.

P.S. The guy building the house had his home done this way, he also had his doubts and asked quite a few questions. He has had no problems with leakage and his house is built into a hill.

Thanks again.

[Rich]

ic what they're doing. Hmmm.. not a bad idea I guess. I still don't really see the benefit to me as a contractor though. Especially the drilling rebar into the footer - I could pay a guy 1 hour instead of 20 hours to place rebar into footer before placing it - in lieu of drilling and epoxying. If they do drill - make sure they use epoxy and don't just stuff it.

[mjpliv]

Rich, how do you suppose they handle the expansion gap?

[Rich]

Excellent point mj. They will get somewhat of a thermal break but the expansion will have to happen inside the wall... nowhere else for it to go.
I guess I'm just happy that I'm not in the OP 's situation... like I stated before 9 out of 10 it will probably work but that 10th one will be a disaster.

[mneuner]

Guys, sorry about my attempt at drawing, unless it shows up right on your pc's, cause on mine it is a jumbled mess.
I think you guys get the general idea behind this now. I also still don't understand why they didn't set the rebar right away, doesn't seem cost effective at all.
Could you explain a little more about the thermal break and the expansion joint...more new terms to me.
Thanks again, I'm learning all the time.

[mjpliv]

A thermal break is a very general term in construction. It refers to any separation between two heat conductors. In this case the thermal break would be a separation between the concrete exposed to the outside elements (footing and foundation walls) and the interior element (the slab).

The expansion joint is a space between the slab and any other concrete structures like the foundation walls, interior piers, interior concrete grade changes, etc. The slab will expand and contract with temperature changes so the space creates a little (about 1/2") room for the slab to move.