# Minimum Roof Curb Height?



## mtlogcabin

Contractor called yesterday about the minimum curb height on a roof top unit. The mall re-roofed about 2 years ago and as is typical they installed about 4 inches of insulation which lowered the curbs height from 12 to about 8 inches. Now the HVAC contractor is going to be replacing some of the rooftop units and is asking if he need to extend the curbs. I can't find a minimum curb height requirement. Is there one and if there is would he be required to raise the existing curbs to that requirement? 2009 I-Codes


----------



## ICE

I haven't heard of a minimum height or even a code requirement to have a curb.  The installation instructions might have a dimension.


----------



## Pcinspector1

mtlogcabin,

2006IBC 1507.3.9 Flashing. flashing and counter flashing shall be provided in accordance with the manufacture's install instructions.

Only thing I can find.

Is curb a construction "slang" term?


----------



## mark handler

Florida requirs 8"  FBC 1509.7

But nothing that i have found in unmodified IBC


----------



## Pcinspector1

Skylight 2405.4 framing 4" min above the roof plane. Still can't find a requirement for RTU's

pc1


----------



## mtlogcabin

I can't find anything in the code. I do find an 8" minimum recommendation from the (NRCA) National Roofing Contractors Association. Spoke with one of our better commercial roofing contractors and he said an 8" curb would be minimum for most roofing manufactures warranty requirements for flashing the curbs. Search some curb manufactures and 8" was the minimum that the majority had.


----------



## Builder Bob

What is the typical snow load??? I know that the curbing is an easy place for moisture to form and freeze, which translates into water when the temperatures rise....


----------



## James Davis

mtlogcabin said:


> Contractor called yesterday about the minimum curb height on a roof top unit. The mall re-roofed about 2 years ago and as is typical they installed about 4 inches of insulation which lowered the curbs height from 12 to about 8 inches. Now the HVAC contractor is going to be replacing some of the rooftop units and is asking if he need to extend the curbs. I can't find a minimum curb height requirement. Is there one and if there is would he be required to raise the existing curbs to that requirement? 2009 I-Codes


fire code 7.8.2.1 requires a minimum 18" from roof surface


----------



## ADAguy

"Best" practice? It depends, doesn't it?


----------



## linnrg

IMC 304.10 clearances from grade


----------



## mark handler

linnrg said:


> IMC 304.10 clearances from grade


And that relates to "Roof Curb Height", How?
Underground structure?


----------



## mark handler

"...Units using curbs that have built in metal base flashing flanges are difficult to seal and, therefore. are not recommended for use. Composition base flashing should extend a minimum of (a nominal)
*8-inches above the roof *line. Wood or fiber cants must be provided at any good angle created by rectangular curbs or projections. Wood nailers should be provided on all prefabricated curbs. The composition flashing (base flashing) should be fastened either with 1 inch, solid cap-head nails or with nails driven through tin discs. The nails should be spaced approximately 8 inches apart. (See NRCA Detail "R" in the Appendix.)…"

*National Roofing Contractors Association*
Handbook of Accepted Roofing Knowledge
http://docserver.nrca.net/technical/7901.pdf


----------



## Builder Bob

traditional design will use a minimum of 12 inches / better with 18 inches for curbing here - you have to take into account any roofing insulation applied on deck as well. The height of the curb will vary upon the amount of roof insulation applied.


----------



## north star

*@ ~ @ ~ @*

Are there any rooftop drains ?......If so, is there the correct
amount of slope on the condensation drain piping from
the HVAC units ?

*@ ~ @ ~ @*


----------



## ADAguy

How deep does the snow get in your area?
Sloped roof or flat?


----------



## Yikes

My firm is doing a re-roofing project.  NRCA recommends 8" curbs, mainly as a condition of honoring a roof warranty.
I have learned that if both the contractor and the manufacturer are willing to warranty a lower curb than 8" there is nothing in our local codes that prohibit lower curbs.
In fact, the NRCA manual has "gravel stop" details that are much lower than 8".


----------



## ADAguy

Yes but, gravel stops serve a different purpose and are typically installed downslope.


----------



## HVACMAC

mark handler said:


> "...Units using curbs that have built in metal base flashing flanges are difficult to seal and, therefore. are not recommended for use. Composition base flashing should extend a minimum of (a nominal)
> *8-inches above the roof *line. Wood or fiber cants must be provided at any good angle created by rectangular curbs or projections. Wood nailers should be provided on all prefabricated curbs. The composition flashing (base flashing) should be fastened either with 1 inch, solid cap-head nails or with nails driven through tin discs. The nails should be spaced approximately 8 inches apart. (See NRCA Detail "R" in the Appendix.)…"
> 
> *National Roofing Contractors Association*
> Handbook of Accepted Roofing Knowledge
> http://docserver.nrca.net/technical/7901.pdf


⬆️This. I’m in the HVAC industry and I haven’t ever heard of a code for curb height. The 8” height is to make sure the roofing contractor has enough space to securely seal penetrations for warranty purposes.


----------



## fatboy

Yeah, not in the code........manufactures specs....


----------



## mark handler

fatboy said:


> Yeah, not in the code........manufactures specs....


Notice that I highlighted the source National Roofing Contractors Association
I did not say it was code


----------



## fatboy

mark handler said:


> Notice that I highlighted the source National Roofing Contractors Association
> I did not say it was code



I didn't mean to infer that you said that, it is a great reference, I was merely pointing out that it is not in the code.

This is because recently I had a roofing company come to me, that had a picture of an RTU, basically sitting on the roof. At some point, someone had done a roof without a permit, added enough insulation that it eliminated whatever curb was there before. The company wanted me to require they build a curb for this existing RTU, that was not getting replaced, I couldn't do it as there are no curb requirements in the code. Manufacturers specs rule it, but this was existing, had nothing to do with the reroof.


----------



## ADAguy

Best practice, Duh!, code is only a minimum.


----------

