I’m interested in having a home built that is 1 1/2 stories tall that will have dormers on the second floor. I can’t seem to find anything about the proper way to frame the second floor knee wall using conventional lumber. All I’ve found is that “balloon framing” is an old way of doing that but is no longer used due to problems with fire codes. The only other thing I could think of would be to use posts to support the main roof beam and have the rafters hang from that or to have a timber frame home built. Any help would be great.
Thanks.
Replies
There are lots of ways to do it. Attic trusses, I-joists with a structural ridge beam, conventional lumber, etc.
Some more details might help - Width of the house, pitch of the roof you're going for, how wide you want the upstairs to be, how big the dormers are, etc.
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Thanks for the reply. It's a small cottage style house that is only 16 feet wide and 42 feet long so obviously I want to get as much space as possible out of the upstairs. There will be dormers on both sides of the roof that are roughly eight feet wide in two locations on each side.
What Boss said, lots of different ways. But just to clarify, Balloon framing is not a problem with fire codes, just have to add blocking every 8 feet vertically, depending on local code. I usually platform frame the kneewalls and haven't had strength/support issues partially because I've always used 2x6 walls and if not full collar ties somewhere up there, at least some visible beams to hold the roof system together.
I like the idea of the 2x6's and the visible beams. How often would a cross beam be required and at what height would it be required for a 12/12 roof (for example, < 1/2 the height of the roof, etc.)?
I mentioned the collar tie idea to a builder I know and he said that collar ties more than a few feet above the base of the ceiling offer almost no resistance to spreading. Does that sound right?
He's basically right. The higher up the collar ties are, the less work they can do to help keep the walls from spreading out. The general rule of thumb is anything in the bottom 1/3 from plate to ridge is effective. Anything higher loses effectiveness progressively, though it seems like if you glue and through bolt the collar ties they can work a fair amount higher. No matter how you go, I'd use a structural ridge.
A really good article you may want to check out is in the May 2000 Fine Homebuilding #130. "Framing an elegant Dormer".
MD