helping working out my pitch 15 degrees

Discussion in 'Carpenters' Talk' started by tiler7777, Jan 26, 2016.

  1. dwlondon

    dwlondon Active Member

    thats right you are given 2 values 15 deg and 3.3m. in this case its the sin function as s=opp/hyp

    so sin15=x/3300
    or 0.258=x/3300
    or x= 3300x0.258

    = 854
  2. chippie244

    chippie244 Super Member

    This is what was said at the beginning, it's only HA who can't do the maths and then tries to wriggle out of it.
  3. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    :) I'm still confused actually. Reason being, I can't figure out how the OP knows the length of the joists, but not the height(or the difference) of the two walls. coz change the height of the wall relative to each other, then the joist length will be different.

    If however his initial 'length of joist' was wall to wall(3.3m), then the roof joists would be longer and the high wall would be 1100mm for the 15º.

    Mr. HandyAndy - Really
  4. Phil the Paver

    Phil the Paver Screwfix Select

    That's very good question, I only worked the 850mm out on the fact the joists were in fact 3.3m in length, if as you say the distance between the walls is 3.3m, then the joists will indeed be longer and to achieve 15° would need the rear wall to be higher than 850mm.
  5. chippie244

    chippie244 Super Member

    If the distance between the walls is 3.3M then the rise will be 884mm and the length 3616mm not including any over lap,
    to keep the rise down to 500mm the acute angle is only 8.6 degs.
  6. Phil the Paver

    Phil the Paver Screwfix Select

    Looks like a flat roof is going to be his preferred choice at this rate. :D:D
  7. chippie244

    chippie244 Super Member

    Looks like it :):)
  8. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    The rise will surely be 1100mm?

    Mr. HandyAndy - Really
  9. Phil the Paver

    Phil the Paver Screwfix Select

    No, Chippie pretty much on the money.
  10. dwlondon

    dwlondon Active Member

    for a difference (rise) of 1100 you can work out from both sin and tan but not cos

    sin15 = 1100/joist on an angle (hyp)
    or 0.259 =1100/x
    or x= 1100/0.259
    = 4247mm

    tan15 = 1100/joist level wall to wall (adj)
    or 0.268 = 1100/x
    or x= 1100/0.268

    proving with pyhtag to approx figures

    4247^2 = (4104^2)+(1100^2)
  11. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    That's all very well, but with two sides of equal length at 90º perpendicular, the third side having angles of 45º, it follows that 15º will be one-third length of the second side
  12. dwlondon

    dwlondon Active Member

    it doesn't work like that..............

    the 45 deg is correct but the 1100 is actually 884 and the 2200 is 1900.

    this is because the sin tan and cos functions follow a sinusoidal shape ie like a wave form and their mathematical values are not linear. which means when you are working to rule of thumb, you won't be able to expect them to be of predictably nice and easy proportions.
    Last edited: Jan 27, 2016
  13. tiler7777

    tiler7777 Member

    The measurement is 3.3meters from the external side of the outerbrickwork.

    Ive allready bought the roof tiles off gum tree for a fiver so they are going up. I dont like wasting five pounds. lol

    So is 884 the correct measurement or is it more with the joists being more than 3.3meters.

    At the momnet the wall is 1.8meters high so if I add the 884 thats 2.64 at the front and 1.8 meters at the back. Then I was going to build up the corners of the front of the building with soil so any measurment comes out 2.5meters
  14. Jitender

    Jitender Screwfix Select

    Does depend on the tiles used, as most have a minimum pitch to work to.

    If you draw it out using a scale of 1:2 and protractor.

    Then the height comes to about 650mm
  15. Jitender

    Jitender Screwfix Select

    Scale should be 1:20
  16. chippie244

    chippie244 Super Member

    If you are having problems there is a trig app, all you have to enter are 2 values and it works out the rest.
  17. tiler7777

    tiler7777 Member

    so is it 650 or 884?
  18. Jitender

    Jitender Screwfix Select

    Roof Angle.jpg Ok worked it out now.

    The trig function is Tan.

    so Tan15°=opp/adj

    3.3m x tan15° = 884mm

    But it is probably best to lay the rafter according to the course of bricks taking into account the battens and roof tile, as you want a 100mm lead flashing.
  19. CraigMcK

    CraigMcK Screwfix Select

    If 3300mm is the horizontal distance: Tan 15 * 3300 = 884mm
    If 3300mm is the slope length: Sin 15 * 3300 = 854mm

    650 does not come into it anywhere
  20. Jitender

    Jitender Screwfix Select

    I though I would draw it out and see it would work.

    I did another drawing with a scale of 1:10, ans it still came out to 650mm:(

    The Tan function is the one to use as you have the adj and need to work out the opp; opp/adj

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