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-   -   Open cornice rafter bay blocking. (http://www.diychatroom.com/f19/open-cornice-rafter-bay-blocking-116029/)

diyroofer 09-02-2011 04:55 PM

Open cornice rafter bay blocking.
 
So I have a gabled roof with open cornice overhangs of about 2.5 ft. Above the exterior wall there is blocking that closes off the rafter bays. I want to put attic intake vents in the blocking by cutting an inch or so off the top and screening, or replace some pieces of blocking with screen. But I don't know if the blocking is structurally necessary. If it is, would cutting an inch off the top of all of them pose a problem?

Ron6519 09-02-2011 06:18 PM

Try posting some photos.

diyroofer 09-02-2011 06:39 PM

General roof/overhang view:

https://lh4.googleusercontent.com/-M...0/IMG_5517.JPG

Currently there are two rafter bay openings (total) that are screened, here is a detail on one of them...you can see the blocking between the rafters in the bays bordering the screened one:

https://lh3.googleusercontent.com/-S...0/IMG_5520.JPG

And here is a picture from the attic looking toward the overhang:

https://lh6.googleusercontent.com/-B...0/IMG_5522.JPG

kwikfishron 09-02-2011 06:56 PM

Do the screen every third bay, that with a ridge vent and you should be fine.

Gary in WA 09-03-2011 11:36 PM

Ask your Building Department as in a seismic zone, full height blocking is required. If not, they are less than minimum so you would be alright without them: http://publicecodes.citation.com/ico...002_par028.htm

To remove every third bay would give you plenty (3.75 times) more venting than minimum, but the bays between would not get much venting and could result in ice dams or frosting inside, depending on location. Figure 9 sq. in. per foot, so your 2' o.c. rafters would need 18 sq. in. of NFVA or about a 1-1/8" slot in each bay with 1/4" hardware cloth.
You'll want some baffles in front of them as the attic insulation appears below minimum for most areas. http://www.airvent.com/homeowner/pro...it-specs.shtml

Where are you located?

Gary

diyroofer 09-04-2011 04:47 PM

I am located in Portland, OR. Within the state codes the only thing my untrained eye can find that may be applicable is in reference to lateral bracing of rafters:

http://ecodes.biz/ecodes_support/fre...nstruction.pdf

section R802.8 Lateral support

Basically, it says lateral support is required where rafter depth-to-thickness ratio exceeds 5:1 based on nominal dimensions, given 2x6 rafters my depth-to-thickness ratio would be 3:1, if I understand this correctly, and therefore lateral support would not be required. Additionally, I checked the City of Portland's Residential Seismic strengthening page and there is no reference to special requirements (over and above those listed in the state codes) with regard to lateral bracing of rafters:

http://www.portlandonline.com/bds/index.cfm?c=53562

Am I missing something?

Ventilating through the under-eave blocking would certainly be the most waterproof option for me to add intake vents to my attic. I could install an airvent/smartvent/coravent type product on the roof to provide intake vents, but it would have to be installed at least 3 feet up from the eave edge given that I have open cornice overhangs. To me, this seems like a weak point in the roof's "armor". Additionally, even though it is extra work, I would rather install continuous under-eave vents through the blocking than completely removing every third piece, since this continuous venting would provide the most thorough ventilation for the whole roof deck.

GBR, you mentioned installing baffles due to sub-par ceiling insulation in the attic. Did you mention this because if insulation was added the baffles would be needed to keep the vents clear? OR did you mention this because installing baffles would help keep the intake air directed up the rafter bays rather than over the ceiling?

Thanks for the replies.

Gary in WA 09-04-2011 11:05 PM

"GBR, you mentioned installing baffles due to sub-par ceiling insulation in the attic. Did you mention this because if insulation was added the baffles would be needed to keep the vents clear? OR did you mention this because installing baffles would help keep the intake air directed up the rafter bays rather than over the ceiling?" ----

Both. You don't want wind-washing to negate or move the loose-fill insulation.
Sounds like the blocking is a non-issue, check locally if you go that route.

Gary

diyroofer 09-05-2011 12:55 AM

Great, thanks for the good info. I've got an inquiry in with the cities bureau of developmental services regarding code compliance if I make changes to the described blocking...I will proceed from there.

Thanks again!


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