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chrisser
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# Posted: 19 Feb 2014 19:15
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We're thinking about building a cabin on our rural property this season.
So I've been working through ideas.
I was going to go with 10x12, but the loft would be on the cramped side.
Going 12x12 makes the loft a little taller.
Now, a second story knee wall would be an easy way to gain headroom, but using rafters, I understand that's not the best approach, even if the kneewall studs were continuation of the first floor studs.
I understand why the kneewall is bad - the forces of the rafter want to push it outwards.
But it got me thinking - even with, say a 2 or 3 foot kneewal, that area of the loft is going to be of limited use.
So I was wondering if I could run boards from the top of the kneewall to the floor joists under the loft, creating a tie to resist the force of the rafters trying to bend the kneewall.
I did a search and couldn't find anything like this.
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chrisser
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# Posted: 19 Feb 2014 20:37
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After a little more googling, I found what I had in mind:
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MtnDon
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# Posted: 19 Feb 2014 21:48
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Quoting: chrisser I understand why the kneewall is bad - the forces of the rafter want to push it outwards.
You know the answer to the question you asked.
Building codes do not recognize what is shown. That doers not immediately mean is won't work. But we can't tell if it will be strong enough to prevent problems without some engineering analysis... what will the actual forces be? Is there snow or special wind loads? How many nails will be required to safely secure those braces? How big should the bracing be?
Lofts always have been of limited use, IMO. When people wanted more truly livable space they built on a bigger footprint or they built up with a true 1 1/2 or two story, neither of which had kneewalls. They are bad design as you already noted. Some others will argue that they are okay in a small building. That may be true. But to be sure it needs an engineers look and approval.
Note going higher in a building on a small footprint carries other concerns too. Wind loads on the narrow foundation increase quicker than on a wider building.
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bldginsp
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# Posted: 19 Feb 2014 21:48
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I don't see how that cross brace really gains you anything. If the floor joists extend from wall to wall they will do the job, you just need to secure them to the studs.
Another alternative is to use a beam at the ridge, supported on each end. This supports the roof and prevents the rafters pushing outward.
But the floor joists ought to do the job. Only question is how many of them are missing in front of the loft. Where there is no joist to tie the walls the roof will push outward. But in a 12x12 there's not much to worry about.
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OwenChristensen
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# Posted: 20 Feb 2014 07:30 - Edited by: OwenChristensen
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It's not a problem on such a small building. There are a couple ways to make the knee wall sturdy. One is a dormer made from the floor of the loft upwards, or another gable roof perpendicular to the main roof. Your idea is ok and you could expand on that by just adding another short stud. With a few braces near the middle the knee wall won't bend outward even with a northern snow load.
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chrisser
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# Posted: 20 Feb 2014 09:10
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Thanks all.
The cabin site is outside Parkersburg WV, so snow load is a consideration as is wind, to a degree.
I have some unusual constraints. Building codes aren't an issue, however we have an old shack on the property that is going to provide a fair amount of material for the cabin, so there are some things I can change and others I can't.
As an aside, both the shack and the barn on the property were built using a very unusual method of construction which I haven't found a name to describe.
The floor is somewhat conventional - planks over beams and/or logs.
The roof is also somewhat conventional - 2x4 rafters with horizontal runners and corrugated metal roofing.
However, the siding is simply nailed into the sides of the outer floor members and then a type of header board was nailed at the top onto which the rafters sit. There is no framing inside - the siding is the structure, and if I understand the forces involved, the whole structure is held up by the nails in sheer that attach the siding to the floor framing members. The siding itself is some sort of hardwood, and a mix of 1x6, 1x8 and some larger in a sort of board and batten configuration.
Both structures have withstood decades even with little maintenance over the last few. I estimate they were built in the 20s or 30s. It's hard to argue with the method given how well they've survived, but I have reservations about trying to renovate them.
So, for the cabin, I'm planning on building a floor base of dimensional lumber and a conventional frame. Take the entire roof apart from the shack and refit and reassemble it onto the cabin as it's in good shape. The framing will be sheathed in plywood or OSB with tar paper, and then I will use the siding from the shack as non-structural siding as we like the look of the weathered wood.
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wirivercabin
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# Posted: 20 Feb 2014 10:04
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My first job in the '80's was at a lumberyard that opened around early 1900. I worked with an retired Architect, we had files full of plans and estimates from back when this place started.
It's closed now, but he gave me a book of old barn plans. There was not much to they way they built then. Also a book from the 50's that has some old house and cabins plans in it.
I would build the wall full height, fasten the floor to it. "let in" a ledger to sit the joist on. And nail the joists into the stud sides. 12x12 it would never go anyplace. but that is just me.
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cbright
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# Posted: 24 Feb 2014 10:01 - Edited by: cbright
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On a building that size just frame your side walls with 2x6s such that they are one piece for the whole height... i.e. 10' for a 8' loft with 2' knee wall.
My place (a 13'x16') has a 4 ft knee wall on a 8 ft loft (12' 2x6s). My place is overbuilt and is rock solid. I do have it tied across at 8' so it is basically a 4' cantilever with minimal outward forces. I did the math a long time ago and the forces needed to deflect the walls was outside anything there ever would be under snow loads in my area.
Back to your picture.... I think that is a great idea to add strength. You could even make then triangles extend further to the center of the loft and incorporate them into a railing that would make them not seem out of place at all ... see picture of what I'm thinking.
(note, my place is 13' not the normal 12', so the picture is a little off, but you get the idea)
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Don_P
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# Posted: 26 Feb 2014 14:38
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In WV I've heard the construction style of your shed referred to as a Jenny Lind cabin. Jenny Lind was an opera singer in the mid 1800's known as the swedish nightingale... how that relates to a construction style I've never heard  .
In New Orleans it was called Bargeboard construction, barges delivering goods from upstream on the Miss were disassembled and sold for the lumber by folks from upstream before they took their payment for goods and bargeboards and hiked back home along the Natchez trace.
Late in his career Frank Lloyd Wright used the same technique in some of his Usonian homes, affordable stylish homes for the masses. He was also not too worried about energy efficiency.
If your design is only 10x12 or 12x 12 it'll be easy to hang the roof off a structural ridgebeam and that beam would likely be just one or two plies of dimensional lumber.
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Jim in NB
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# Posted: 27 Feb 2014 05:03
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cbright has the rigth idea - it is called ballon framing - tonnes of info on it - just google it
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MtnDon
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# Posted: 27 Feb 2014 09:55
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You will notice in that balloon framing example that there are still rafters ties connecting each rafter at the wall top plate. Also note the balloon framing example is a full two story design; no kneewalls. Also note the ribbon or ribband appears to be a 1x, which is still code and does not cut the stud by more than 25%; also code.
Old drawing. There is an omission by todays standards. There should be fire blocking in the wall. Without blocking a fire uses the space between the wall studs as a chimney to spread fire rapidly from below to up top.
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Jim in NB
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# Posted: 28 Feb 2014 05:24
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Pts raised by MtnDon are things that must be considered. I just posted the first pic of balloon framing I came across. I would imagine 90+% of remote small cabins do not meet code in many ways. More of a common sense approach than an in town going to sell this to some dumbass who either knows very little and must rely on building codes which are in many many cases a terribly excess means of eliminating one issue that affects one home in a million. Think of the wasted resources put into buildings in NA that could be better used in Africa or other places where any kind of shelter is a luxury - okay enough of a rant - I work with engineers all day and that can drive you nuts at times - yesterday was one of those days - have a good weekend everyone!
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Don_P
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# Posted: 28 Feb 2014 08:18 - Edited by: Don_P
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Assuming Parkersburg is in WV's 30 psf or below snowload zone, a double 2x10 ridgebeam with support posts at each end down to the foundation would carry the 12x12 roof. Then rafter ties are redundant or unneccessary since the rafters are hanging from a beam at their top ends rather than pushing against the walls or ties with their bottoms. This eliminates the question of whether a projecting kneewall can resist the horizontal thrust, there is no horizontal thrust. The bottoms of the rafters are only bearing vertically on the kneewall. The kneewall could then be of any height under 10' and can be platform framed on top of the loft floor if desired.
The pic of an inboard brace in your second post brings up one of the weaknesses of wood construction. Typically wood shines in compression but has been harder to make good tension connections in the field. Notice in old timberframes braces are generally used in pairs, they were only counting on the brace that was in compression. By contrast steel shines in tension. A single strand of high tensile wire that wrapped around the joist and kneewall and were somehow well connected would do more than a nailed on 2x4 brace in tension.
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