Minimum roof slope for adequate roof venting. CA zone 4B
abnorcal
| Posted in Building Code Questions on
Hi, new to GBA. Was hoping to get some advice.
Located in CA, zone 4B. Can I get away with 2/12 pitch on single sloped roof using fiberglass insulation with 1.5″ air gap between insulation and sheathing with vents placed top and bottom of roof under eaves? If not, how about 2.5/12? What does building code require?
Read somewhere 3/12 is preferred, but doesn’t fit our clearstory home design very well. Any suggestions would be greatly appreciated.
Thank you
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
abnorcal,
You may find this discussion and the links in it useful: https://www.greenbuildingadvisor.com/question/venting-a-low-slope-shed-roof#:~:text=For%20ventilation%20channels%20to%20be%20effective%2C%20you%20need,insulation%20layer%2C%20plus%20a%20few%20vented%20%22doghouses%22%20%28cupolas%29.
I would check what your specific local building code requires:
https://up.codes/codes/california
With any roof, air sealing the ceiling is always more important than venting. A well sealed house needs very little venting, whereas no amount of venting will get you out of trouble with a leaky ceiling.
Our code requires 2x the vent opening for a low slope roof (anything less than 1:6). The vent gap between insulation and roof deck is always 2.5" except at exterior wall where it can be 1".
Since you are in a pretty dry area, if you seal your ceiling and build the vent gap and vent opening per your local code, I doubt you'll have problems.