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If I understand correctly, when using a monolithic slab (where the footers and slab are poured as one piece), the vapor barrier is installed under the slab and the footers.

The NEC does not consider concrete that is over a vapor barrier to be in direct contact with the earth. ).

Are there any common practices or workarounds that are code compliant and used?

Note: I don't mean using a different grounding method, or skipping a vapor barrier. I'm asking if there's some clever way to get enough of the footer to contact the earth while still protecting the slab from moisture. Or something else along those lines.

NEC 250.52 (3)

Concrete-Encased Electrode. A concrete-encased electrode shall consist of at least 20 ft of [...] One or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm (1/2 in.) in diameter, installed in one continuous 20 ft length, or if in multiple pieces connected together by the usual steel tie wires, exothermic welding, welding, or other effective means to create a 20 ft or greater length.

Metallic components shall be encased by at least 2 in of concrete and shall be located horizontally within that portion of a concrete foundation or footing that is in direct contact with the earth or within vertical foundations or structural components or members that are in direct contact with the earth. If multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system.

Informational Note: Concrete installed with insulation, vapor barriers, films or similar items separating the concrete from the earth is not considered to be in "direct contact" with the earth.

  • what about good old couple of ground rods outside the perimeter of the building? Or metal water pipe? Etc. – Harper Sep 22 '17 at 1:07
  • @Harper -- Ufers are basically the best ground electrode one can get ("ground enhancement material" you see sometimes in telecoms etal work is really a specified form of concrete), and some jurisdictions actually require them in new construction (I live in a place that does that) – ThreePhaseEel Sep 22 '17 at 1:25
  • @ThreePhaseEel Do they need to be your foundation, or can they be any concrete work? – Harper Sep 22 '17 at 2:03
  • @Harper: I added the relevant NEC paragraph about it above. It's not exactly clear on if it has to be part of the foundation or not. – Nick Sep 22 '17 at 2:14
  • @Harper -- the more area you have in contact, the better it works, but the the internal rebar system needs to be 20' or longer for it to count. I suppose you could build one into a porch or the likes... – ThreePhaseEel Sep 22 '17 at 2:23
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Front Porch Grounding Electrode

An Ufer ground (concrete-encased electrode) can be made from any reinforced foundation or footing that is in direct contact with the earth, whether it is supporting a structure or not. This means that not only is the house foundation eligible provided it is in direct contact with the earth, but a front or back porch or stoop can be used in addition or instead provided it has a sufficient length of suitable rebar or wire inside and has a footing that is in direct contact with the earth. Since there's no need for a vapor barrier on a porch or stoop where the concrete's wearing surface is exposed to the elements anyway, this means that with perhaps a bit of trickery, you can get the benefits of an Ufer ground without having to give up a sub-slab vapor barrier for the main foundation and footings.

  • Might I suggest to go ahead and use the Ufer ground on the slab, but install a ground rod as a supplemental ground. That way the Ufer will definitely take care of the grounding inside the dwelling and the ground rod will protect you on the exterior if the Ufer is somehow insulated from the exterior dirt. – Retired Master Electrician Sep 22 '17 at 12:49
  • The "trickery", I guess, is figuring out how to route the front or back porch footer to where the main panel is, and how to hide the rebar sticking out of the porch. I suppose the rebar could come out the side of the slab or footer underground and connect to the panel via copper wire. I was thinking there would be something that was simple and standard practice, like cut a 20' slot in the vapor barrier under the footer where the ufer is located. – Nick Sep 22 '17 at 19:09
  • Also, can the rebar in the slab be bent? ie. if you have a 4' x 5' porch outside the door, can the Ufer be a piece of rebar that is bent like a rectangle and goes all the way around? Or would it be less effective because it's not spread out enough? – Nick Sep 22 '17 at 19:11
  • @Nick -- you can bend the rebar – ThreePhaseEel Sep 22 '17 at 22:08

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