Rafter bay venting on buildings with parapets

[Yikes]

Picture a conventional wood-framed building with parapets and a “flat” (1/2” per foot low slope) roof. The roof assembly is built-up roofing over plywood deck, over 2x or TJIs with batt insulation between, over a fire-rated gyp board membrane. In other words, the indoor ceiling is directly attached to the underside of the rafters.

I see this all the time on apartment buildigns in the southern California area, right? But how come I almost never see ventilation for the individual rafter spaces?

In re-reading IBC 1203.2 in sounds like I should have to individually vent each rafter bay. Yet when you walk these roofs, I typically don’t see ridge vents, nor rows of mushroom vents (one per bay), nor do I see a louver vent on the outside face of the parapet. The IBC commentary does not directly address parapet building situations; the illustrations only show buildings with eaves.

What am I missing here?

I know such ventilation was at the building official’s discretion in the 2001 UBC 1505.3. Was it simply a non-issue on parapet buildings?

Does that continue to be the case in the 2009 IBC?

 

[MarkRandall]

This is exactly why rigid insulation is common on top of the deck. I see no way around the venting requirements. I can remember a discussion with another Architect several years ago and she was saying they wanted to use batt insulation and eliminate the venting requirements by not providing the airspace and I did not agree that it was allowable per code. I don't doubt you see some built that way, but it doesn't make it right.

 

[Yikes]

Thanks, Mark. The crazy thing is, we're not just talking about an isolated roof here and there - - I'm talking about the vast majority of roofs. This is not a case of "might makes right", but it does cause me to woder if they know something I don't.

 

[MarkRandall]

I'm surprised that it is more of the norm for your area. I don't see that being done much in Washington State, but then again, I don't get onto other architect's job sites too often, so maybe it does happen more than I think.

 

[pwood]

it is becoming more common to have buildings totally conditioned space with no ventilation. i am seeing this pattern more in commercial construction. it's a green thing!

 

[Examiner]

http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation/?searchterm=attic ventilation

The above link or site maybe of some help. Attic venting flat or slope roofs will depend on where the insulation is.

 

[Yikes]

Thanks, Examiner, this is exactly what I needed! The literature led me to links at the DOE that prove this particular project is in a Hot + Dry area, minimizing the potential for condensation. (Indeed, tearing off portions of 30 year old unvented roof diaphragm next door reveals no mositure damage.)