I have a 10 x 10 slab (4 inches thick) that is already in my back yard in the corner. I bought a kit to build a 10 x 10 shed on it, and then realized that would put it right up to my fence. So my thought is to essentially extend the size of the slab to a 11 x 11. This should give me enough space around it.

Since I have not really tried to attach new concrete to existing concrete before, what is the best method for this. Two of the four walls of the shed will be resting on the new portion of concrete I pour, so I want to make sure they do not pull apart over time.

Any suggestions?

3 Answers 3


Your best bet for a good finish relies on doing three things:

  • Making sure that the foundation for the extension is well compacted and onto good material
  • Cut back and scable the joint with the existing 10x10 slab so that there is a good bond when the new concrete is cast.
  • Drill and fix dowel bars into the existing slab across the joint with the extension.

The number and type of bars will vary depending on the load from the new shed but you should be fine with something like 8mm (3/8") bars at 200mm (8") centres.

You should probably note that this will give you a full structural connection and so may be too much for your particular problem unless you want to be absolutely sure that the slab and the extension don't separate.

  • Any special considerations for the surface? Seems like there would be a line/crack between the old and new. My thought is the only way to avoid that is to add a new 1/2" (?) layer to the whole surface, so it appears as one continuous slab. Obviously this is a lot more work and concrete (and $), so it may not be worth it, but there may be an easier/cheaper way that is worthwhile..
    – gregmac
    Apr 29, 2011 at 20:33
  • 1
    The dowels should prevent large shrinkage cracks etc. so that shouldn't be a problem. If you are after a good finish then a thin screed should help you avoid that.
    – Ian Turner
    Apr 29, 2011 at 21:35
  • thanks, I just finished pouring earlier today. Will see how it looks tommorrow. While forming it all up, I actually discovered the original was 6 inches thick. So drilled a 1/2 inch hole and used 1/2 rebar as dowels to tie it all together. Thanks for the suggestions. May 1, 2011 at 3:04
  • How did it turn out
    – LBJ33
    Apr 25, 2020 at 12:56

can you adjust the plans to make it a 9x10 shed? Might be easier than attaching concrete to concrete. Also, be sure to check local setback code...you sometimes have to have your out buildings several feet from the property line.


Expose the existing iron rods and weld them with new extension part. It will surely not show cracks.

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