Is grounding rebar in concrete slab required by NEC?

[kwired]

You didn't mention the slab location. There are a few cases where bonding would be required. Art 547 & 680 for example.

Those applications are not for the purpose of creating a grounding electrode though.

 

[kwired]

Seems to me I've seen some metal barns that were built on top of only a slab, or at any rate any difference between slab and footing was invisible from the top side.

The portion supporting the structure is generally thicker than the rest of the slab. It may be all in one pour or a footing pour then the slab.

I've mostly only seen the all in one pour on smaller buildings. They commonly do all in one pour on grain storage bin slabs around here though.

 

[JohnE]

Rebar grids in slabs are not required to be bonded. They are not “likely to become energized”.

A CEE (concrete encased electrode) is a different thing entirely. The NEC now requires 20’ of 1/2” rebar, OR, 20’ of #4 solid copper wire, to be embedded in a *footing or foundation*, if the concrete is in contact with earth, and to be used as the grounding electrode.

“In direct contact with earth” is key here. The building code requires all heated structures to have a 10 mil vapor barrier below slabs. Some residential, and even commercial buildings, will have insulated slabs, especially if they have radiant floor heating. As both vapor barriers and polyisocyanurate foam board insulation, which is used in insulated slabs, break the direct contact with earth, the CEE is virtually never used in new slab-on-grade construction.

I'm not sure if I'm reading your post right. Are you saying that the NEC requires a CEE, or are you describing what it must consist of to be qualified as a CEE. The wording makes it seem that you are representing that the CEE is required. I'm not reading it the same way. I read it that it is required to be included as part of the grounding electrode system, if present. (Granted, rebar is included in most new construction footings in this area) Oddly, a number of building inspectors are telling contractors that a CEE is required, and are inspecting for a rebar to be brought out of the foundation or footing.

 

[Seven-Delta-FortyOne]

Since everything we build here has rebar, unless it does not qualify for the reasons I mentioned earlier, we are required to use it.

I have heard of some areas where they interpret that to mean that it’s now required to be installed, but I’m not sure about that.

 

[Seven-Delta-FortyOne]

Seems to me I've seen some metal barns that were built on top of only a slab, or at any rate any difference between slab and footing was invisible from the top side.

No one I know of builds anything on top of a simple slab, not even a pergola. It would require at least point footings, if not an entire perimeter.

 

[ptonsparky]

No one I know of builds anything on top of a simple slab, not even a pergola. It would require at least point footings, if not an entire perimeter.

You need to expand your horizons. I would venture to guess that almost all of the metal sided, wood framed, ag buildings in this area have no footings.

 

[kwired]

You need to expand your horizons. I would venture to guess that almost all of the metal sided, wood framed, ag buildings in this area have no footings.

Newer ones probably do, unless it is a pole building or some small accessory building.

A building on my property I use for a shop is an old farm building probably built in the 1950's. Most framing is still original lumber, I have modified a little to put some doors in or move a door, and I have put sheet metal over the original dutch lap wood siding. Roof is still the old galvanized sheet metal from back when it was built but probably could use a little TLC at the very least. has pretty shallow footings definitely not below typical frost depth. It is still standing but if some guys were to see someone construct a building the same way this one is put together they would tell you it would not last very long.

 

[Fred B]

The portion supporting the structure is generally thicker than the rest of the slab. It may be all in one pour or a footing pour then the slab.

I've mostly only seen the all in one pour on smaller buildings. They commonly do all in one pour on grain storage bin slabs around here though.

Sounds like you are describing an Alaskan Slab. The typical installation would require vapor barrier and insulation thus become non usable for a CEE. See Typical installation illustrations:

 

[jaggedben]

I'm not sure if I'm reading your post right. Are you saying that the NEC requires a CEE, or are you describing what it must consist of to be qualified as a CEE. The wording makes it seem that you are representing that the CEE is required. I'm not reading it the same way. I read it that it is required to be included as part of the grounding electrode system, if present. (Granted, rebar is included in most new construction footings in this area) Oddly, a number of building inspectors are telling contractors that a CEE is required, and are inspecting for a rebar to be brought out of the foundation or footing.

The NEC does not require a CEE because the NEC does not dictate how buildings or structures are built. You can have a building or structure without a concrete foundation (built on steel pilings, for example).

At least one AHJ I have worked on had its own local ordinance that requiref installing a CEE if pouring 20ft of new footing. If an AHJ explicity requires the CEE, or if the buidling construction follows a code that requires the equivalent of a CEE, then the NEC requires it to be used.

 

[Seven-Delta-FortyOne]

You need to expand your horizons. I would venture to guess that almost all of the metal sided, wood framed, ag buildings in this area have no footings.

Here is one I did, residential. Hard to tell, but the slab is 5”, footing are up to 36” deep. After it’s built it’s hard to tell how deep it goes. This did not have a vapor barrier, as it is not going to be heated. This one got a CEE.





Here is a commercial one I did, this got a vapor barrier wrapped all the way under the footing. The CEE was not present. This had a 24” wide footing, average depth 30”.







This is another residential, but this had a separate stem wall, with a slab poured inside. Footing was poured first, 16”x16”, then stem wall, then slab with vapor barrier. This had a CEE. This was a wood framed shop.





I’ve never built anything on a slab without a footing. Not even a water tank.

 

[electrofelon]

I built my solar shed on a uniform 16X50 slab 8 inches thick. My opinion is most people are irrationally averse to slabs on grade and worry about things heaving. This has #5 rebar on a 12" grid. It heaved maybe 3/4" on one side one winter, so what. No cracks whatsoever. Use a good amount of rebar and use water reducer or plasticizer in the mix so you dont have junk watered down concrete and it will be fine.

 

[JohnE]

The NEC does not require a CEE because the NEC does not dictate how buildings or structures are built. You can have a building or structure without a concrete foundation (built on steel pilings, for example).

if the building construction follows a code that requires the equivalent of a CEE, then the NEC requires it to be used.

Those are the points I was trying to make.