I’m doing a major house re-model and addition in which I have roof slopes ranging from 4:12 down to 1/2″:12 with several in between. The roof shapes are gables with some shed roofs attached (one shed roof ends at a wall). The roof material will eithr be asphalt shingle or metal. Most of the ceilings are vaulted, so there are almost no attics. I’ll leave a 1″ minimum gap between the insulation between the rafters and the roof decking, and this cavity needs to be ventilated. I had expected to use continuous ridge vents, but they seem to require a minimum slope of 3:12, which is more than several of my roofs. So what is the most effective and best looking way to ventilate the roof cavities?
Thanks!
Replies
You have a bigger problem than venting.
There are no shingles that can be safely instaled at less than 3/12 pitch and not metal roofs that should go to less than 2/12, as far as i know.
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Thank you all for your input! I'm new to Breaktime but a longtime reader of FHB and am delighted to be participating!I did realize that lower than 2.5:12 slope would require a cap sheet, which was one reason I thought metal would look less like a patchwork quilt of different materials. But I thought I needed to ventilate the highest point of the roof cavity where there was a vaulted ceiling in order to 1) facilitate cooling on hot days (I should have mentioned I am in the San Diego area) and 2) to prevent moisture and condensation from damaging the roof structure. Am I not correct here? I knew of a house less than 10 years old where there was no air space between the top of the insulation and the roof deck and the roof deck was rotting out. Is a continuous soffit vent adequate for my venting needs?Thanks again, Barbara
there are methods for doing rof cavity insulation with no ventilation. it involves making sure that the insulation is good enough that cenvection currents within that material are effectivly eliminated, and applying a vapour barrier with good integrity to the inside of the living space - as a general rule. I see the climate you have as being unlikely to even need the VB. That could end up traping moisture and accelerating any potential problems like your neighbor had. A cooling climate such as yours is more likely to need the VB on the exterior side of the assembly - in your case, the roof would do that job. As for insulation choices, search the archives here for foam discussion or for cellulose, denspak cells.The best choice you could make is to have a site applied foam sprayed into the frame cavities. The foam itself is not only the best insulation money can buy, but many foams are also a VB in place.the worst insulation you could choose is fibreglass batts, though the weakest pouint about them is negligible in your climatic zone.blown cellulose would oplace in the middle.
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yes, I have applied shingles down to a 2/12 pitch and would not be concerned about it in this climate she has with the double ply of felt or with I&W under it all, but I was refering to the standard application , and there are several roof sections she has that are lower pitched than that.
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I think you have to ask yourself the question, "what will be the advantage of ventilating the low-slope roofs?" Secondly, you really have two different roof types working here, low-slope, or flat roofs, and pitched roofs.
I have seen ventilated flat roofs but they are usually used in conjunction with attic spaces, and you mentioned finished vaulted ceilings in your post. If you have finished ceiling to the underside of say a 10" deep roof structure then it is unlikely that you will need to ventilate this space. What is usually the case, and this is seen in lots of commercial work where flat roofs are more common, is that you have insulation between structure and/or atop the roof deck with the modified bit. roof or single membrane roof atop it or atop a protective sheathing. A BIG INSULATED WALL TURNED HORIZONTAL. Where you have larger/deeper overhangs (soffits or eaves) you could ventilate the eaves with a vent strip near the roof edge (at the underside) and a vent strip near the wall at the underside of the roof. This is circumstantial though because you are really addressing the issue of saving on insulation (installation) -- why insulate where there is no lower floor space?
By all means ventilate the 4/12 pitched roof even if it does have a finished space beneath. The ventilating is better than no ventilation because heat loss is contributing to ice dams or degradation to shingles is a concern. The best way to achieve this in common construction of course is using plastic baffles to hold down the insulation between the structural members.
The asphalt shingles you mention are probably not suitable for less than 3.5/12 pitch. Check the specs for the exact minimum. So that probably means you are going to use a modified bitumen roof or something like that for the .5/12 low slope (flat roofs). I don't know what the minimum slopes allowed for your type of metal roof materials are...
If your low roof that ends at a wall (high side) is a pitched roof (say 4/12). Ventilate at the gable ends and insulate the ceilings of the space (if not vaulted).
Edited 2/13/2005 11:17 pm ET by houseman
When she stated, "needs to be ventilated." I assume that sjhe is in a jurisdiction where the local codes require ventilated attics.but in a space where no attic exists...it has to depend on the wording of that code - if the roof itself is required to be ventilated.In practical terms, there are many ways of edealing with unvented roofs but the insopectors have to be dealt with. Some are still living in the seventies and eighties
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Piffin - in my jurisdiction, on the back of the shingle wrappers for most brands, the instructions say shingles can be used on 2/12 thru 4/12 roofs with half lapped #30 felt or ice barrier membrane underlayment. I just check the GAF site and verified this. I'd post a link, but the application instructions are a downloadable pdf.Cheers.Scissors cut paper. Rock breaks scissors. Paper wraps rock.
When I was retailing shingles(till 2002)..the lowest pitch I recall seeing on a spec sheet was 2-1/2/12. But GAF was not in my area!
I live in a weather zone that does about 44 inches of rain a year and temps fluxuate from zero to 100......I strongly discourage the use of shingles below 3/12......I even discouraged it for long valleys on 4/12's.
Low (under 2.5/12) slope and flat are best serviced by membrane systems in my environment.
So the thought of warranteed shingles on a 2/12.....gives me the wet willies!
..............Iron Helix
I'm pretty sure ( I haven't had time to look ) that everything sold here will go down to 2/12 with special underlayment requirements. I don't like to do it, but the manufacturers seem to be comfortable with it. In my book anything under a 144/12 ought to get copper on it..........................Scissors cut paper. Rock breaks scissors. Paper wraps rock.