Dormer (roof style)

[Thor]

Okay Rich I'm gonna test your knowledge on dormer roofs :lol:
My current home is an older (55 years) 1- 1/2 story house with inadequate roof insulation that causes some severe icing problems up here in the frigid north. I want to add a dormer and am debating on a shed style roof or a gable style. Of course the shed style is easier to build , however I'm worried about having enough R value and breathing space that would eliminate icing problems. So for a problem free dormer I presume a gable style would be more appropriate for an area with heavy snow falls and extreme cold temperatures.
What I'm wondering is if there is a way to construct a shed style dormer roof that will provide enough R value (approx. 40) and still have enough space in there for proper ventilation that will avoid ice build up ?

[Rich]

Actually there is typically more problems with a gable type roof because of the 2 additional valleys. But anyway with a shed style dormer you can get both good insulating and proper ventilation - if the conditions allow it.. haha.
Let's look at insulation which is typically the driving force for depth of framing members. R-40 with high density batt insulation is about 11" thick which would bring your framing members to at least a 2x12 to get any ventilation (you will have to compress it some to get baffles in). You may want to look at a rigid type insulation to get the rafters down to a 2x10 with plenty of air space. Some of the rigid type will give you R-30 in just 2" - go 3-4" for R-40.
Now on to the framing. There are several things that are unknown right now so I'll try and cover both conditions. If you keep the outside wall tall enough to where the ceiling of the interior and the dormer are at the same elevation you can lay the insulation in the top of the ceiling joists and have plenty of area for ventilation above it. Add eave vents and you're pretty much done.
If you have to vault the ceiling at the pitch of the rafters then you will need exterior vents and baffles to get the air up to the header. At the header you will have to provide another means of ventilation through that - at least 2" holes in each space.
If you vault the ceiling it then becomes an engineering problem. Can I put X number of holes in this header with this area above it and have it hold what I need it to? Is there enough room below the ceiling line to put a beam in and still have head clearance to allow the existing roof rafters and new roof rafters to rest on it?
Also - you will probably need to double the 2 existing side rafters up to carry the new weight of the header and associated rafter loads.
Hope that gets you started anyway.

[Thor]

The existing interior vaulted roof line doesn't provide a whole lot of headroom. (I'll get some measurements for you later) I can see this project will turn into one of those engineering nightmares. However I have plenty of time to ask questions and make up a solid plan as this project probably won't take place in the very near future (probably not til next year). Currently planning for a massive outdoor firepit/poolheater combination that I'll be starting as soon as all this snow melts......wait til you hear the questions I'll have for that project :lol:
Anyway I'm expecting a digital camera from Sears within the next few weeks , so I'll be able to provide you with some photos and measurements of the existing roof and some of the other plans that I have for my roofing project.....tinkering with the idea of extending the roof over top of a 12 X 30 deck , so it could be in my best interest to totally remove the existing roof and start from scratch but with my luck it will rain for 40 days and nights while I have the roof completely removed.
I like your recommendation of adding rigid foam to increase R value especially since my favourite brother in law just gave me 55 sheets of 2'X8'X 2" rigid foam that he didn't have any use for.........and who says in-laws are a pain in the butt :lol:
We'll put this subject on hold for a few weeks til I provide you with photos and measurements , that way it will be easier to come up with a definate plan. I have some webspace where I'll be able to put up the pics and link you to them as soon as they're available.

[Rich]

Sounds like you're on the right track for everything anyway... plan plan plan. Then go for it.
Yeah if you get me some dimensions I can put together a quick drawing for you.
As long as you are registered you can use the album on the forum to upload pictures to also. Just click the Album link in the navigation above.