enter image description hereI have a suspended 4 feet high concrete deck 36x 12 and 4 inches thick. It is suspended every 6 feet with concrete blocks in both directions with a center beam.So I have the outside walls and a center beam 6 feet from each side. I am wanting to take it out and put in a wooden deck. How deep does the saw cut need to be to control the breakage. I am wanting to keep the brick outside wall. There is no connection of the concrete to the brick. The slab was poured on top concrete blocks with sheet metal under it and then the brick was added later from the footing. This was done so the concrete could be poured from the truck otherwise the wall would have been to high. Then the brick was installed to a height of 6 feet. I want to cut it about 18-24 inches from the brick wall so it will all fall to the inside when hit with an excavator with jack hammer. What I need to know is how deep should the cut be?https://www.flickr.com/photos/77159845@N07/51718218396/in/dateposted-public/

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    Sorry, but it's not at all clear what you're asking. If you're demolishing the thing, why is "breakage" a concern? You may need to post a diagram or photos. Imagine reading that with no prior knowledge.
    – isherwood
    Commented Dec 1, 2021 at 21:48
  • Your revisions help, but the process is what's still unclear. Maybe less important than depth is angle, so that when you crack it the portion to be removed actually falls out. Are you asking how far you can cut it without it collapsing under its own weight?
    – isherwood
    Commented Dec 1, 2021 at 21:55
  • Sure looks like a great way to get yourself killed...sawing a partially collapsed masonry mess that could easily decide to collapse further while you are sawing it. I think I have seen diamond saws that mount on an excavator arm, which would improve survival statistics.
    – Ecnerwal
    Commented Dec 2, 2021 at 16:25

2 Answers 2


I believe you're asking how to control the concrete slab breakage during demolition without destroying the house to the right of the picture or the wall to the left and back of the picture.

I would cut all the way through the slab, completely isolating the part to be removed from the parts that will remain. That way, when you hit it with a jack hammer, sledge, or whatever, any breakage will stop at your cut line.

If I understand correctly, once you've got the center portion of the slab cut free, it's basically going to be balancing on a series of piers spaced 6 feet apart. Since you've already broken up some of the slab, closest to the camera, the part to the left will cause the now free slab to immediately tip left. It will, possibly, knock over the piers (they're concrete block, right? May or may not be mortared together) and move to the left as it falls, possibly taking out that brick wall to the left of your photo as it goes.

Cutting the slab so that it isn't a multi-ton block of concrete attempting to balance (in a now very unbalanced configuration) itself on some piers and in a way that if when it moves in an unexpected way it doesn't kill you might not be a DIY job.

This site is all about DIY, and I'm a firm believer in and supporter of DIY, but this one might just be beyond DIY scope. If you bring in a professional demolition company, they will have experience in handling situations like this, and, perhaps more importantly, they'll be insured to cover any damage that might result from things going awry.


We allow control joint cutting in concrete slabs in lieu of installation of plastic or metal control joint material. If cutting is used, we require a cut 2/3 the depth of the slab.

I think the difference between what I’m doing and what you’re doing is my cut is for “temperature control” and “shrinkage control”. Temperature control can be for expansion or contraction and shrinkage control occurs shortly after the placement of the concrete.

You are trying to break off a section of concrete leaving a clean edge.

When my control crack occurs, it occurs slowly over time. Your crack will occur instantly when pressure is applied.

I still think the 2/3 rule will work, but you may want to test it in an off site location.

  • I don't care if it is a clean cut. A jagged edge is ok just so it breaks within 1-2 inches of the saw cut. Commented Dec 2, 2021 at 0:11
  • Concrete will break at its weak point. That’s why I’m suggesting a saw cut 2/3 the way through the slab.
    – Lee Sam
    Commented Dec 2, 2021 at 0:32

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