CBC 1203.2 Attic Ventilation

[C & A]

I have a dilemma in how to ventilate the "attic" space formed by 15" - 21" deep preengineered roof trusses. Ceiling finish (5/8" Type X Gyp. Bd.) is applied directly to the underside of the trusses. The roof is flat, only sloped enough to drain, and does not have overhangs. We were planning to use gravity ventilators, as there is no location for eave or soffit vents. How can we get the high and low ventilation required by CBC 1203.2, as the lowest point of the roof is only 6" lower than the highest point? The project is Type VA, sprinklered, 4-story, R-2 apartment building. Thanks for any help you can give!

 

[mtlogcabin]

What about draftstopping? Are you using a NFPA 13 or NFPA 13R system? It will make a difference

 

[mark handler]

C&A

do you have parapets?

Somewhere I have a detail for vents in parapets, to ventout a flat roof.

 

[C & A]

Draftstopping: The roof truss (attic) space is sprinklered NFPA 13, so we don't need draftstops. Yes, we have parapets in most locations. We are thinking of using wall vents at the perimeter (low point of roof) between the roof trusses and gravity ventilators at the high point of the roof to get the cross-ventilation. However, we cannot get the wall vents to be 36" lower than the gravity ventilators due to the flat roof. I have heard that some jurisdictions will approve this solution, as there is no way to strictly comply with the letter of the code when you have a flat roof. Thoughts?

 

[mark handler]

we cannot get the wall vents to be 36" lower

If you cannot get the wall vents to be 36" lower, You use the 1/150 factor instead of the 1/300.

 

[mtlogcabin]

Draftstopping: The roof truss (attic) space is sprinklered NFPA 13, so we don't need draftstops

Correct NFPA 13 no draft stopping required

NFPA 13 R draftstopping would have been required

 

[alora]

Is there an argument to be made about attaching the building envelope insulation to the underside of the roof deck, thereby eliminating the need for ventilation?

In our climate in AZ, we do this all day long.

 

[C & A]

I just saw Alora's question -- does anyone know if this would be OK in (San Jose) California? We would love to eliminate all the roof penetrations from gravity ventilators.