Raising Floor in a Depressed Room

[TRAK.Structural]

Working on a situation where I've got to raise the floor level in an existing depressed room at a residential property. It's old construction from the 60's. Existing floor is slab on grade of unknown thickness poured separate from the perimeter walls (floating slab). Perimeter walls in the space are (2) wythes of brick, with the outer course continuing up as the veneer. Foundation under the masonry walls is unknown. In the adjacent area that has a higher floor elevation, the 2x8 joists bear on top of the masonry stem wall, so I need to raise the floor to match. Debating whether I should just span new joists on top of the masonry stem which increases the load on unknown foundations............or make a couple short wood framed knee walls on top of the slab and support joists off those. It would make line loads on the slab but probably small enough magnitude to be ok. See image below, this is the depressed room, the stud walls are exterior and must remain. Any thoughts?

 

[XR250]

I usually do the kneewalls on the slab.

 

[ANE91]

Owner wouldn’t let you dig with a hand shovel? Knee walls it is!

 

[TRAK.Structural]

Thanks for the input!

 

[jerseyshore]

I'd also vote for kneewalls while keeping in mind the direction and span of the adjacent joists. You want to match the feeling of the other space as best you can so when walking from old to new it's not that noticeable.

 

[phamENG]

I'd vote for digging up the footings and checking them for the load and spanning from wall to wall. If you need an intermediate support, cut out a strip of slab and put in a footing. Otherwise, be sure to check the slab as plain concrete. It's probably a 4" slab, so you'll get a 2" thickness for your calculation. That's not going to give you very much.

 

[jayrod12]

I'm more in the camp of trying to confirm the existing footing size. It would be odd that it would be a different size than the rest of the perimeter footing in my experience. Most home builders I've seen prefer to keep the perimeter footing one size all the way around. It removes the chances for errors.

I'm not a fan of using the existing slab on grade due to the potential for differential movement of the slab compared to the perimeter foundation. You could end up with this portion of infill floor moving differentially from the remaining floor that is presumably supported on the perimeter footing and localized pads.

 

[XR250]

I'd vote for digging up the footings and checking them for the load and spanning from wall to wall. If you need an intermediate support, cut out a strip of slab and put in a footing. Otherwise, be sure to check the slab as plain concrete. It's probably a 4" slab, so you'll get a 2" thickness for your calculation. That's not going to give you very much.

Kneewalls distribute the load out pretty good so the bearing pressure is small. Much less than the car tires it has been supporting.
Many times I have to use shallow joists so they can get mechanicals under there. The kneewalls can help with that but also get in the way at the same time.
Have also done flush beams with posts on footings as a last resort if the other two cannot be made to work with the mechanicals.

 

[phamENG]

With lots of kneewalls it can work out, but I still don't like it. I've seen these go bad - poor compaction under parts of the slab, thinner than expected slabs, etc. Then you have the wood on the concrete. That problem can be detailed out, but most of the contractors doing these kinds of jobs don't pay much attention to details around here.

Much less than the car tires it has been supporting.

Really? Come on, XR. We're in America. You don't park a car in a garage. If you did something silly like that, where would you store all your crap you don't need and never use?

 

[TRAK.Structural]

With lots of kneewalls it can work out, but I still don't like it. I've seen these go bad - poor compaction under parts of the slab, thinner than expected slabs, etc. Then you have the wood on the concrete. That problem can be detailed out, but most of the contractors doing these kinds of jobs don't pay much attention to details around here.


Really? Come on, XR. We're in America. You don't park a car in a garage. If you did something silly like that, where would you store all your crap you don't need and never use?

What's the issue with wood on concrete?

Note from the picture that the 2x4 sill plate for the walls to remain hangs over the inside of the stem wall by about 1", a little more in some places. Would you just thread floor joists in between the wall studs and bear them on the existing sill plate?

Another thing I noted was that I could not find any anchors from the sill plate to the stem wall. Part of me just wants to not touch that at all because the construction just isn't great.

Kneewalls distribute the load out pretty good so the bearing pressure is small. Much less than the car tires it has been supporting.
Many times I have to use shallow joists so they can get mechanicals under there. The kneewalls can help with that but also get in the way at the same time.
Have also done flush beams with posts on footings as a last resort if the other two cannot be made to work with the mechanicals.

It isn't currently a garage, just a storage room at the back of the home. I guess putting load on the slab even though small may result in some vertical displacement.

 

[phamENG]

What's the issue with wood on concrete?

Water + wood = bad. Even if it's treated. Personal anecdote time: last weekend, I was working on replacing the flooring in a room in my house where the previous owner had the garage closed in by a contractor. I was removing baseboards at the infill wall where the garage door was, and instead of the baseboard coming off the flatbar pushed into the unpleasantly spongy drywall. Got it off, and there was mold growing behind the baseboard. Cut into the wall, and the wall cavities were covered with mold in the bottom 6" of the wall. The sole plate was treated and in good shape...but it set my moisture meter off at about 62%. So while there's no real concern about rot in the wood because it is well treated, I have a huge moisture issue that, if I'd let it go, could have opened me up to termites in the non-treated wood above that also had elevated moisture. Not to mention the stuff we've been breathing down there. So, wood + concrete needs to be carefully considered. Details can solve it, but like I said - this kind of work is rarely done by contractors that pay attention to details in my area.

Would you just thread floor joists in between the wall studs and bear them on the existing sill plate?

Part of it depends on what the wall composition is. If that's a veneer and they just filled in an old patio and put a roof over it, I'd have them rip it down and start over. But if that's a structural brick foundation wall, I'd add retrofit plates between that sole plate and the wall and bear joists on the sole plate baloon-frame style. Just be sure you provide adequate fire blocking.

 

[TRAK.Structural]

Part of it depends on what the wall composition is. If that's a veneer and they just filled in an old patio and put a roof over it, I'd have them rip it down and start over. But if that's a structural brick foundation wall, I'd add retrofit plates between that sole plate and the wall and bear joists on the sole plate baloon-frame style. Just be sure you provide adequate fire blocking.

Not sure I understand how that looks, see section sketch below