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Discussion Starter #1
I am building a large workshop for myself in the Cape Cod style. It will have two dormers on the front and a shed dormer in the rear of the top floor. I plan to build a two foot high knee wall on the front and back of the upper floor, than install the rafters and dormers. I will use engineered wood I-beams for rafters, and hopefully a wood I-beam for the main ridge beam. The dimensions of the workshop are 16 ft across the front and 20 ft deep. The ridge beam will span the 16 ft dimension. I want to have as much head room as possible on the upper floor, hence the knee walls. I do not have architectural plans. I want to have at least an eight foot high ceiling on the upper level.
My queston is this: Am I correct in assuming that I will have to install horizontal members between the rafters to prevent the upper floor knee walls from spreading? Since the rafters will be resting on knee walls, there will not be anything to hold the knee walls vertical without bracing the rafters with horizontal members, front to back. I don't have the exact pitch of the roof calculated yet, but it will be fairly steep, since it will be a Cape Cod. Can I just install the horizontal members at the eight foot or higher level, as long as they are present to tie the rafters together across the 20 ft dimension? Will I have to put horizontal members on every rafter. or can I use another method?

The shop will look similar the one at this link.
http://www.thehousedesigners.com/plan_details.asp?PlanNum=3800
 

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Not an architect but one solution may be to build the walls higher and install the joists for the upper floor on a ledger board. That way the joists would help hold the walls in instaed of just a 2 foot wall nailed on top of the joists.
 

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Discussion Starter #3
I thought a ledger board was for supporting horizontal joists vertically. My rafters will be going up at an angle, therefore creating outward pressure on the knee walls.
 

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Instead of doing an 8ft wall with joists on top and then a knee wall make the frame 14 ft tall, install a ledger board and put your joists on the ledger board. You can nail through the joist into the studs which will then act as a cross brace. Then you can cross brace the rafters. It would be much stronger than a knee wall.
 

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Discussion Starter #9
Actually, my second floor joists will be paralell to the walls that will support my rafters. The width is 16' and the depth is 20', so I am putting the joists across the 16' dimension. The rafters will be front to back, so they will span the 20' dimension. I will have to figure out how to attach the front and rear 12' high stud walls to the second floor decking at 8' above the first floor decking, so that the rafters will not cause the front and rear walls to spread. I guess that I could attach a ledger to those walls, and then use the floor decking to hold the walls vertical. Is that stong enough?
This method you suggested sounds like what I have read about as "balloon framing" versus "platform framing", which is what I am used to.
 

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Actually, my second floor joists will be paralell to the walls that will support my rafters. The width is 16' and the depth is 20', so I am putting the joists across the 16' dimension. The rafters will be front to back, so they will span the 20' dimension. I will have to figure out how to attach the front and rear 12' high stud walls to the second floor decking at 8' above the first floor decking, so that the rafters will not cause the front and rear walls to spread. I guess that I could attach a ledger to those walls, and then use the floor decking to hold the walls vertical. Is that stong enough?
This method you suggested sounds like what I have read about as "balloon framing" versus "platform framing", which is what I am used to.
Have you considered a Gambrell roof? Sometimes called a 'barn style' roof!
It would give you more head room on the second floor! And would save the bother of the dormers!
 

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Discussion Starter #11
Not an option.

Actually I did consider a Gambrell roof, but my wife wants my shop to look like our house. Only two dormers instead of three. End of discussion!

The upper floor will be built with rafters supported on 2' high knee walls. Two dormers in the front, and a shed dormer in the rear.
The problem is: how to install the wood i-beam rafters on the knee walls without the knee walls spreading. I need as much headroom under the roof as possible. I don't know if I can use "balloon" framing because it will be too high for me to reach.
 

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