looking for tips on simplifying the rafter cutting for my 8 equal sided turret (15″-3 1/2) span, 12/12 pitch
I am making a 3 1/2″x3 1/2″ octagonal ridge block and cutting eight equal commons for the center of each wall
I have the commons all figured on paper, and wanted to figure the pitch of my 8 equal hips on the desk instead of trial and error on the wall
Thanks in advance
Mike
mike
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Replies
Hips will be 13/12 not seventeen. Is that what you needed?
Hips will be cut to 12/13 or 7/13 ect. whenever octogon use thirteen instead of seventeen. You should pick up a roof framers bible. I guess some guys would tell you that you could figure it out with math. I started framing for my self after only a year with my boss. Books like roof framers bible, complete roof framer, A roof cutters secrets ect. Any resource you can get. The last book I mentioned I think would serve you well. It's a great book to help you figure things out on the ground with math. All though there will be times when it's faster just to use your tape.
Roofcutter's Secrets is a great book. A suggestions I can make is to snap it out on the ground. Even though it takes a few minutes, it does make things simple.
I've got a house coming up with 2 octagon bays (half octagon that dive into the main roof).
Another edit :-) The diagonal for a triangle (13" run, 12" rise) is 17 11/16".
Take (17 11/16 /13)= 1.360908 mulitply that by the run of the hip and you've got the unadjusted hip length
Edit: here is the link to Will's book. http://www.amazon.com/exec/obidos/tg/detail/-/1928580149/qid=1108429631/sr=1-1/ref=sr_1_1/103-6681692-3609405?v=glance&s=books
Edited 2/14/2005 8:07 pm ET by TIMUHLER
Edited 2/14/2005 8:12 pm ET by TIMUHLER
Edited 2/14/2005 8:20 pm ET by TIMUHLER
Thanks to all
I know in the past i have just used a 12/12 and adjusted the seat cut
12/13 it shall be
mike
Using a block at the turret peak doesn't completely eliminate angled rafter cuts as you would hope. The octagon hips will run into the corners of the block, requiring a reversed double cheek cut to get good bearing. I still do it the way we were taught at hammer school:
There are 4 different rafters in an octagon roof.
'first commons' - these two butt directly to each other at the peak. They sit square across the wall plate like a regular common rafter. No shortening. Unit of run:12. Cut 2 pieces.
'second commons' - these butt in square to the first commons, and are the same theoretical length. Shorten by half the thickness of the first common stock, square to the 12/12 plumb cut. Unit of run: 12. Cut 2 pieces.
'third commons' - same theoretical length as first and second commons, but have a double cheek cut at the top to let them into the right angle formed between the first and second commons. Shorten by half the diagonal thickness of the common stock. Unit of run: 12. Cut 4 pieces.
'octagon hips' - these fit in the 45 degree angles formed between the third commons and the other commons. They sit on the plates where the walls intersect. They also have a very acute double cheek cut that will need to be cut with a handsaw or sawzall. The easiest way to determine the shortening and cheek cuts is to do a full size layout of the turret peak on a piece of plywood and measure them directly. Unit of run: 13. Cut 8 pieces.
Calculate line length of common rafter:
12/12 roof with span of 15'3-1/2".
Run of roof 7'8-3/4"=91.75"
91.75/12=7.6458 units of run
unit line length per unit run for 12/12=16.97
16.97*7.6458=129.75"
so line length of common is 10' 9-3/4"
Remember this is the theoretical length from centre point of turret peak to outside edge of wall plate. Some of the rafters need to be shortened as mentioned before, plus you still need to add the overhang.
Calculate line length of octagon hip:
run of roof = 91.75"
91.75/12=7.6458 units of run
hip unit = 13", so run of hip = 7.6458*13
run of hip = 99.395"
rise of roof = 91.75"
line length of hip=sqrt((run of hip)^2 + (rise of roof)^2)
=sqrt((99.395)^2 + (91.75)^2)
=135.268"
so line length of hip= 11' 3-1/4"
I like to do a full size layout of the turret peak so I can see the sidecut and shortening for each member and directly measure them. Octagons take a little while to figure out but they make an interesting change from the ordinary.
Wally
Lignum est bonum.
Sly, I use this same method except that I install all the hips first.
The common jacks are very easy to install then. I never have a common that reaches the peak.
blueJust because you can, doesn't mean you should!
Warning! Be cautious when taking any framing advice from me. There are some in here who think I'm a hackmeister...they might be right! Of course, they might be wrong too!
So you cut the hips square on the peak ends and start that way? Then what I call commons - because they run square across the wall plates - would have the 22.5 degree angles on the ends. I guess it doesn't make much difference which order it's done in... or does it? Do you cheat a bit and just butt the commons into the angle without bothering with a double cheek cut? That would be a real time saver if the peak's not visible from below. Hmmm, file that one away for next turret, gazebo or bay window.
Wally
Lignum est bonum.
Sly, when I've done true octogons, I usually start with two opposing hips. I square cut them and lay them against each other. The next pair of hips are also square cut and lay against the first two. Of course, this second pair needs to be shortened 1/2 thickness of the first pair of rafters.
The third group of hips would all have double bevels and layinto the four standing rafters.
After getting those eight hips standing, I then install the commons. I intentionally start my layout 11.5" off center and install the two longest jacks. This eliminates a common hitting into the already crowded center peak area.
In most of these octogonal roofs, I usually only have to put one pair of jacks in each bay. Occasionally, I will need to put a second pair of jacks in each bay, but the second pair will normally be very short.....less than 48".
I typically bevel the jacks starting at the center of the member. Experience has shown that if I tip the saw (remember, I'm not fighting any guards or extended tables) running the blade as deep as I can, the tip of the blade will just peek out and be carving out a nice fit. If I try to bevel the entire rafter thickness, the bottom of my blade will be buried somewhere inside of the cut and I'll have to flip the rafter over and shave off a significant amount to make the rafter lay against the hip in a normal manner. I don't see any significant gain in strength doing all that extra work, so I simply bevel only the amount that my 7 1/4" saw blade allows me.
Frank won't even bother to bevel most of his octogonal rafters, but since I'm an oldtimer and had a lot of stuff brainwashed into my head, I continue to do it my way, even though some oldtimers would look down on my metholodgy.
I think my stuff looks fine and I know it will do the job that it's expected to do...both methods are relying on the same shear strength of nails to keep the rafters standing.
blueJust because you can, doesn't mean you should!
Warning! Be cautious when taking any framing advice from me. There are some in here who think I'm a hackmeister...they might be right! Of course, they might be wrong too!
I'm not an oldtimer but I still like to bevel it all and make it fit right. However the method you describe sounds like a good time saver. As you say, nail shear strength is the key here, not exact fits - unless the work is visible from below like a gazebo.
I've filed this one away for future use. Not sure when it will get used; just found out today my next job is a big box hardware store of 60,000 SF with a row of smaller rental stores in front, another 13,000 SF. All tilt-up slabs with structural steel roof. Opening date October 1. There goes the summer!
Lignum est bonum.
MRjesse, I always install the hips first. Many years ago, on my first octogon, I used a built up centerpost like you are suggesting and ran my commons into it. The cutting of the hips was brutal. I never did that again. Now, I run all the hips and I usually only have to put two "jacks" in each bay.
To get a good sense of how I attack these beasts, check out my Boogerin thread about octogon roofs. It has pictures.
That's my only tip although I could go on for hours.
blue
Just because you can, doesn't mean you should!
Warning! Be cautious when taking any framing advice from me. There are some in here who think I'm a hackmeister...they might be right! Of course, they might be wrong too!