I’m closing on a new house in a couple of weeks and it will need a new roof immediately. It’s a small (25X25) single-story cottage with a hip roof that is a pyramid – it comes to a point with no ridge at all. I will be gutting the house and doing a cathedral ceiling. I’ve read lots of the posts about insulating cathedral ceilings but haven’t found one that has this particular issue of venting the underside of the roof when there’s no ridge. Since I’ll be tearing the old roof off, I’m considering using outside rigid foam topped with a new plywood deck and leaving the old underside of the roof exposed (it’s a look I like). I know I can’t get top R-values this way and I know it’s pricey, but I’m drawn to the seamless envelope it provides while giving me an interior look that will require almost no effort or expense since I’d be just leaving it alone. The house is in Pittsburgh and gets full sun all day. Also, has anyone had any experience with the Ondura/Onduline roofing in terms of it’s ability to self-venilate due to its corrugated design? I know they sell vent-strips that are perforated for that purpose, but by the time they make those holes small enough to keep out insects, it’s hard to believe much air actually makes it through.
Thanks in advance for any input. Due to its effect on the interior of the roof, this decision will set the design tone for the whole renovation, so I really appreciate having the chance to hear what the Breaktime community has to say while I’m still in the planning stage.
John
Replies
You are wise to have come here first, because a lot of what you are considering can cause you great problems.
First is the engineering that you will need to pay attention to to hold the walls when removing ties to create a cathedral ceiling. The hips will have to support the loads.
Second - not all roofs need ventilation, unless your local codes absolutely require it. You could use spray foam to gain R-value ( up to R-7/inch) and eliminate dew point condensation and inflitration/exfiltration.
Third, if you do need venting, a cupola could be the answer. You could also increase space available by framing an inside ceiling at a lower pitch than the exterior roof. For instance a 112/12 rof with an 8/12 ceiling.
Finally, ondura is junk. I would not use it for anything but a woodshed or a goat pen. Never on a roof. If you are thinking of a shimed/furred up venting roof surfacce which we call a clod roof, that can be accomplished for any kind of material, but an easy is using metal panels.
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Well, I was waiting to see if others would weigh in, but I guess everyone feels you've covered it. You've confirmed my hunch about Ondura. There's a house nearby that looks like #### due to the deterioration of that stuff, but I thought maybe it was just very old. At this point I just need to close on the place and get it gutted so I can see if the underside of the roof is worth leaving exposed. Thanks much for the input.
John
I think Piffin covered it surprisingly well, considering his youth and inexperience.(-:Structural concerns are first in my mind too. a 4 sided hip has thrust at all 4 walls. You have to deal with that somehow.The venting I'll leave to others...
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An unvented roof without adequate insulation doesn't sound like a good combination at all.
Seems to me structural shouldn't be a problem ... assuming the existing is cathedral and that the 4 diagonal rafters are carrying the loads to the corners.
I thought of the cupola style vent as well.
If you go foam/rigid insulation, you can get descent R-values ... it just takes more thickness ... 4" buys you R-28. You have edge details to think about, but you have to do that nomatter what. If you do it ... buy as much as you can afford ... you will only do it once. IMO
Sheathing nails at the corners are not sufficient for the loads involved, even at a 12 pitch. Cableing diagonal at the hips is a good start...it could be made somewhat attractive.
From this web page:
"Interestingly, a square hip roof, in which all the hips meet at a common point at the peak of the roof, doesn't need ties because the sheathing (the boards, plywood, or OSB that cover the rafters) connects the hips and keeps them from spreading."
Not sure what you are saying ... ?? Sheathing nails?
Maybe he meant "Sheathing nailed...."