Joined at the Hip

It turns out that a hip roof (think of a four-sided pyramid) is harder to frame than one might think. Much of this post is cribbed from an email exchange with Andy, one of the projects expert advisors.

I planned to have the four main rafters (one from each corner of the tower) ascend to meet each other at a "capstone" piece. This was a non-starter, though it did waste a few hours, as the capstone eventually splintered to pieces as I tried to drill through it.

In the end I laid two hip rafters from opposite corners, meeting at the peak. These were straightforward to cut and measure; I did it the same way I framed the gable rafters on the main room. (Of course, the birds-mouth notches are not angled on their interior to match the external corner. See photo. But I figure they'll be mostly invisible, shielded by fascia on the outside and by the interior framing on the inside, so good enough.) I've now cut 15 rafters like this, including the four I had to discard, so I'm getting a little better at it.

What I wanted to match up well was the peak, where the hip and common rafters come together, and (eventually) the joints with the jack rafters that will fall from them to the wall. Because these joints are all visible from inside the tower.

I calculated the angles you would see if you looked directly down on the roof from above (a 5'x3' rectangle, with two diagonals -- plain old trigonometry). The height of the roof I determined by fiddling around until it looked right; it was quite pleasing to discover that the vertical angles (ie, looking straight in from the side) are EXACTLY the same 28 degrees as the horizontal.

So why did it take another hour, and five false tries, to cut the last rafter I got done this morning? Well, there's this little angle adjust on the skilsaw that runs from 0 to 45 degrees. I set it at 28 degrees, and the cut was way off. OK, I decided I calculated wrong, and just kept trying different measurements until I finally hit on the right one, which turned out to be 17 degrees . . . see where this is going?

17 = 45 - 28

If you've actually followed this, you can see why it took me a while to stop swearing when I realized.

The only other difficulty is making straight cuts with the skilsaw. It tends to move around on me, especially when the blade is angled. Clamping rails down, as I did for all the plywood cuts, doesn't work when the blade is angled (the motor body interferes).

Is there a power miter saw that rotates along TWO axes? That would sure make this whole thing easier.