Junction boxes on roof roof decking

[Mikejet]

I understand that you cannot fasten conduit directly on the underside of corrugated roof decking. What does this mean in regards to mounting junction boxes? Do they have to be mounted on strut? Everywhere I go I see boxes mounted directly onto the corrugated deck.
Does the new code prohibit this method? It seems very labor intensive to have to mount every box on a piece of unistrut.

 

[rcarroll]

Boxes are required to have 1.5" separation from the lowest surface of the deck. Boxes were added in the 2011 code. 300.4(E)

 

[kwired]

Boxes are required to have 1.5" separation from the lowest surface of the deck. Boxes were added in the 2011 code. 300.4(E)

Even if they had not been added, it becomes fairly impractical to use small dimension boxes as any raceways that enter the box are going to be in violation at least near the point of entry.

 

[infinity]

You could always use RMC or IMC.

 

[kwired]

You could always use RMC or IMC.

How about malleable iron boxes to go with the heavy wall raceways?

 

[Smart $]

How about malleable iron boxes to go with the heavy wall raceways?

The exception says nothing about boxes.

 

[infinity]

The exception says nothing about boxes.

I agree and IMO this is a terrible code requirement. For decades EMT and metal boxes were installed on the decking without much of a problem. Yes it's possible that sometime in the future a roofing fastener may penetrate the EMT but odds are slim. I would put it up there with the odds of a homeowner using a 3" nail to hang a picture and hitting an NM cable. :slaphead:

 

[kwired]

The exception says nothing about boxes.

I know, I was making a suggestion that if we can use a heavy wall raceway why not a heavy wall box?

 

[infinity]

I know, I was making a suggestion that if we can use a heavy wall raceway why not a heavy wall box?

Sounds like a good proposal for the 2017 NEC.

 

[kwired]

Sounds like a good proposal for the 2017 NEC.

I won't say I never work in places with this kind of roofing, but don't do it often enough that it is worth my time to deal with proposals related to this.

 

[don_resqcapt19]

I agree and IMO this is a terrible code requirement. For decades EMT and metal boxes were installed on the decking without much of a problem. Yes it's possible that sometime in the future a roofing fastener may penetrate the EMT but odds are slim. I would put it up there with the odds of a homeowner using a 3" nail to hang a picture and hitting an NM cable. :slaphead:

Rob,
I think for decades the roofers used appropriate length screws. Now it seems everywhere I go with this type of roof construction, either new or replacement I see the screws sticking through the bottom of the deck 2 or more inches. In a recent re-roof job, most of the screws were sticking through more than 4".

 

[iwire]

Rob,
I think for decades the roofers used appropriate length screws. Now it seems everywhere I go with this type of roof construction, either new or replacement I see the screws sticking through the bottom of the deck 2 or more inches. In a recent re-roof job, most of the screws were sticking through more than 4".

I think that coincides with increased insulation requirements requiring longer screws. Often the insulation boards are tapered in thickness for roof drains etc meaning they either use long screws every where or carefully lay out which length screws must be used in each areas

 

[infinity]

Rob,
I think for decades the roofers used appropriate length screws. Now it seems everywhere I go with this type of roof construction, either new or replacement I see the screws sticking through the bottom of the deck 2 or more inches. In a recent re-roof job, most of the screws were sticking through more than 4".

I've seen that too, where the screw protrudes much farther then the 1.5" minimum requirement which makes running the EMT at least 1.5" below the deck not much help anyway. :happyno:

 

[don_resqcapt19]

I think that coincides with increased insulation requirements requiring longer screws. Often the insulation boards are tapered in thickness for roof drains etc meaning they either use long screws every where or carefully lay out which length screws must be used in each areas

They were using the tapered insulation when I started, but it seemed that they used different lengths of screws to match up with the thickness...now it is one size fits all and often well over 2" too long.

 

[don_resqcapt19]

I've seen that too, where the screw protrudes much farther then the 1.5" minimum requirement which makes running the EMT at least 1.5" below the deck not much help anyway. :happyno:

I wonder if, once you are 1.5" away, if it is more likely that the screw will deflect when it hits the curved surface of the conduit than is would be when the conduit is closer?

 

[kwired]

I wonder if, once you are 1.5" away, if it is more likely that the screw will deflect when it hits the curved surface of the conduit than is would be when the conduit is closer?

I was going to suggest the chance of deflection is probably much greater in those circumstances, but you posted first.

 

[infinity]

I wonder if, once you are 1.5" away, if it is more likely that the screw will deflect when it hits the curved surface of the conduit than is would be when the conduit is closer?

You're probably correct but a solidly mounted box would have no chance at deflection. As you can tell I loathe this requirement. We've been wiring warehouses this way with EMT since EMT was invented. There was no high occurrence statistical analysis added to the substantiation of this code change proving that it is likely to happen. You have a better statistical chance of a carpenter with a zip tool destroying the conductors in a wall mounted box then a screw penetrating the EMT. :rant:

 

[ActionDave]

You're probably correct but a solidly mounted box would have no chance at deflection. As you can tell I loathe this requirement. We've been wiring warehouses this way with EMT since EMT was invented. There was no high occurrence statistical analysis added to the substantiation of this code change proving that it is likely to happen. You have a better statistical chance of a carpenter with a zip tool destroying the conductors in a wall mounted box then a screw penetrating the EMT. :rant:

I agree with you, but you could say the same thing about a whole host of code changes.

 

[novemberaudi]

Although it seems unlikely I worked for a contractor that repiped a warehouse with emt above the bar joists, and the roofer ended up
hitting the pipes in excess of 20 times. Lots of overtime!!!

 

[KWH]

Leave it there

Leave it there

I agree and IMO this is a terrible code requirement. For decades EMT and metal boxes were installed on the decking without much of a problem. Yes it's possible that sometime in the future a roofing fastener may penetrate the EMT but odds are slim. I would put it up there with the odds of a homeowner using a 3" nail to hang a picture and hitting an NM cable. :slaphead:

I have had to deal with both do not put any more possibility's out there.