Layout the Foundation Outline
Regardless of the type of foundation you choose, you’re faced with the challenge of creating an outline for your cabin that has truly square corners. Measuring with a tape and your eye alone won’t deliver corners close enough to 90º. And since the scale of the work is large, you need geometry to deliver accuracy. A carpenter’s square just won’t do it.
Start by laying out one of the 16’ ends of your building, with one spike or stake at one pair of end-wall corners. Next, grab two large tape measures and a couple of helpers to hold the tape ends on the spike heads. You’re about to locate one of the 25’ sides of the cabin so it forms a square corner with the end wall of the cabin you just marked as your starting point.
The overall width of the cabin is 192” (16’) and the overall length (including porch) is 300” (25’). And according to the Pythagorean Theorem (remember high school geometry class?), the corners of this rectangular footprint will be 90º when opposite sides are the same length, and diagonal measurements taken corner to corner are equal. So what should both these diagonal lengths be? Here’s the formula, with the numbers for your cabin plugged in:
Square root of the length of diagonal = Length of one side squared + Length of adjoining side squared. This sounds more complicated than it is. Watch the video up next to see how simple it is.
VIDEO: Geometry is Your Friend
In the case of this 16’ x 25’ cabin, the diagonal (also called the hypothenuse) works out to be almost exactly 356 3/16” or 29’ 8 3/16” when the corner is square. Hook one of your tape measures over each corner spike (this is where the helpers come in), then extend both tapes out at an angle so the 25’ mark on one tape rests on top of the 29’ 8 3/16” mark on the other. The spot where this intersection takes place is the spot where one corner of the remaining cabin side must be located to create a square corner at the other end. Sink a 12” spike or wooden stake here. Repeat the process for the other corner, then double-check that the opposite two corners you just located are the proper 16’ length apart from each other, just like they’re supposed to be. You’ve now got a truly rectangular outline for your cabin, with 90º corners guaranteed. The holes you need to dig for the foundation will need to be larger than this, of course, but at least now you know exactly where your cabin foundation piers need to go.
Be sure to watch the next video to see how all this double-tape business plays out in the real world. It’s simple but powerful.