roof venting/ blocking

[dug]

I'm a little stuck on the proper way to vent my roof and close off between the rafter bays.

I want an open soffit. Not sure if that is the correct term, but basically I want to leave the rafters (overhang) exposed. I just like the more rustic look it lends.

To close the rafter bays I am thinking about something like this



I am going to stucco the exterior so there is a 1 inch relief at the bottom. I will paint it to match the trim.

From what I understand, there are usually a couple of V notches at the top of the block for venting.

How big to notch? I was thinking about 3 inches

Screening on the interior side I assume. What type of screen is used?

Is every rafter bay vented?

Any and all help appreciated!

[ScottA]

You can buy some round vents that can be pushed into drilled holes. I saw some at the lumberyard the other day. Just another option.

[davidj]

There's a formula for the size of the vents.  I think it's something like 1 sq ft of vent for each 300 ft of roof, split between upper and lower (i.e. 1 sq ft for 600 sq ft at the eave).  And this is based on "net free area" of the vent, which is less than the actual size of the vent.  Here are mine (can't remember the actual size, but you can probably guess based on 24" oc rafters).  We just put some 1/8" screen over the inside to keep out the bugs.

[Redoverfarm]

Doug you are on the right track and the info from others pretty well sum it up. But yes you need to vent each bay in an open soffit. Closed soffit you can just vent the soffit area and it will find it's way to each individual bay.  I had always heard that 1'@200 sq ft but I don't recall the reference that I saw.   

[MountainDon]

The building codes rule is usually 1 sq foot of net vent area per 150 sq feet of attic or floor area, not the roof unless it's a flat roof.  There are exceptions that reduce that to a 1:300 ratio. The simple way is to just go with 1:150.  And that is listed as a minimum, more is good.

So a 20x30 = 600 / 150 = 4 sq ft of free vent area required.
Divide that by 2 = 75 sq ft. 75 sq ft vent at the ridge and 75 sq ft at the soffits. That's soffit area total = 75 sq ft. Put half on each side.  Purchased vents I've seen all have insect screening and have the actual free area stamped or on a label.



It is important though to ensure that insulation in the ceiling does not block the soffit vents. That's what those foam corrugated looking things called air chutes are for.


BTW, if you are building as you described you should be using plywood, not OSB for the roof sheathing in the eve area where the underside will be exposed. Maybe that's been mentioned to you before, but I don't recall.

[MountainDon]

FYI....

Section 1505 subsection 1503 of the UBC, states as follows:
"Ventilation, where determined necessary by the building official due to atmospheric or climate conditions, enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the rain and snow.  The net free ventilation area shall not be less than 1/150 of the area of the space ventilated."

         EXCEPTIONS: 1. the area may be 1/300 of the area of the space ventilated provided 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914mm) above eaves or cornice vents with the balance of the required ventilation provided by eaves or cornice vents.
    2. The area may be 1/300 of the area of the space ventilated provided a vapor barrier having a transmission rate not exceeding 1 perm is installed on the warm side of the attic insulation.

Translated?  1 divided by 150 x 1000 square feet of attic equates to 6.6 square feet of venting per 1000 square feet of attic.  Place half of your vents at the eaves and half at the peak and you can get away with 3.3 square feet of venting.   Keep in mind this is the minimum requirement to meet code.

[dug]

Wow! Just got home from 'work'  (as opposed to the actual work I do on the house) and am very pleased to have gotten the answers I needed. Now I can move on.

Thanks to everyone on this forum for all the help, it really makes things easier for a novice like myself. As I've heard here, and elsewhere, I'll start to figure out what I'm doing on about my 3rd house!

more questions are sure to come.