Expert: Dan Griffin - 12/28/2008

Question
We live in MN and have a slab foundation.  We are looking at remodeling a bathroom in our house.  couple questions:
1. if we move the tub and toilet to new locations and have to break up the slab to relocate pipes, how far away from the outside wall must we be to avoid upsetting the footing?  It's a 12" floating footing (I had a specialist come in and confirm I didn't have any problems).
2.  in pouring new concrete - how would I tie in the new concrete with the existing slab?
3.  what is the best way to cut into the slab?
thank you!  

Answer
Steve, I'm not quite sure what you mean by a "floating" footing.  I'll take a guess that you are referencing what I would call a monolithic footing where the footing and slab were all poured at one time.  These are fairly common on metal buildings, but pretty rare here for residential.  Texas uses a lot of post tension slabs designed for extreme clay upheaval that are similar.  I thought your frost depth was  close to 48" which would seem awkward for this type footing.

If I am correct, the only limiting factor you will encounter is the thickness of the monolithic footing.  Here it would be 12-16" due to the width of a backhoe bucket.  This would be from the outside surface of the concrete.  YOu can chip and gouge your way just as far as you need to go for your rough in, but it will be miserable as the concrete will be full depth.

I have had better luck going after residential slabs with a sledge hammer or large electric jack hammer.  Breaking a rough trench leaves the sides rough and the trench edges NOT straight.  YOu will be surprised how easy it is to break a typical slab once you have the hole started.  Spend plenty of time beating the fill back into the trench and/or use sand and water, you sure don't want the fill to settle under your new slab.   I do believe in drilling into the edges of the new trench about every foot to insert steel transfer dowels to prevent settlement of the slab.  I have used Tapcons screws, short rebar dowels, scrap bolts, pencil rod drilled a few inches into the old and stick a few inches into the new.  The rough and jagged edges will help disguise the patch and hold the patch better than saw cut concrete.  Clean and moisten the edges when you get ready to pour.