Home>Discussions>BATHROOMS>Cutting into a concrete slab for remodel?
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Cutting into a concrete slab for remodel?

Home is slab on grade, no evidence of post tensioning, nor any existing cracks in drywall.
We need to remodel the master bathroom. There is an existing garden tub and an adjacent shower stall.
We would like to remove both the tub & shower, and convert it to a large roll-in shower.

One contractor wants to cut out a 39" x 89" section of the slab, excavate a few inches, move the drains, then pour concrete into the opening, so that the top of the sunken area would be a few inches lower than the existing slab. He claims he will use rebar to anchor the new "sunken" area slab to the existing slab. He then wants to use a prefab, pre-sloped, pan which he will drop into the sunken area, with the entrance area at the level of the existing slab.
Then based on using Schluter Kerdi for the walls, extend the kerdi over the prefab pan, with 2x2 tile over.
The long dimension of the cut would be parallel to the exterior wall, and about 7'6" from the edge of the slab.

Other contractors advise against cutting the slab as they say it could lead to major cracking of the main slab in the future. They say that cutting the rebar that's in the floor is not a good idea. They also say it would be difficult to prevent leaks where the main slab meets the sunken area. Basically, they don't want future liability for slab damage.

:confused: What say you all? Is it acceptable to cut the section out or should we look for a better plan, possibly doing away with the roll-in concept and using a conventional curb?

Thanks in advance.

Re: Cutting into a concrete slab for remodel?

It's hard to say without knowing how your slab was poured. But, keep in mind that the slab and the footing is one pour. Therefore, and here is where the danger is, cutting into the slab will undoubtedly compromise the integrity of the footing. Future possible leaks is another problem.

It's also true that by cutting the existing re-bars you will weaken the slab.

My recommendation: find another bathroom design. Let us know.

Re: Cutting into a concrete slab for remodel?

Been there done that more than a few times. First, most home slabs have rebar only at the perimeter since that's where the house load is carried. Some home slabs don't even have wire mesh in the pour. Most of the time this works just fine provided the slab was laid on a properly prepared base (which is NOT common in slab homes but again it's usually good enough). Can cutting a slab initiate a crack? Yes, especially on a poor base or near the edge but this almost never happens with a good slab. For all that I've seen more than ten uncut cracked slabs for each one cracked possibly due to cutting. I wouldn't worry about it myself, especially if the new pour ties in with rebar doweling. A larger concern for me would be groundwater ingression, likely with slabs set in a depression, where high water tables are present, or where natural soil drainage is poor and rainfall heavy. That can be somewhat alleviated with proper technique but I'd rather not cut in those cases- otherwise you don't often have a problem. And when you must, you cut- it's either that or replace the slab in it's entirety or a large part of it.

The center of most residential slabs is poured at a nominal thickness of 4". Actual measure is usually closer to 3 1/2" unless the inspector is sharp. With the rest of the slab undisturbed you can usually reduce this in non-house-bearing areas to 2" without any problems as long as you don't induce cracks while doing it. Undamaged, 2" will support a full tub. So you can probably safely remove 1 1/2" of concrete from the top of the existing slab and work with that never having a problem. Lots of cuts with a diamond blade in a skilsaw will get you there, break each 3/4"-1" wide 'sliver' off by hitting nearly horizontally with hammer and chisel. Carefully work the bottom similarly. Do not hit downward. Now you've got your depression. If more depth is needed for piping use a similar technique; hopefully you won't break through but if you do it's not the end of the world. Cut what and where you must.
Once the piping is in place, brush on a bonding agent (like Acryl 60) on the exposed chipped slab, then pour a skim coat of grout (high strength concrete without gravel) to finish level and flat at the needed elevation. Once that's cured you're good to go.

With existing residential slabs you cannot know how thick they are without drilling or cutting into them. You cannot know how good the soil base under them is. You cannot know by sight where the rebar is or whether wire mesh was used (but a metal detector will tell). You're guessing and hoping you guess right. With knowledge most of the time you'll be right but you can never really know for sure unless it was you who laid the slab in the first place. With commercial and industrial slabs there is closer oversight and more certainty (and usually drawings) to indicate what's in the slab and where. As I've said here before, most residential builders know almost nothing about concrete so you have to operate on the assumption that any cost-cutting measures possible were taken and that things are minimal at best. To be honest I've never had any problems with cut slabs but I've known some knowledgeable guys who have. None were major issues but anything can happen and it sometimes does. That's what insurance is for so as long as the contractor knows his stuff and is insured well enough to restore a badly cracked slab, that's the best you can get. Go for it.


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