Construction Question

[Jeff]

Tammy and I are putting up a 12 by 16 building.  We are using 2 by 6 rafters and a ridge board, for roof construction. The pitch we want is 6/12 or a simple 45% cut. My question is, is there a forumla to figureing out where exactly to cut the "birds mouth" where the rafters sit on the wall. We will be spanning the 16 foot direction. The ridge board will be a 2 by 6 as well. It would be nice to cut all of these pieces in advance.  So far we have our bearing walls up. Still need to build the gable end walls before we get to the roof.

[DouginUtah]

Jeff,

If your rise over run is 6/12 then a cut of 45° is not going to work. That would be for a 12/12 pitch.

For a 6/12 I think you will need a 22 1/2° cut. If I understand what you are saying. 

A framing square is used to lay out the bird's mouth cut--held at 6" and 12".

Laying it out in AutoCAD would give you exact numbers but I don't have Acad on this computer. I suspect someone will have it for you before long though. If not, I will lay it out for you.

-Doug

[DonE911]

Good question....  Last time I  did a roof I cut 4 rafters to death until I got one right...   then I put it down on the floor and made a jig so every one was exactly the same....  I still messed up a couple of em... no idea how that happened.

[mike_van]

Doug is correct, a 45 will make the pitch 12/12.  Jeff, I use a framing square, they make 2 small brass knobs for it, on the outside edge you tighten one a 6" and on the long edge, one goes at 12".  Hold these against the edge of the rafter, you have your pitch angle. Works for stairs too.   There is a method for the length of the rafter called stepping off, I never learned it though. Easier for me was to cut the ridge cuts, lay the 2 rafters out flat with a piece as thick as the ridge board between them, then if your walls are 16' apart, lay a 16 footer across them, line it up good & even, mark the ends, thats where the birds mouth is.  The same angle set on the framing square will give you  both cuts for the birdsmouth.  Help any??

[Gilman]

Here's a 6-12



here, I bolded the dim I think you are looking for.

Note: 2 x 4 walls


Here's a 12-12 Ski Lodge, 2 x 4 walls.


Let me know if you need any changes to wall thickness, etc.

[Jeff]

I cant read it.     I can make out 144" but I need it for the 16 foot, outside to outside.  THe ridge beamd will be 1.5 inches thick.

[Gilman]

 

[Jeff]

I'm pretty dense when it comes to that stuff, but I bet if I study this long enough I'll figure it out.

Thanks!  BTW, is it 22.5 degrees on my chop saw?

[woodbeard]

BTW, is it 22.5 degrees on my chop saw?

Probably, but the angle is not going to be 22.5 deg.
You will have to lay it out the way Mike explained.
Then you can measure the angle, and set your chop saw to it.

[Furby]

Yup, ya need to double check with a framing square and set the saw to that. Remember the angle though in case ya need it later.

[Don P]

Jeff, a 6/12 is 26.5 degrees...5/12 would be 22.5 (half a 45)...which are you wanting?

[Gilman]

Yep, it's 26.6 degrees





I like pictures 

[Don P]

Try this calc out, I entered 95.25 (outside to outside 192" - 1.5" ridge) / 2.
http://www.ls.net/~windyhill/Calcs/raftercalc.htm
I then punched in 6...for 6/12
Then 0
0
0
and Show result

The plumb to plumb line length measured along the top or bottom edge is then given. It does give the same numbers as in a rafter table. Let me know if it doesn't make sense.

I like to make "yokes" to hold my ridge in place temporarily while I build the roof. Setting them at the right height was always a little tricky for me.
This calc will give the height to cut a prop under the ridge. It places the bottom edge of the ridge flush to the Bottom edge of the rafter's plumb cut. That's the "correct" location for inspector types.

[Furby]

Sorry, I'm just a simple framer..........it doesn't make sense.

[DouginUtah]

Jeff,

Do you want to have the inside open to the ridge or do you want to put a ceiling in so that you have a floor for an attic, with that additional storage space.

If you don't put a floor in, there is nothing tieing the bearing walls together. Weight on the roof (snow load) would try to squash the roof down and push out the walls.  The ridge board, supported at each end, will be all that is supporting the weight of the roof. It is only 12' but I doubt that a 2x8 would be adequate.

Now it's getting trickier. Attic floor joists would have to run the 12' direction so they wouldn't tie the walls together--just the flooring (OSB) would tie it. Maybe, this just being a shed, you could run the joists 16' but it would definitely be a spongy attic floor. Decision time!

-Doug
(I suspected that cutting the slope in half wouldn't make the angle half. Glad I said "I think you will need..." I always plot these out in CAD and dimension the angle.

[isawlogs]

Doug
  that could be solved with some collar ties .

[Jeff]

there is not going to be "attic" but there will be cross beams down about one and a half feet from the peak leaving a cathedral like ceiling.  I want a 6/12 pitch as that is supposedly what our house is.

[Don P]

Furby, try this one.
http://www.ls.net/~windyhill/Calcs/rftllclc.htm
It was an "earlier" model
If its still not clear give me stronger hints  .

If those purlins are well supported in the endwalls and stout enough they can act as structural support and do the job just fine.

106.5" plumb to plumb, Jeff.

[isawlogs]

  How many collar ties you putting in  Jeff 

[Jeff]

Rafters every 16 inches and coller tie on every rafter. Plan on insulating, and finishing with t and g pine at some point on the inside.