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Community and Q&A

Slab edge question

user-6948949 | Posted in General Questions on


I’m hoping to find more information regarding the differences between a sloped thickened slab edge versus a uniform slab when doing frost-protected stem walls. I’ve included the two details for reference.

I’ve found innumerable references to details for the uniform decoupled slab (no slope/thickened) with recommended vertical and horizontal rigid foam, both on Fine Homebuilding, Green Building Advisor, and many others; however, finding the thickened sloped detail or any “concrete” information (no pun intended) is scarce.

At a glance, the sloped thickened edge detail seems to be a nightmare from a backfill/subgrade/rigid installation standpoint. Are its benefits (which I am hard-pressed to find any online resources attesting to such) worth the seemingly excessive additional backfill/subgrade/rigid install process? A few forum discussions mention alleviating slab curling as one such benefit but don’t go into detail if this is for code, best practice, or more of a commercial application for heavy traffic (as opposed to interior residential space).

Any information would be much appreciated on this subject.

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  1. Expert Member
    RICHARD EVANS | | #1

    My understanding is that you don't need a thickened slab edge if you have frost walls. Combining the two looks a little silly to me...

    In an Alaskan Slab or Frost Protected Slab, a thickened slab edge allows space for a rebar beam and serves as a footing, spreading the load out over two feet or so. In a slab on grade design, the slab is not structural so no thickened edge is needed. The frost walls take the load (not the slab) and spread it out over the footing.

  2. Expert Member


    As Rick said, there is no benefit I can think of to thickening the slab edge where it will not be load-bearing. What will help alleviate potential cracks due to differential settling, is making the step in your foundation wall deep enough that there are several inches of fill between the concrete ledge and the foam under the slab.

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