Lean to rafter calculation question


  #1  
Old 08-18-21, 08:21 PM
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Lean to rafter calculation question

Hey, so in theory I understand how to cut a rafter... but for some reason, I am losing my mind. My main issue is that I am building a lean to, not a gable, so I need 2 birdmouths and I keep misaligning them for some reason.

the building is an 8x12 coop, slope goes down the short side. There is 5/8 siding on the exterior, so that would affect the short side.

short side is 5'6. Tall side is 7'.

I was calculating my pitch at 2.25/12 because I'm rising 18 inches over 8 feet. Though typing this, perhaps the rise is wrong due to the exterior siding? But my big yellow speed square isn't exactly precise at 2.25 anyway...

The rafter is 12' and we want about 3 feet on the high side, 1 foot on the low.

if I measure from inside the high stud horizontally to the end of the low side siding (what i believe should be where the rafters fall), I get 93 1/8

Anyway, I have been measuring 3 foot down the top of the rafter, dropping the 2.25 line down, then measuring 1 1/8 up for the first cut, and using the square to get the horizontal part of the cut.

but i keep messing up because I am getting the distance to my other birdsmouth wrong. I have tried calculating it from the top, from the bottom, and every which way. I 'get' the geometry and math in theory (like, I understand the math in theory) but I'm always off.

help, please?



 
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Old 08-19-21, 05:30 AM
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If you can't figure it out, set the rafter up on top of the wall. Then measure down. Maybe you will figure out what you're doing wrong.
 
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Old 08-20-21, 05:40 AM
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I would cut the top bird mouth, set the rafter in place as XSleeper suggested, and mark where the bottom birds mouth will be located. Cut the bottom birds mouth. Once you are satisfied with the fit, use that rafter as a pattern to mark all the other rafters.
 
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Old 08-22-21, 10:02 AM
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OK, well I figured it out... sort of.

First, I redid all of my calculations and measurements... drew my lines and was STILL off on the bottom. ThankfullyI had made no cuts this time.

For anyone reading this in the future, the solution is simple. Draw your first birdsmouth, but unlike suggested above, do NOT cut it. The slanted line should sit perfectly vertical by design. If you do the higher first, it will sit on the inside edge. Now just mark the vertical line on the other side where you want it to fall (remember to space for siding if necessary on the outer bottom edge).

If you cut the birdsmouth before you mark the second, it changes the position the other end lands.
in fact, doing it this way allows you to skip pythagorean theorem altogether. You can just sit the rafter on your lean to and mark both since you are not worried about a gable. Then, use your speed square for the vertical. Take your board off, and finish marking your birdsmouth cut based on slope.

I am still unsure why my math doesn't work out. I have the right measurements and whatnot, so it's something simple and stupid I am doing wrong... but whatever
 
 

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