Weird existing slab condition - explanations why it looks like this?

Does anyone know what is happening here? We pulled up carpet tiles and away drywall on an interior partition to find this: the partition wall is framed down into a wall-width trench in the floor slab. Structural concrete below, lightweight fill on top. This is under an open stair.

Was this a slab height mistake?

Mar 13, 15 11:21 am

...and if it was a slab height mistake wouldn't it have been cheaper to tear out the dozen metal studs and pour a flat slab instead of form up the slab around them? I don't think the concrete was actually poured with the studs in place, I think they were inserted after, but I've never seen anything like this before ?

Mar 13, 15 11:24 am

Could they have been using sliding doors with a sill track they wanted to have flush? Seems silly.

Mar 13, 15 11:27 am
el jeffe
that's a standard detail for gypcrete overlay. is there radiant heating the topping slab?
Mar 13, 15 11:38 am

Looks like top slabs were poured over the original after the fact (evidence = left in place wood forms), question is why? Concrete on left looks new; better ask the night-shift:)

Mar 13, 15 11:44 am

Nope, jeffe, no radiant heating. Hmmm.

Mar 13, 15 12:03 pm

Is it an old building?  Did someone think rather than demo and replace an old slab they would top over it?

Regardless, its a shortcut someone took, somewhere at sometime.  Even with Radiant, you'd have a plate on the sub floor as a pour stop - I don't think I've ever seen anyone bury studs.

Mar 13, 15 12:39 pm

My best guess.  They poured the structural slab first.  So the framer was there first.  That was followed by the topping subcontractor.  Just a sequencing thing.  Not really an issue in commercial where the normal base is glued on rubber and you don't need a plate for attachment.

Otherwise, I've seen a ton of 'fixes' like this.  Slab uneven?  Use a self-leveling topping.  Curling?  Also a topping covers it up.  Even weird stuff like a riser height issue or short stair that shows up in the field... instead of sending it back, they go through various convulsions to make it work.  Even other things like if it's a structural slab with 2-way reinforcing or post tensioned you can't cut or core easily... So you use a topping to make a trench or conduit trench to run things.  Maybe you don't have access from the bottom side for whatever reason and need to conduct all the work from this space.  So you lay the conduit down and pour a new topping... hard to pull off though because of fixed elevations like stairs/doors/etc.  Also seen other oddball stuff like they wanted a floor recession but structurally, it's not easy, so you build up the rest on a platform.  Radient as someone mentioned...  Even seen it done to add acoustic mass between floors.  Who knows for sure?...  no biggy though, you just fill in the gaps. 

Could even be adventitious like if you wanted a floor box somewhere now you know you can cut a thin trench out of that sacrificial layer.

Mar 13, 15 12:59 pm
go do it

What is that 2' recess that is 90* to the main recess? Another wall?

My guess is that Jimmy Hoffa is in there somewhere.

Mar 13, 15 1:17 pm

go do it, the wall had two pay phone niches behind it, so it was a 10' long partition wall with two perpendicular  2' long stub walls coming out of it with the phones hung within those niches, all under a stair.  We're turning the wall and niches into an information desk, so cabinetry goes in over the weird trench. This was all built 14 years ago.

Mar 13, 15 1:48 pm

Construction often happens in spite of plans, which more often than not are deficient to begin with.

Mar 13, 15 3:24 pm
Olaf Design Ninja_

really not enough info to make a guess, but an interesting problem...

Option 1:  I am assuming the stairs end at the slab nicely and were pre-fabricated?  If so, they delivered the stairs to the site and realized the stair guy had taken measurements from the floor on the left.

It appears the concrete may have been poured at two (2) different times and the 90 degrees is not within the form work, so Option 1 is probably not right....

but two (2) pours is a good possibility.

Option 2: The schedule required one side to be in operation prior to the other side, so they built a wall, put up form work and poured.  Perhaps the vendor for the phone booths was still up in the air as they negotiated for the best deal....

I like that one the best if it's a building that is constantly changing.

Questions: what I can't see in the photo is the bottom track, there is one right?

At any point are we at the structural slab height?

14 years ago how much thickness was needed for Hydronic radiant heat?  It seems a bit thick for hydronic....


had this client once, he literally sounded like the godfather as he yelled at waiters all day long.  He told the owner of the shopping center his guys would pour the concrete, not the entire 100+ store contractor, just little him poured his own concrete.

maybe the framer was a real douche bag with a cocaine habit and wanted to get in and out and told the GC to go to hell, did his framing and said fuck you very much...

Mar 13, 15 9:13 pm

Here's a crazy thought. The wall was in place, sheathed, painted, and everything. The space developed some kind of moisture intrusion, high water table, localized flooding, something like that. Rather than fix the problem, some dummy - likely insurance dummy - comes in and says, cap the existing slab, and you'll solve the flooding basement. I say this because, someone just told me a story, where the insurance company told a homeowner, to fill their basement, to 4' below the floor framing, with concrete, to keep their basement from getting flooded. I shit you not.

Mar 13, 15 10:13 pm
go do it

Whatever was the issue just imagine a bunch of guys or girls standing around trying solve a problem. After much debate they all settle on this "fix"  and someone invariably walking away muttering to themselves "those guys are idiots"!


The only reason to place a slab on a slab would be for an elevation problem.

Mar 14, 15 2:09 am
Olaf Design Ninja_

go do it, I think you might be right, unless it was NYC, there are many reasons to do that, but given I think Donna is midwest and it's 14 years old I agree Elevation problem

Now where is the Elevation problem, or was?

Mar 14, 15 7:53 am
Olaf Design Ninja_

Donna can we get more info on this mystery?  or do we have a cause?

Mar 14, 15 2:59 pm

What mighty said. It's a topping slab. I doubt radiant heat. Plenty of other reasons to pour a topping slab. 

Mar 14, 15 11:30 pm

Are there floor boxes somewhere in the space? My guessis they needed to run floor outlets and didn't want to interfere with the structural slab.

Mar 15, 15 9:11 am

Donna, maybe its just the photo but the second stud from the right looks like the concrete was poured into connecting with it over the wood form? is that true?

Mar 15, 15 10:00 am

Nope, but good question BenC. The studs got all bent out of shape when we demo'd the wall; prior to demo they were neatly slotted between the (fire resistant) plywood lining the concrete trench. The wall was sheathed in fire-resistant plywood. So, now that I consider that detail, that means the wall was built on the lower slab and the topping slab was poured around it. 

Mar 15, 15 10:14 am

I have no guess to make here.  

But these are the threads that make Archinect an interesting place to visit for people who actually build things.

Mar 16, 15 5:29 pm

i agree with citizen on both accounts!

it would be interesting to find out what they were thinking whenever they did that, or what sort of existing conditions caused that to be a good idea.

Mar 16, 15 5:40 pm

It's hard to tell from the photo, but it looks like a self-levelling topping (Gypcrete, Agilia, etc).  We use it fairly frequently, and is sometimes required for fire code where I live, depending on construction type.  Nothing unusual.

Mar 16, 15 6:24 pm

Oh hell, we can get into conspiracy theories as well.  Basically, the original construction wasn't a Union job and used scabs. The Union was pissed.  Twisted some arms, broke some bones, and the concrete union landed a contract where they were able to charge a $1k per cubic yard.  The building owner thought he outwitted them though because they'd already poured the slabs.  Did that stop the Union boys?  Nope, the union showed up and poured that topping slab over the top of the other slab so they'd get their cut of the project. 

Did you notice the oddly placed urinals that were scabbed in place like an afterthought?  Redundant wiring and conduit?  Welded and bolted steel? Why do it once when you could charge double?  :P

Mar 17, 15 10:51 am

if that's the case mightyaa, wouldn't the simple case suggest (occam's razor) that they screwed it up in the first place because they hired scabs instead of union workers?  the rest is unnecessary assumptions.

lesson is, unions=good :)

Mar 17, 15 11:06 am

Good point curtkram; The original construction was obviously defective as was the design and that is what happens when you cut corners and try to save a buck or two.  So they were damn lucky the highly skilled and trained Union boys were there to sort it out for the owner who was rather shady by bypassing 'process' and trying to think he's smarter than the pros. 

Obviously, the framer's union performed the framing (or issued a very expensive review report to 'satisfy the inspectors'), thus burying the track plate was hunky dory and the walls didn't need replaced as a part of this correction.

Mar 17, 15 11:53 am

^^mightyaa- good point about the studs

Probably they exceeded the allowable deflection for the dimension/gauge provided.  Solution; "brace" them with the topping slab.

Mar 17, 15 11:58 am

i'm guessing there was a wood floor on sleepers previously. is it close to 1.5"?

it was removed and a topping was used to bring the floor level again


what do i win?

Mar 17, 15 12:30 pm

Donna - I'm just curious ... don't you have any drawings in the files that might describe what was intended when this work was done originally ?

Mar 17, 15 1:09 pm

I know you guys are just waiting with bated breath on this, so to update: I'm currently on the phone with the structural engineer who is tracking through his records.....

Mar 18, 15 1:39 pm

And......?  I'm going with topping slab to cover electrical, or to level floor from previous wood. 

Mar 18, 15 1:53 pm

i like proto's theory.  seems pouring that much concrete would have been a huge pain in the ass, so they wouldn't have done if it there wasn't a really good reason.  i would think it's too think for a self-leveling concrete application.  if they were running conduit or something, i would think they would just cut a trench.  if it's not on the ground floor, they would have poked through.

i'd give you a cookie proto, but i'm pretty sure you're not within reach.

Mar 18, 15 2:11 pm

I'm liking the Hoffa-disposal theory... though they'd've had to steamroll him first...

Mar 18, 15 2:51 pm

Result: inconclusive. I'm really sorry.

The only thing we were able to determine is that the level is intentionally made up of a structural slab with a 2-1/2" topping slab over it. We can't find any record of why the studs in this location were placed first. Being a structural engineer, my guy (who is the best PE ever, I adore him, he's so rigorous) is concerned that the stair manufacturer may have made a field decision that the landing stringer/beam needed support here, without telling anyone. If that's the case, I just yanked a structural partition out LOL! I don't think the studs were supporting anything, they were just knocked into place, but I can't help but be a little worried. So I'm off to look for shop drawings....

Mar 18, 15 3:01 pm

2.5 sounds good for electrical pans

could still be wood flrg

heat sounded good too but i'm guessing this isn't residential

Mar 18, 15 4:21 pm

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