Overpour a concrete slab

[yeldogt]

I'm trying to understand how thin and proper way to pour a slab ontop of an old one?

The older part of my outbuilding is about 24" wide x 32 deep -- it was built as a garage. Typical block foundation 2x walls. When they built it back in the 60's they just graded the floor with a slope to the front a bit over 4" down -- poured concrete slab. It's sort of flat .. lots of small cracks now. It's solid. Have no idea how thick or if it has any stones under it -- I'm sure no VB.

6 or 7 years ago I got a price from a local guy to rip it out -- dig down and do a proper base -- foam insulation/ VB .... PEX in the new slab. He was around 15k.

My plan now is to have a wood floor

Currently the slab is at the top of the foundation in the back and it drops the 4+ inches over the 32 feet.

I would like lay down a VB. Over-pour the old slab -- make a flat surface. That way I can lay 2x4 on the flats w/ glue to the slab -- some power nails to hold in place. Foam between the sleepers -- floor on top. Possibly a simple radiant.

If I over pour the thin area 1" is that enough -- or do we just feather to the old slab. I don't know if it's worth trying to fill in the deep area with anything .. or just pour the concrete thick

It does not have to look pretty -- it does need to be flat

 

[ConCretin]

The risk with placing the overlay as thin as an inch is that any movement in the lower slab is likely to telegraph thru the new concrete. It's probably not a big deal if it just cracks and doesn't displace vertically but I think I'd take a different approach altogether. Rather than go to the time and expense of leveling up the concrete, I'd just shim the sleepers off the existing concrete slab.

 

[Hilltopmasonry]

You can overpour a slab, as a matter fact a concrete contractor did this in the building he purchased because the old floor was rough and nasty and impossible to roll any sort of jack on it however they went with 4 inches thick. they pull and park dump trucks on it so its good but i wouldnt think that an inch would be thick enough


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[Leaflessshadetree]

For a wood floor I wouldn't bother with over pouring the slab.
Are you trying to have a level floor, leveler floor (say 2" over 32ft), or just flat?

 

[JbTech]

The risk with placing the overlay as thin as an inch is that any movement in the lower slab is likely to telegraph thru the new concrete. It's probably not a big deal if it just cracks and doesn't displace vertically but I think I'd take a different approach altogether. Rather than go to the time and expense of leveling up the concrete, I'd just shim the sleepers off the existing concrete slab.

Respected advice here.

Thanks LL!

 

[Dzlpete]

Over pour is not needed. Just set up a level grid with strings around the perimeter, up 1 1/2” on the high side and take measurements and rip 2x pressure treated lumber and set them on edge with a few flat blocks pinned down to existing floor to side nail the new “joists” into. Lay ridgid foam in the joist bays, lay a vapor barrier over top, and fasten plywood down to joists.

 

[brownbagg]

you cant, where ever the old slab is crack the new one will crack is same area.

 

[yeldogt]

The old concrete is solid -- but it's not flat. I can only assume it was a utility pour -- get it down and smooth it out ..enough. The slope is not uniform back to front and it's not flat side to side. I could never taper 2x material .... plus 1/4 of it space would only be 1.5 inches or less.

 

[HoosierMark]

Find the highest part of the concrete, add the thickness of the 2x4 flat or on edge and that will be the rough in height of your new wood floor. Use some shims underneath as needed. Make treated shims out of deck boards, pieces of treated 2xs etc. I did this with a wood floor in a cabin I have, simple and easy. No need to pour concrete just to cover it up.

 

[yeldogt]

The only way to build a wooden frame is to place them on top of the block foundation ..jsut above the high point of the slab at the back of the space. It would work .. but, it will cost me a couple inches in height. My side walls are only 8'

 

[DFB]

I did the same as in post #9 at work we had a garage floor that was sloped 2" over 20 with the worst pour I had even seen I'm life I don't believe was even screeded certainly wasn't trowled, just spread with a rake is my best guess.

The owners wanted a level floor VCT covered floor to meet the requirements of GAP inspected clean room for produce handling.

Also like you I hade only about inch to work with at the high point as there were two existing cooler entrance doors on the interior back and sidewall. I made it as heavy duty as possible as it was commercial oriented Just as described above PT sleepers shimmed and leveled across the area. Used 6 inch wide under the ply in my case. Foam insul in between. Come out good, very sturdy and virtually no squeeks even after 7 years of total abuse now. There's been over 2 ton of material load and up to 7-8 people on it all at any one given time too.

Even built and framed off closet areas with doors for storage and electrical power services on top the sub floor covered the interior block walls with ply and installed FRP too.

 

[Jackfre]

If the wall height is the concern then I think you have to go with the new slab. The old slab, while deteriorated is probably not going anywhere further than it has, but I don't know of any decent leveler that you could pour as thin as you want given that it is the base for a new structure. I guess you have some concrete research to do.
As it is, You could shoot some lines, put the right size rot resistant blank every 16-24" under a 2x4 laid flat and your sub-flooring, with a generous application of PL-400, over that. It will be tedious laying it out, but that is the way I'd go about it and just leave the old slab alone. I think that will give you maximum ceiling height.

 

[TractorJeff]

Over pour is not needed. Just set up a level grid with strings around the perimeter, up 1 1/2” on the high side and take measurements and rip 2x pressure treated lumber and set them on edge with a few flat blocks pinned down to existing floor to side nail the new “joists” into. Lay ridgid foam in the joist bays, lay a vapor barrier over top, and fasten plywood down to joists.

As it sounds like he is creating a finished room similar to a basement.
The Vapor Barrier goes on top next to the Plywood and not on the bottom next to the Concrete?

 

[Lynden]

Concrete slabs can be leveled using a self-leveling concrete overlay. Lots of good information on ConcreteNetwork.com.

"Self-leveling overlay systems for concrete floors offer numerous advantages. They can correct uneven floors, repair damaged concrete, and provide a smooth and durable new surface for decorative treatments. These flowable polymer-modified toppings have the ability to self level without troweling, making them a quick solution for smoothing and leveling worn or uneven concrete."

https://www.concretenetwork.com/overlays-resurfacing-buyers-guide/self-leveling.html

https://www.concretenetwork.com/fix-concrete-repair/factors.html

https://www.youtube.com/results?search_query=bob+harris+concrete

 

[DFB]

We were going to go with the self leveling thinset refinisher route on my job at first. In fact had ordered the stuff about dozen bags and the bonding agent. I got started in one small corner but my boss pulled the plug on that idea fast. Would have took a long time to get the results we needed things were so rough. Got to move quick too.

 

[memphisnate]

There is a build that's similar that would probably be beneficial to look at: https://www.garagejournal.com/forum/showthread.php?t=393264

 

[paredown]

We were going to go with the self leveling thinset refinisher route on my job at first. In fact had ordered the stuff about dozen bags and the bonding agent. I got started in one small corner but my boss pulled the plug on that idea fast. Would have took a long time to get the results we needed things were so rough. Got to move quick too.

I've never worked with SLC on concrete, but have for tile jobs. The first thing that you find out is that it isn't actually self-leveling.

The second thing that you find out is that you have move very quickly to get the stuff in place--it is a two or three person job to get a nice pour and level. You need to have all the materials out, staged, water for mixing--all ready, and someone good doing the mixing...

 

[NUTTSGT]

The thinnest spot on my floor, for the new concrete, was 3 1/2" and the deepest part was 6 or 6 1/2". I did, however, cut, break the front area down to the footer and add rebar. It's near the front of the refurb thread.

 

[DC73]

Concrete slabs can be leveled using a self-leveling concrete overlay. Lots of good information on ConcreteNetwork.com.

Self-leveling compounds generally have a maximum thickness of pour which can vary quite a bit by brand and even by products within the same brand. SLCs require a bonding primer be applied to the existing surface. SLCs are generally not wear rated and so must be protected after curing. Not a problem for the OP since he's putting down a wood floor.

Ardex would be a good choice. They make a variety of SLC products.

DC

 

[Ironcrow]

I'm looking at a similar problem. Want to poor over an old floor. In my case the old floor has some spalling, but is level, no sags, cracks, heaves, etc. It is an old enclosed patio. So the floor level needs to come up about 7 inches. I want to do under floor heat and ceramic tile for finish. Definitely prefer concrete to put tile on. No moisture issues, trivial bit of effervescence only. So, thinking vapor barrier, couple inches of rigid foam, tie the PEX to wire mesh and place 4 inches of concrete?