How to frame walls? Shed dormers with engineered roof trusses

Started by DutchMo, January 06, 2016, 12:42:12 PM

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DutchMo

Hi all,

My wife and I have the 20x30 1.5 story plans, and we are debating changing them a bit by adding double-shed dormers in the loft area.  I'm thinking the easiest way to frame this might be to get two different types of engineered trusses - 12-12 parallel chord or scissor trusses for most of the roof, and switch to 4-12 or 5-12 trusses in the dormer area.  I've seen others recommend this, so I'm assuming it is a valid approach from a roofing standpoint.  I'm thinking about going 8 foot wide on the dormer at this point.

My question is - what is the best way to frame the walls.  I've made a super-quick diagram to help describe my question:


The way I see it, there are 3 options:
1) Use a balloon framed wall section to go the entire height (red, blue, and green sections).  These would have to be 16' studs (2x6). 

2) Balloon frame the wall at 10 feet (green and blue sections, like the original plans call for), with the loft floor at 8 ft).  I could then build a small wall section (red) onto the top plate of the 10 foot wall.  However, I'm concerned that this may be a bad idea since there wouldn't be anything other than sheathing to brace the hinge point between red and blue sections.

3) I could build the shed dormer section (green) as an 8 foot wall, then do a traditional platform frame on top of that for the blue and red sections.

Any ideas on which makes the most sense?  Or, is there another way of doing this that I'm overlooking?  Maybe engineered trusses are not a good idea for what I'm considering?  I'm open to any and all ideas.  Thanks.


Don_P

I'd use cantilevered attic trusses 24' wide on 8' walls, then platform frame the dormer area to the same floor level, 8' 2nd walls, and then the lower pitched trusses, or field frame that section.


DutchMo

Thanks, Don.  I always appreciate reading your suggestions and taking advantage of your experience.

I hadn't even considered attic trusses because I figured they wouldn't give me enough clear space in the loft.  I'd never even heard of cantilevering them.  After thinking about it, it seems like a great idea.

Just to make sure I understand you, does the model below accurately capture your suggestion?

Also, do you see any problem with transitioning from 8 foot walls (green section) to 10 foot walls in the open area (blue wall section), and using scissor trusses that maintain the same 12-12 roof pitch and outside roof line as the attic trusses?

Thanks again!


Don_P

Thanks, I appreciate that.
Yes the model is what I was thinking. Scissors would work in the cathedral area but it'll leave a funky step inside. Also pay attention to the designers deflections, he'll go cheap first pass but it can be more horizontal movement than you want, if it is tell him to run it again but to stiffen it up. Grades or member sizes will increase as will cost. It plays out more as you get to lower slopes or ask for a thinner scissor depth.  Another way, they can pack several attic trusses together to get support to carry the end of a ridgebeam in a hanger at the loft end, a post can support the gable end of the beam and then rafters of the same dimension as the top chord of the attic truss can hang from the ridgebeam. A double 14" LVL should be in the range, the truss shop will do the engineering on that with the package.