Reentrant Corners and Other 'Flamboozlements' - 2/21/2005 - Home Foundation Structure Framing

Reentrant Corners and Other 'Flamboozlements'

Dear Builder’s Engineer,

[In response to your column regarding 4- or 5-inch slabs,] could you better explain the placement of #4 bars at the reentrant corners?

A few other questions regarding thickness: if using radiant heating in a slab, does it need to be thicker? Are there any other concerns relating to the heat in the slab or the pipes that might cause cracking or weakness? — Jeff Williams, Williams Development, Inc., Bakersfield, Calif.

Jeff,

Don’t feel alone about not understanding “reentrant corners.” Most of my mail over the past few weeks has puzzled over the same question.

A reentrant corner is nothing more than a corner poking into the building footprint. You could also call it an interior corner. I don’t know why the code folks don’t just call them that: interior, as opposed to, say, exterior, but both the Uniform Building Code (UBC) and the International Building Code (IBC) like “reentrant” instead.

Maybe it’s because “reentrant” is not really a word? It wouldn’t be the first time code writers dreamed up fake “codespeak” to “flamboozle” us, ensuring their masterwork remains veiled in a shroud of mystery that only they and their best buddies understand. Not that I don’t trust them, but just to be sure I looked up “reentrant” in Webster’s Collegiate and was shocked to find it, right there between reenforce (I always thought it was “reinforce”), and reest (chiefly a Scottish word, short for areest; to arrest). “Sheesh.” Clearly this is not a well-used section of the dictionary. Anyway, here is our definition: reentrant — directed inward.

Before moving on to something actually useful, here is an interesting side note. The new International Residential Code (IRC) doesn’t call them “reentrant”, but rather, interior, and for that matter, exterior; and actually has a diagram of same, Figure R602.10.5. This is such a critical piece of information, and I assume most readers of this column don’t walk around with a copy of the IRC strapped to their belt, that I have reproduced that sketch here (courtesy of the International Code Council, 2003 International Residential Code).

You will note the striking similarity between these two sketches. Detail (b) is the “reentrant” corner because gypsum wall board should always be on the “inside” of the building.

Now, lets say you’re pouring a slab with a “reentrant” corner. Here is a sketch showing where my previously mentioned two sticks of rebar should go to minimize the crack. (To avoid confusion, the grass and shrubs go on the “outside” of the building.)

Another thing you should know about “reentrant” corners: if you have one and it is a “bad” one (i.e. pokes more than 15% of the building’s overall dimension inward), and you are in seismic zone D, E or F (the “bad” seismic zones), you will have to overdesign certain earthquake resisting elements by 25% (IBC1620.4.1).

To answer the question about in-floor heating (a.k.a. “hydronic”) and slab thickness, I offer my own experience. The main floor of my house is wood-framed with a 1-1/2-inch thick stained concrete cap with hydronic heating tubes throughout. There are 3/8-inch deep sawcuts at 3-feet, each way. Other than some aesthetic stain/sealer problems that had nothing to do with thickness, my floor has performed beautifully. I know others insist on using thicker slabs with hydronic, but my experience doesn’t support that notion. My greatest concern was grazing a heating tube while saw cutting, but I had a real pro on the saw, and it didn’t happen.

Postscript. All this business about “reentrant” brings to mind a couple other important terms you can impress your friends with: minutiae – trifles, details; and esoteric – used and understood by a select few.

Tim Garrison of ConstructionCalc.com, is a professional engineer, author and software producer for the building industry. Send e-mail to buildersengineer@constructioncalc.com. Tim reads every one.

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