Opinions on plans for adding a dormer?

18 years ago

Hello,

This is my first post here and also my first dormer project. I recently purchased a small (24x36) cape with a 12/12 pitch roof. One end of the house (16') has a cathedral ceil. The stairs to the second floor leads to a small open loft area, and off of that through a doorway is one bedroom (~11'x14') with knee walls (44") on both sides. My plan is to add a dormer on the back side of the roof that spans the bedroom and the loft area. The current roof is 2x8 rafters into a 2x8 ridge beam. I plan to leave a 2' rabbit run on the far end which will end-up with ~20' dormer including the soffit overhangs. Seeing this in the greatwhitenorth there is a considerable snow load issue, especially given this will end-up being a 3/12 pitch. I plan to use 2x10 rafters spanning ~13', connnecting them directly to the 2x8 ridge beam. So do I need to (somehow?) add to the ridge bean or possible just nail to the exiting 2x8 and add support to the 2x8 raft on the opposite side?. There is no center supporting wall. The ceiling supports/collar ties will run from the opposite 2x8 rafter to a new 8' dormer wall. the current ceiling is only 7'. So is this enough information to tell me if my plan sounds sane?

BTW - I plan to buy one big a$$ tarp to gaurd against the elements as this will be more than a one day event. I am actually thinking of doing it in two separate 10' wall sections stages just to be safe(r).

TIA

Jack

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Comments (5)

ron6519
18 years ago
Wouldn't you need architectual plans for a permit? One issue I see is connecting the 2x10's to the 2x8 ridge. The bottom part of the 2x10's are unsupported. You don't mention the floor joist size. Are they sized properly for this?
Ron
sombreuil_mongrel
18 years ago
Is the new dormer wall going to be right over the first floor wall or stepped back? I don't think you can step back more than a foot or two, but you probably wouldn't want to any way.
You'll need to double the rafters on either side of the roof cut. You need a rafter table to tell you what size is needed for a 3/12 roof factoring in the required snow load for your locality. The fact that the new rafters will be deeper than the existing ridge can be surmounted by a special type of joist hanger made just for this purpose. You can always make the span less by moving the wall in a foot to bring it down to a smaller member, if you are on the margins. Structurally, you may need to make the ridge (or a portion thereof) load-bearing, but this definitely requires engineering.
Casey
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sierraeast
18 years ago
It's always a safe bet to get an onsite consultation from a licensed structural engineer.

In some cases, the dormer can be framed and bucked up tight to the weather cutting the opening out later in order to keep weather intrusion to a minimum, but you still need to keep the tarps handy.
jackinvt
Original Author
18 years ago
I appreciate everyone response. They have give me some things to consider and look into. Normally here in northern vermont there is no need to get a permit unless you are changing the footprint of your home. However this is a new town for me and a dormer does add square footage. Connecting the 2x10s into the 2x8 ridge was a concern of mine as well. The information about the special type of joist hanger is very helpful. The second floor is already a living area so the floor joists should be fine, and the new dormer wall will go directly over the first floor wall. The ridge beam is unsupported over a span of 11 feet and then is supported by the wall between the bedroom and open loft area. I generally tend to over engineer these types of things. To be safe I am planning to put the 2x10 rafters 16" o.c. I am basically copying a dormer on my existing home, also in snowy norther VT. The real issue/concern is tying it back into the smaller framing on the other side. Any additional insight is appreciated.

TIA
Jack
sombreuil_mongrel
18 years ago
This is the hanger I was thinking of. We used these on a project last summer where additional strength was called for.
Casey

Here is a link that might be useful: rafter tie