Finding the angle of the dangle.
blownonfuel
| Posted in Construction Techniques on
When supporting a roof ridge with a purlin how do you find the angle that the purlin ends should be cut at? Ex. The purlin you are going to use will be tied into a bearing wall below that is a foot or so to the side (running parallel) of the ridge board.
What tool or technique do you use to find the angles.
Since i’m not a framer I hope I have used enough correct terminology to explain myself.
Thanks
Replies
Rise and run.
The run will be the offset from the ridge to the wall. The rise will be the height from the top of the wall to the bottom of the ridge. Find the totals of those numbers, then divide them by a commmon denominator small enough to get the numbers to fit to a framng square.
EX: the ridge is offset by 17.5 inches. The height differential is 76.75 inches.
If you had a framing square that had a tongue of 20" and a blade of 80", it would be easy to mark it full scale. Since you don't, you have to fit the numbers onto your manly "Speedier Square" otherwise known as the framing square.
I would typically (mentally) divide both of those numbers by ten. That would give me the following results: 1.75" and 7.675". You could hold those two number and get your cuts. Me? I don't like working with those small numbers becuase the square wouldn't be balanced. I'd double them again in my head and lay the square down using the numbers 3.5 and 15 1/4 for the cuts/angles.
Bob's next test date: 12/10/07
I forgot to mention. I probably wouldn't go through all of that unless there was some pressing need for an exact fit that needed to be exposed for aesthetic purposes. I'd typically be making that type of a cut up in the ceiling and have my saw with me. I'd make the test cut by eye and sometimes I'd be wrong on the first attempt. The recut would be perfect though.
If I was precutting these braces as part of the installation parts, then I would use the geometry in the first post to get all my lengths and cuts.
Bob's next test date: 12/10/07
Ah Jim so you kinda do it like using the framing square for your seat cuts on the birdsmouth?
Yes, it's the exact same principle. I used the total rise, total run equation in many, many different applications all day every day when I framed. It was as routine as pouring the coffee every morning. The carpentry instruction books always used the same technique but most often converted the numbers using a divisor of 12 to show the numbers in a foot/inch relationship. But, that's really not necessary. I always found that step to be a useless extra step except in some circumstances. Bob's next test date: 12/10/07
Whatever the angle is for the pitch of your roof. It's the same saw setting as the gable stud. Example, if your pitch is a 5/12, the saw setting is 22.62°. That would also be the angle you set with a speedsquare or framing square for the plumbcut on the rafter. When you scribe the framing square set at 5/12, if you take a speedsquare, it will read 22.62°.
You set your saw at 22.62° (22.5°) and lay the beam on a flat and cut it.
If your pitch was a 12/12, the saw setting would be 45°.
I've never used a "purlin" before. Why are you using one?
There are many ways to determine the pitch using math, framing square or a speedsquare. What are you using to layout your rafters? Are you comfortable with math, and if so are you using a construction master calculator, scientific calculator?
I probably did not make myself clear on this one Joe. I understand pitches of roofs and how to get that but I wanted to know how to figure different angles out.If you wanted to support something from underneath but there was nothing directly below only off to the sides how would you figure out the angles to make the cuts for the support?I attached a cheesy power point to show what I mean. Sorry I don't have sketch up here.
Yep, that's exactly what I was picturing Blownonfuel.I converted your picture to a PDF file using the free openoffice PPT program. Bob's next test date: 12/10/07
Remember, I said I'd eyeball that cut first if I was adding the stud to the existing purlin or ridge? My eyeball guess would be about a 6/12 ratio with the picture you are showing. A quick cut would tell me how much my guess was off. Of course, I wouldn't measure my first cut, I'd be doing it by eye but I doubt that I would be more than 5 degrees off on my fist go at it. A quick measure with my scale gave me a ratio of 7 1/2 and 3 7/8. If I was field cutting that member, I wouldn't bother converting those numbers into a base 12 number. I'd hold 15" and 7 3/4" on the framing square.My first eyeball guess was actually a 4/12 till I rotated it. I then thought it was a 6/12 or lower but more than a 5/12. In the field, where my eyes are filled with the exact ratios, I'm actually pretty accurate on my first attempt cuts. I don't mentally think of any numbers or ratios, I just hold the saw and lumber to the same angle I'm gazing at. Bob's next test date: 12/10/07