Roof Vent Calcs — What areas of the House are included?
idahobuild
| Posted in General Questions on
Hey all,
We have our front and back porch roofs integrated with the house’s roof.
My questions are:”
Do we count that space in the square footage for the roof venting?
Do we count the garage?
See attached images/plans.
Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Yes
The required ratios (1/300 or 1/150) are not very scientifically derived; they were created as placeholders in the 1940s with the explicit intention of being updated as testing showed more precision. But nobody has done that testing, and the ratios continue to work well, so they are a good guideline. In most cases you can't go wrong with additional ventilation, while skimping on ventilation can definitely lead to problems, so I would include all of the area that the roof covers.
I recall that you are concerned with wildfires--are you planning on using soffit vents that are fire-resistant? Or is my recollection wrong?
I was using an online vent calculator and the first question was square footage. I immediately looked at the plans and added all the numbers up (dwelling, garage, porches).
After watching a Joe Lstiburek video on GBA related to venting attics, I wanted to get some more opinions/info. Many key take-aways in the video, not the least of which was more vents at bottom than at top.
No real fire hazards for our build. We are high desert.
thanks.
You basically count the square footage of the floor in the entirety of the attic spaces that will be vented. More vents doesn't hurt, so you don't have to worry. It's usually desireable to have a little more intake (soffit) venting than exhaust (ridge) venting too, to help avoid keeping the attic under a slight vacuum, which makes stack effect worse.
The easiest is probably to put soffit vents under all connected soffits at the low portion of the roof (parts parallel to the ground), and a ridge vent along the entire part of the ridge over the attic spaces. That usually works pretty well, and it looks consistent too, which is a good thing. If you do it that way, just use the NFA (Net Free Area) ratings on the vents you use to make sure you have enough for the space you're venting.
Bill
Got it. Thanks All!