Hello, long-time lurker. Designing next house. Zone 5 Tree fire hazard in neighboring lots, 50' or so, my lot is cleared. Also concerned with wind blown water & snow into vents. Simple rectangle gable end house. ICF walkout basement will be my single level house. Ridge beam down center 8:12 pitch, 14" TJI rafters 16" OC seated on top of ridge beam, 5/8" CDX sheathing, 30# roofing felt , steel roof. I plan on vented roof with chutes against bottom of TJI top chord. It seems that the lookout barge rafters will block this channel at both gable ends of house. Should I SPF insulate these end rafter bays & not vent them? OR drop down my vent chutes 2" & loose R-value? Fire resistant vented soffits look easy. I am concerned with upper vent design. Ridge or gable-end vents both have potential problems with wind blown water & fire. My planned ridge beam will be a 32" deep engineered parallel chord truss, 3 x wide so 4.5" wide, 2 sections totaling 49' I will have a 12' wide flat ceiling down the middle, giving me room for 20" of blown in cellulose and air space above that for gable-end venting. Solutions, Ideas? Thanks BBP

Looks like my spaces between paragraphs didn't work. Sorry for the hard to read run-on look of this.

I currently have large gable vents and they are nothing but problems. They let water in during wind driven rain events, there is no way to stop it. They are louvered and angled but that only works up to a light rain.

The home is not located in a forest fire area but if it was, there would be no problem with the embers getting in through that gable end.

Any roof venting scheme increase the hazard during forest fires. The steeper the pitch, the greater the risk of sucking in embers. In zone 5 using R20 in rigid foam above the roof deck, and taking it the rest of the way up to R49 on the underside meets IRC code for unvented roof assemblies.

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_par100.htm

If you're taking it up to R65, make that R25 in exterior foam.

Be sure to provide the proper slip-surface underlayment for the steel roof, but mount it tight to the rigid insulation, not up on purlins, to eliminate ember penetration under the steel.

Then insulate & air seal the gable ends, bringing your attic fully into conditioned space.

One way to do gable vents that are more secure is to put in double vent louvers/screens. Say that you have a two foot over hang on the gable, put your first vent assembly just in front of the sheathing plane. Then build a "bird house" out 12" and put the second vent assembly on the face of the "bird house". As the air moves through the first set of screens it slows down and then drops it load prior to entering the second set.

Dana, I don't understand the stack-up. Say I put 4" of XPS on top of 1st 5/8" roof sheathing, then a 2nd layer of 5/8" roof sheathing over that, attached with 5.5" screws through into 1st sheathing. Then I can put 30# roofing felt & steel roof.
Then on underside of 1st sheathing, 2# SPF at R39 thickness, which costs about $1.50 / board foot here.

Sounds like it would work GREAT, but Seems like MUCH more time & money?

FBBP, I like the idea of gable vent with a DIY internal steel duct, closed on 4 sides with internal screens. I saw a DIY design on some website that drained water back out the bottom of store bought steel gable vent.

Thanks guys, BBP

Dana, I think you are saying, use long screws to attach the steel roof directly to the foam down into sheathing.

I don't like it. I have a steel roof on my current house with 5/8" CDX sheathing, & the screws work loose about every 10 years. I think that they would work loose much faster if they were cantilevered 4 more inches out, much more leverage as steel roofing moves from thermal cycling. Lately, as mine worked loose I have been replacing them with larger screws.

BBP

You can never really get away with long-screwing siding or roofing through foam- it always fails. In your case you have two options, one more labor intensive, the other with higher material cost:

Dado into the foam for some 2x furring through screwed to the rafters 24" o.c. with pancake head timber screws, making the furring flush with the foam. Apply the appropriate slip-surface/underlayment then steel is then screwed to the furring.

Alternative to the furring approach, a layer of CDX through-screwed to the rafters with pancake head timber screws 24" o.c. works.

Found some fire resistant gable vents

http://store.primopumps.com/Flame_Ember-Resistant-Gable-End-Vent-14-x-24-Retrofit-0125-Screened/productinfo/BGV-GE1424-R-125/#.UYqHrY7FWM5