One structure on our property is built slab on grade. Located in the US in climate zone 6. This is a heated space which we will be removing part of. The removal will expose a lot of the slab to the elements.

Now, I assume that the slab is correctly constructed and has appropriate footings and is probably insulated underneath. (Assumed because we've had zero problems with it in 20+ years). However, we intend to cut away portions of the to-be exterior slab. The concern is that this will expose the slab to frost heave.

I am looking for suggestions as to how to protect it from this. Options I can think of include: - Adjust drainage to make sure water flows away. - Dig a new trench "footing" under the new slab perimeter, which presumably has to be down below front depth. (Not a load bearing footing, however - just a water diversion) - Dig a trench and use plastic or some other impermeable material to create a sloping surface to channel groundwater away from the slab. This would also have to go below frost depth.

These may not necessarily be mutually exclusive options. Cost is a factor, of course, but mainly I want to make sure I know how to do it correctly.

Edit - I think the foundation is a typical footing-below-frost-depth type. Not a shallow frost-protected design. This assumption is based on the age of the structure - built around 1990. According to this HUD report, at that time the shallow type was extremely uncommon in the US.

  • Is it slab on grade or is it a footing with a slab on top(you mention 'new trench')? – Chris Jul 22 '16 at 3:37
  • It is slab on grade... but I believe the way it is constructed is with a perimeter footing down below frost line (or at least deep enough for a heated slab). By cutting away some of the slab, we would also be removing some of that perimeter footing. Thanks – StayOnTarget Jul 22 '16 at 11:38
  • You expect it to be like the image on the left side rather than the right side? Also, pictures would help! – Chris Jul 22 '16 at 22:07
  • @Chris actually I am not sure if it is conventional below-frost-depth type or a shallow type. But since we would be removing a portion of the existing slab perimeter (whatever it is) I don't think it matters in that section. It will matter though in the remaining original perimeter which will then be unheated. I'll try to cook up a diagram for this question. Thanks – StayOnTarget Jul 23 '16 at 1:48
  • From what you're explaining, the only guaranteed items you'll need are a) re-bar b) structural epoxy and c) either a concrete saw, or seam foam for joining the new footing/slab edge. Don't over think it, generally match what's there, or do better. With more information I can give a full explanation which is why I'm leaving this as a comment. – Chris Jul 24 '16 at 1:41

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