Cutting out a concrete floor to lower it

4 Apr 2021
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United Kingdom
We are installing an MOT bay.

We are doing it ourselves with the help of a builder.

I will obviously discuss all this with him but I am trying to work out ROUGH costs and he is away at present.

We have a concrete floor that is approx 6-8 inches thick.

We need to make a bay approx 7 inches lower than the current floor. The lift has to be recessed in for compliance purposes. This will be 6 meters by 3.5

In my head we are going to cut the slab out with a petrol cutter, cut it and smash it and get it removed.

Then I'm looking at sub base, Type 1? The aggregate company calculates 6.9m/s for a 150mm deep bed.

Regarding rebar, would you double layer it or one be enough? I was pricing up the mesh sheets. I'm not sure how it would attach if possible to the exisiting floor as we will be going lower than the floor is deep.

I also can't get my head around how the damp course would work.

Any thoughts/views?

Please believe me, when it comes to it, although we will be labouring, it will be skilled builders on site! I'm just being impatient with ideas.
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You need to know if the existing floor is ground bearing or suspended for a start. Most likely ground bearing, but it may have been designed to span nominally over soft spots etc. Cutting a large hole into the existing slab may affect its structural integrity.

You will also be adding point loads close to the edge of the cut slab (I assume it's a 2 post lift you'll be using?) which will concentrate high loads in one area of the slab.

Ideally you'd get the slab designed to ensure continuity with the existing slab. This might mean scabbling back the concrete 500mm or so past the cut reinforcement and then installing bent rebar to join the existing slab to the new one before shuttering and pouring the concrete.

I don't know how the existing slab is waterproofed, but if there is a DPM I'd suggest lapping a new DPM with the existing - not sure exactly how much lap would be required.
Brill thanks.

Sounds a little out of my depth so I will just have to wait for the skilled guys. It is a four post lift.

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