Planning a 10'x12' x 20' roof (!) A-Frame

[Chet Wesley]

Hi There,

First post here.

I have 20 acres in Northern CA. In the last year I have built a very small un-insulated A-Frame (8ftx4ft!), a small bathing hut, and an outhouse.

This year I'd like to build a building that is big enough to also do basic things inside of in the day time if it is cold or raining. Like cook, poop, sit/read/etc. The little cabin I have now is really only for sleeping because of how small it is, and I only can heat it with a little propane space heater.

Code enforcement in the area is basically non-existent, but I would like to avoid having to tear it down if someone does notice.

I am pretty set on an A-Frame. I am aware of the drawbacks, but I am attracted to the structure for various reasons. My home in the city is actually an A-Frame with a 5' knee wall.

Here is what I am thinking:

- 12' wide, 10' deep, 20ft rafters, making it about 19' tall at the peak.



- 7.5' deep loft 10' up, with collar beams across at that height. This gives about a 6'x7' loft, and 2.5' of space that is open all the way to the ceiling, and nice tall 10ft ceiling below the loft.

- Just building the 12x10 deck platform/floor on blocks, possibly with holes dug and gravel under  to ensure some stability. I'm not really too worried about it; it's a small footprint.

- Lots of window on the front and back

- 1' of roof overhang on front and back, making roof depth 12'

- Corrugated plastic roof

- Very small wood stove.

- Insulated somehow.

My questions:

- Is 20' rafters/19' height insane on this small of a deck? I haven't seen anyone else build an A-Frame that tall at that footprint size. I like the shape, the height, and it would give more usable space without having to build knee walls.

- I am thinking about the rafter spacing and dimensions of lumber for the rafters. I am thinking about doing double 2x6 possibly at 2.5ft spacing so that is 4 "A's." I really don't know what is safe. There will be no worry of snow loading because that is a very steep roof. The only thing the roof will need to support is its own weight and that of the loft with two people sleeping inside.

- The A-Frame house I live in the city in is built with T&G 2x6 boards as horizontal support, and a ridge beam on top. Big 4x12 rafters. I don't know for my design how necessary a ridge beam is, nor whether purlins are really necessary or alternately whether it could be OSB sheathing tying the rafters together.


I will be building this mostly on my own.


For your gratification, here are some pics of the projects on the property so far, which has been 99% built with no assistance and mostly with reused wood.









And what it's looking like there right now!

[flyingvan]

Does the wind blow there?

[Chet Wesley]

Yes

[Don_P]

At 16' wide it would be 18' tall, closer to a 1:1 aspect ratio. That would give a loft at 8' up about 7'6" width. The higher a loft is the longer and more awkward the stair/ladder. It looks like you could find a suitable large foundation rock(s) to pin to. From there some rocks and mortar and it would be pretty simple to build a nice stone foundation.

[Chet Wesley]

 
My real concerns are with the required spacing and dimensions  of rafters and other structural issues. These are the aspects I am most hoping anyone with some experience or theoretical understanding can chime in on.

120 square ft is the max allowable building size when building unpermitted in my county.

I have thought about 15x8 which is also 120 square feet but more unusable floor space due to the more gradual roof slope in a wider A-frame.

I am guessing the question about wind is a smug way of suggesting it is unwise to build without a foundation, but frankly it hasn't seemed that likely that a  very bottom-heavy cabin weighing a few thousand pounds will just blow over. It's 4500 feet up in the hills and surrounded by firs and pines, not Kansas.

However  I only have my own common sense and a 100 year old cabin still sitting on some rocks on a nearby hill to go by. I definitely could anchor into rocks on some parts of the property, while other parts have softer soil.

The proportional  dimensions seem more a matter of personal taste. 10x12 makes sense to me in a floorplan sense given my size restrictions. Building relatively tall I just think will look/feel nice and help mitigate some of the interior space lost to roof slope.

[flyingvan]

  You're basically designing a big sail, so I'm a little concerned about the wind load.  If you have a good foundation, I think your corrugated plastic roofing would be the first thing to go.  You mentioned a 120 square foot limit before requiring permits---is there a height limit that goes along with that?  If so, looks like you have a decent rocky slope.  If you were to dig out a foundation, you'd gain a solid footing, add interior height while decreasing exterior height, and add space without much expense--just labor.  It would also help with climate control, a challenge with A frames due to their large surface area

[Chet Wesley]

I don't understand. It seems less a sail than most box-like houses.  The amount of surface and weight in the higher parts of the structure is very small compared to the amount of surface and weight lower. A sail on a boat is triangular to keep the energy of the wind collected lower to maintain a lower center of gravity and keep the boat from tipping over.  There are plenty of 20ft tall A-frames of various slopes and the roofing materials don't seem any more prone to blowing off of them than on any other design.

I hear what you are saying about the advantages of a foundation in the other senses but this is also not a year round dwelling.  Being able to build it myself with my resources in the time frame I would like to build it are aspects of consideration too.

[flyingvan]

http://www.buildingsguide.com/calculators/structural/ASCE705W/

    Here's a pretty handy wind load calculator.....Put your numbers in and see if they meet acceptable PSF levels.   
    Most sailboats have their center of gravity below the water surface, where a heavy keel does the work.  Another strategy is to keep the CG above water line to reduce drag, and maintain a wide stance--like a catamaran.  Your structure is pretty tall compared to the footprint.   I don't think the weight being low at the higher parts matters much, but the surface area sure does...
    The cottage I just finished was taller than it was wide, too.  I had to go real deep with my concrete foundation and have a considerable amount of shear transfer.
    Stick built structures are very, very lightweight things.  Just some (very) rough estimates---if you stick with 4 'A' s, each made from doubled up 2x6, at 40# each for 20' and 24# for the 12's, I get 208# per 'A', x4 is 832 pounds.  3/8 OSB is 1.2# PSF,  so 960 there, then the floor we'll add 1,000#.  You are now at 2,792 pounds---I'll round up to 3,000.  This gives your structure a density of about 2# per cubic foot, about the same as the heavier styrofoams...
    Is the roof 12' wide?  If so, I get 480 square feet per side.  With a narrower stance on your footprint and no foundation, there's a concern.  (There are formulas online where you can plug in mass, width and height to find a tipping point of something but suffice it to say as the height increases and the width decreases, it takes less force at a given mass.)  When I guessed at your numbers and plugged them in to the wind load calculator I came up with 15.8 PSF...Multiply that by 480 and I get 7,584.  I think I remember structures have to withstand a minimum of 10 psf wind load but I'm not certain about that..
    So I think your options are
1) Do it as planned--tall, skinny, no foundation--and hope for the best.
2) Lower and widen your structure.
3) Secure your structure very well to the ground.

     I'll have to get a picture of the A frame in our neighborhood with anchor cables attached to the ridge and strung off into the ground.  I don't know if it was tipping, scooting, or shaking in the wind but it's a retrofit

[firefox]

Anchor cables are a good idea. Military uses them all the time and pretty easy
to implement with ground anchors similar to what the power companies use to
secure phone poles.
They would also be convenient for hanging outdoor lights  and bird feeders.
Just an idea.

[firefox]

Also in the summer, could be used to support a shade tarp.

[Chet Wesley]

Thank you, yes, a good idea.