An Off-Grid Passive Solar Cabin Design
By Tommy Bailey
In July 2010 I finally had the opportunity to pursue a dream of mine – to begin construction on an off-grid passive solar cabin that I designed and would build myself. It took me about 3 months of several weekends and evenings to rough in the entire structure and I spent the next 6 months working on the interior as I got free time.
For this article I would like to talk about the specifics of the building process and I have included a picture gallery below. Unfortunately, I didn’t know I was going to write a blog post on this at the time or I would have definitely taken a lot more pictures. Also, I did a “prequel” blog post on this passive solar cabin on www.livingasimplelife.com if you would like to read about the philosophy and inner workings on how I got to this point.
PHASE I – Foundation:
When I was planning the cabin I knew I wanted to use Locust posts for the foundation. My property has several large locust trees on it so several months before construction I felled some trees, cut them to approximate lengths and skinned the bark off them. I used the locust for the foundation posts and decided to use southern yellow pine (SYP) for the floor joists and girders. SYP is a bit more expensive (still relatively cheap) than white pine and spruce, pine, fir (SPF) but is a heck of a lot stronger.
Laying out the foundation:
My original cabin plans called for a 16x16ft square. Laying out a square is not rocket science, but from my past experiences I learned you’ve got to get this part as close and humanly possible as to avoid a build-up of problems later on. For instance, it doesn’t seem like a big deal to be an inch or so off on the foundation, but as you continue construction that inch can turn into a lot of headaches when your floor joists and floor are not square, then you get even further off square when you add the walls, then the siding, roof and so on. So it’s best to take one’s time during this step and get as perfect as possible so the rest of the construction process will go smoother.
There are two methods I use to get a foundation “square”. One is to measure and mark off each corner, in my case 16×16, and pound stakes (not deep, you will need to pull them out several times before you are done) at each 16ft corner mark. Then take a tape measure from one corner to the opposite corner – like the line on an X. Then you run the tape from the other corner to its opposite corner. You’re looking for the same number here. If it’s off, as it probably will be, you just start pulling up your stakes and repositioning them until you can get within an inch of square.
This is when I use the 3-4-5 or 6-8-10 triangle method. There is an official name for this but I don’t know what it’s called…something to do with Pythagoras. Here you can build a wooden triangle to these dimensions or you can run a string from each of your corners and mark them accordingly. For a better explanation and diagram of this I encourage you to search the net or check this out, one of the first search pages I came to when looking or pictures: http://www.thisoldhouse.com/
After I paid careful attention to get the foundation square I began marking and digging holes for my locust posts. My cabin base was 16×16 and I planned to use at least 2 – 2×12’s for each girder so I decided to place 3 posts along each side at 8ft. intervals and a line of posts down the center. 2×12 lumber can easily span 8ft. and I could have actually gotten away with smaller lumber, but I like to overbuild because I know it will be solid for years to come.
Without getting too in depth here (because there are numerous ways you can do this next step) I put a little gravel in the bottom of each hole (for water drainage) and installed my posts, making sure to get a good tamp on the dirt every 6 inches or so. Oh, I went down about 24 inches, which for Virginia is enough to get below the frost line. You don’t want the soil freezing and thawing and moving your cabin in the winter. This can lead to settling doors, cracked drywall and broken windows!
PHASE II – Floor and Walls:
After I got all the locust posts in place I then nailed the 2×12 girders to them using 16 penny nails to hold them, then went back with large lag screws to secure them for the long term. Now I was ready to start installing the floor joists. Since my cabin was 16ft. and my posts were inset to about 15ft. I used 8ft. 2×8’s that would rest on the middle and outside girders – about a foot or so on each side cantilevered. Once I got all of my floor joists in place I installed sill boards all the way around and was ready for the floor!
Because I was on a budget and because I planned to lay tile in most of the cabin, I used 1/2 OSB for the subfloor. 3/4 inch would have been better but every dollar counted on this project and I planned to install 1/2 backer board on top of the OSB so this would stiffen up the floor substantially. Installing subfloor is always fun for me, I don’t know if it’s because I am making progress or because I get to hammer a lot of nails! Anyway, I ran a line of glue on the floor joists, place the OSB right side up and secured with ring shank nails. There are a lot of options for subfloor, but OSB is probably the cheapest out there.
After I got all the subfloor down I began immediately on the walls. The thing with construction is you want to get your project under roof as quickly as possible. A few days rain while “roughing in” can really put a damper on the project, especially if you’re not using water resistant subfloors!
About this time in the construction my wife said she wanted to add a small alcove/room! What! I said! We’ve already got the foundation in and the floors joists on and the floor almost done! In reality, it could be done, but at the time I saw it as more of a distraction from my grand plan than a bonus! But, after some griping and a few days digging new holes for posts and building into the current floor plan I’m glad we did it. After all, two days of work for a small room that would eventually become a bedroom nook was well worth it.
Back to walls…I already had the door and windows at the site and knew all of the dimensions of each so I easily laid out and built my walls in a few days. I invited some friends over to help out in this stage. It is nice to have one or two friends to help lift a wall into place and hold as stabilizing nails and bracing are added.
PHASE III – Roof:
With the last minute addition, the cabin was pretty close to 16×20 at this point. There was still a section of the cabin that measured 16ft. across but I planned to add a small covered porch later on that would pretty much make the cabin a 20×20 square. This would also help in my planning to do the main roof and smaller roof offshoots. For ceiling joists I was lucky to find 20 foot 2×8’s! I have never used lumber this long before and didn’t even know lumber came in lengths this long until this project!
The 20ft. rafters were a bit too long, but it would give me plenty of space for the big overhangs I wanted, plus it would save me the trouble of cutting and piecing together two 10 foot boards. These 20ft. SYP boards were hard to handle by myself so I enlisted my wife to help me unload and get them on top of the walls. Oh, by the way, I built the walls so that I could have a sloping shed roof on the cabin. This would make the roof fairly easy to construct and went along well with my skill level of roof construction. I forgot to mention that 20ft boards are nice but there is no way they could carry the load of the roof and extra snow load of winter. So I erected posts and beams on the interior center of the cabin to absorb this extra load. Since these timbers would be exposed I used a nice hickory which was not only strong but aesthetically pleasing.
After I got the ceiling joists nailed and hurricane strapped in place I called my friends again and we installed purlins and metal roofing! Installing the purlins took about a half day of me going up and down the ladder more times than I can count. I think it took pretty much took a full day to install the metal roofing.
That’s it for this post, check back soon for:
PHASE IV – Windows and Siding
PHASE V – Interior
PHASE VI – Porch and Landscaping
PHASE VII – Things to Come
Category: Renewable Energy