Calculating angle cuts

Discussion in 'Shop Talk' started by Phantomfyre, Jul 13, 2007.

  1. Phantomfyre

    Phantomfyre Black Cat Farm Supporter

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    I feel a little silly asking this - I guess I should've paid more attention in geometry class years ago... :rolleyes: And I can't seem to find an answer searching the internet - probably, I'm searching wrong.

    I am building a 4X8' chicken tractor with a triangular roof. I figure that, using 4' 2X4s to make the sides/roof, I'll create an equilateral triangle, so all of my inside angles would be 60*. My bottom frame is a flat 2X4, so to get the inside angle of 60*, I need to cut the side 2X4s at 30* on the bottom to get them to lean in correctly so they meet at the top. At the top, where they meet, they'd need to be cut at 60* to get them to join flush with one another. That's my theory, anyway, and I hope this makes at least a little sense...

    How do I calculate how much to cut off the end of the 2X4 to create the correct angles? I tried calculating this, and was way off the first time, so I recalculated and thought I fixed it, and they still wouldn't meet up correctly. I don't have a miter saw, just a circular saw and a jigsaw.

    I'm starting to think I shoud've just built a rectangular box, but I'm already into this project, and shoot, it can't be that hard...? I'm just a little math-challenged. Last fall, I managed to build the horses a pretty respectable 12X24 run-in shed, but that was all right angles... :shrug:

    Thanks for any help!
    Diana
     
  2. wy_white_wolf

    wy_white_wolf Just howling at the moon

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    Using a framing square. Setting the legs at 12 and 20 3/4 with the edge of the board will give you the 30 and 60 degree angle.

    Cut what ones for the floor with square ends. The legs will be the same length as the floor (add for rim joists if used) but with one end at 60 and one at 30. Stand them up on top of the floor.

    In case your wondering why I know: http://s48.photobucket.com/albums/f220/wy_white_wolf/Cabin/
     

  3. BobK

    BobK Well-Known Member

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    ..while you're probably already there you could also use the law of cosines to calculate length.....
     
  4. Bearfootfarm

    Bearfootfarm This Space For Rent Supporter

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    Get a helper. Hold the bottom end of a long rafter up to the side 2 x 4. Find the center of the span and mark it. Raise your rafter to the height you want and plumb up from the center line and mark it. Mark across the other end for the bottom angle. If youre going to use a ridgepole too, subtract half the width from the top cut .
    You now have a pattern to use to mark the rest.
     
  5. wy_white_wolf

    wy_white_wolf Just howling at the moon

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    Why Bother? With an equilateral triangle, all legs are equal.
     
  6. sammyd

    sammyd Well-Known Member

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    Buy a square...
     
  7. bill not in oh

    bill not in oh Well-Known Member

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    Diana - unless I'm misunderstanding what you're doing, the long side of your rafters should be the same length as the distance that they are spanning. And I believe that the angle at the top will be 30* on each (½ of the 60* that you need there).
     
  8. moopups

    moopups In Remembrance

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    In that your going from top plate to top plate, you covering 180 degrees of a circle, the cuts have to equal that amount. If the top plate cuts are 30 degrees each, 60, then there is 120 degrees left, which makes each rafter 60 degrees.

    Now to figure out where to stop them concerning distance; find the exact center of the gable, both upper and lower. Place a temporary vertical timber so one edge of it corresponds with the vertical center line, obviously making it higher that the estimated stopping point of the rafter. Place the 30 degree cut end on the correct place of the top plate, making sure the fit is exact, now you can scribe a line on the edge of the rafter, at the point it contacts the vertical timber.

    Cut one example of this, test fit it to both sided of the vertical timber, if it fits correctly that problem is no longer of concern. Measure the width of both gable;s top plate spread, any variance will cause this method to not fit correctly.
     
  9. Phantomfyre

    Phantomfyre Black Cat Farm Supporter

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    Thank you for all the suggestions! I admit, several of them were totally over my head though, LOL. I have NO formal knowledge of building. Everything I "know" I learned by watching Dad or looking at something to see how it was put together and replicating that, and sometimes just trying stuff and learning as I went along. Like when the push mower wouldn't stay running, so I decided I could fix it myself and tore it apart... (Why not? It wasn't working anyway...) The carburetor just needed a good cleaning and that old mower ran like a champ again! :D

    Anyway, I will definitely be adding a square to the list of tools I need to acquire!

    In the meantime, I think I was able to get those angles fixed up using Bearfootfarm's suggestion. :)

    Thanks again, everyone!
    Diana