Minimum knee wall height?

[Rocksteady]

I would like to modify the plans I got.  In the plans the knee wall is 2 feet tall in a platform built (not balloon framed) 1.5 story.  I'm going to keep the half story an unconditioned attic and I need enough height between my ceiling and my eaves for R-60 insulation.  I would also like the height of the walls + subfloor + Rim joist + sill plate to equal exactly 10 feet so I can use 10 foot pieces of OSB and be as efficient with the material as possible, not to mention no seams in the sheathing save every 4 feet horizontally.  Is there any minimum requirement on knee wall heights?  I would guess that the knee wall actually becomes stronger the shorter it gets because the height to width ration decreases and the hinging tendency with it, but maybe i'm totally wrong.  Thanks for any answers, sorry about the convoluted nature of the question.

[MountainDon]

Missing detail.... do you have a preference for rafters or trusses? 

Best construction practise would not use rafters with kneewalls of any height. Rafters generate a horizontal outward force; the shallower the roof pitch the more the force. The usual engineered truss does not have any outward horizontal force though and can be placed on top of short walls; kneewalls. Some trusses like scissors trusses may have some horizontal force but limited. Trusses can also be designed with raised heels to help provide space for full insulation right out to the wall.

Trusses can be designed to provide usable attic space down the center of the upstairs.

Unconditioned attic... Question?  What sort of heating system would be planned? Any central A/C?  Forced air heating and A/C are now most efficient when used with a conditioned space, even just a conditioned duct chase.

Once the roof framing / truss has been designed then the lower sections can be sized to accommodate those 10 foot sheets of sheathing. But then how does the interior height of the rooms work out for drywall? Seems to me you might have to juggle a lot of figures to make inside and outside materials work out with zero waste. I'm not sure I'd lose any sleep over trying to avoid cutting of a few inches of OSB from a ten foot long sheet, but that's just me. If there was interior drywall using that with a minimum of sutting would take priority.

[Rocksteady]

Heating with a wood stove, not anticipating using an HVAC system given the small size of the house.  I'm also sheathing the house in 1.5 inch foam so 8 foot lengths plus 2 foot lengths comes out to a very nice 10ft.  I have no problem with the idea of trusses, but I would like to save time and money which is where rafters come into play.  If i have 2x8 rafters, a 2x12 ridge beam and 2x6 collar ties at the midpoints of the rafters on 16 inch centers spaning 7 feet from the midpoint of the roof at a 12/12 pitch, do they still exert that much outward force under a snow load?

[AdironDoc]

Aside from structural considerations, which are Don's forte, there are some practical issues as well. For me, I set the knee wall height in the guest loft to allow for low-profile Ikea style beds to slide in fully and still allow someone in bed enough room to prop themselves up on an elbow. Also, that my foot lockers and low cabinet could be used. Remember, if the knee wall is too low, it will prohibit use for anything but bags and some plastic bins. 30" was what worked for me in the guest camp loft. Yours will unconditioned but may well be put to good use in the near future.

[MountainDon]

....If i have 2x8 rafters, a 2x12 ridge beam and 2x6 collar ties at the midpoints of the rafters on 16 inch centers spaning 7 feet from the midpoint of the roof at a 12/12 pitch, do they still exert that much outward force under a snow load?....

First I don't know  how much snow load you have so can't take snow into account to affirm rafter sizing or comment on how much that will contribute to wall loading as you described the structure.

The 2x12 at the ridge would be called a ridge board, not a ridge beam, unless this is a very small building. As a ridge board it simply is a handy thing to nail the rafter tips to and to provide and maintain the proper spacing.

If you place those 2x6 horizontal "members" half way up (or down) the rafter height they are neither collar ties or rafter ties.

A collar tie should be in the upper third of the rafter triangle and serves to help keep the rafter tips ties together in times of great winds. Hence, to do the best job as a collar toe they should actually be as high as possible. They may be eliminated if appropriate steel straps are used, over the peak.

A rafter tie, on the other hand, should be in the lower third of the rafter triangle (illustrated below).



As the rafter tie moves up from the top of the wall (the ideal location) in that lower third, it affects the load, the stress, on the rafters. Because of that rafters must be de-rated by as much as 1/3 when the rafter tie is at the upper third position. This is in the footnotes for Table R802.5.1(1). Higher up than that their effectiveness as a rafter tie falls considerably and the ties stress the rafters even more. Also note that a ceiling joist, or attic/loft floor joist that is dropped down from the wall top into the stud bays, creates stress on the wall studs as well as not truly effective as a rafter tie. That gets worse as loads go up.

IF, a properly sized ridge BEAM is used and provided with properly sized support columns in each gable end wall all the way down to the ground and perhaps in mid span, if required, then the game changes. The rafters only carry the roof load in a straight down direction. One half the total roof load on the BEAM and one quarter on each wall that the rafter tails rest on. Note that one half the load on the ridge BEAM can add up to large numbers quickly as snow load increases, making the foundation support for the ridge BEAM and columns very important. Collar ties are still required but there is no horizontal outward force on the wall tops so rafter ties are eliminated.

[Rocksteady]

Thanks MountainDon! I really appreciate your knowledge!  I think I'm going to buy the Big Enchilada plans to get a different perspective on how to do things.  The plans I have simply don't seem beefy enough and I love overbuilding things when I find out what it would mean to overbuild something.  Thanks again, I love this place!