trig

Discussion in 'Carpenters' Talk' started by yozman, Mar 26, 2005.

  1. yozman

    yozman New Member

    i know i should have paid attention at school but
    can anyone help me with a bit of trig i need to cut some rafters for my monopitch roof and need to know plumb and seat cuts the measurements are 2990mm span x 1000mm rise
    anybody help please
     
  2. dirtydeeds

    dirtydeeds New Member

    before we can do this what is the thickness of your planted ridge
     
  3. yozman

    yozman New Member

  4. yozman

    yozman New Member

    i will be sitting the top of my rafters onto a 100mmx50mm
    joist bolted to wall making plump and seat cut 1/3 the thickness of rafter which will be 50mm rafters 150m x 50mm
    why do you need ridge thickness?
     
  5. dirtydeeds

    dirtydeeds New Member

    because otherwise the length of the rafter will be wrong and as a consequence the angle of the roof will alter.

    more on the angles to follow
     
  6. dirtydeeds

    dirtydeeds New Member

    This is a mono pitch so you take the length and deduct the total width of the planted "ridge"

    ***************************
    I have assumed that the horizontal measurement is from the wall to the OUTER face of the wall plate. IF IT ISNT AND YOU HAVE MEAUSRED WALL TO INNER FACE OD THE WALL PLATE TESE CALCS WILL HAVE TO BE REDONE
    **********************************

    So 2290-50= 2240
    The rise is 1000

    so using the tangent rule opp/adj (so 1000/2240)=.446428
    using the inverse tan give the angle = 24.057 degrees.

    nobody can work to .057 degrees so 24 degrees is good

    The plumb cut is therefore 24 degrees and so the seat cut is 90-24 = 66 degrees.

    The rafter length from plumbcut at the ridge to plumb on the birdsmouth is therefore 1/cos 24 x 2240

    DONT CUT THE RAFTER THIS LENGTH BECAUSE YOU HAVE TO ALLOW FOR THE FASCIA AND SOFFIT
     
  7. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    because otherwise the length of the rafter will be
    wrong and as a consequence the angle of the roof will
    alter.

    more on the angles to follow

    How come ?

    Handyandy - really
     
  8. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    This is a mono pitch so you take the length and
    deduct the total width of the planted "ridge"

    ***************************
    I have assumed that the horizontal measurement is
    from the wall to the OUTER face of the wall plate. IF
    IT ISNT AND YOU HAVE MEAUSRED WALL TO INNER FACE OD
    THE WALL PLATE TESE CALCS WILL HAVE TO BE REDONE
    **********************************

    So 2290-50= 2240
    The rise is 1000

    so using the tangent rule opp/adj (so
    1000/2240)=.446428
    using the inverse tan give the angle = 24.057
    degrees.

    nobody can work to .057 degrees so 24 degrees is
    good

    The plumb cut is therefore 24 degrees and so the seat
    cut is 90-24 = 66 degrees.

    The rafter length from plumbcut at the ridge to plumb
    on the birdsmouth is therefore 1/cos 24 x 2240

    DONT CUT THE RAFTER THIS LENGTH BECAUSE YOU HAVE TO
    ALLOW FOR THE FASCIA AND SOFFIT

    Did that help ?

    Handyandy - really
     
  9. yozman

    yozman New Member

    sorry just realised i put mono roof should be lean to
    your calc sound right to me and yes the measurement was to the outer edge of wall plate, so if i am not using ridge board just 100mm x 50mm timber for a head with common rafters birdsmouthed on underside to sit on top of head will the calcs be the same
     
  10. dirtydeeds

    dirtydeeds New Member

    yoz, the words dont matter much. mono pitch is a perfectly correct description for a lean to roof.

    by the way handyandy, please check my calculations and tell yozman if they are correct. two people doing a calculation is always better than one
     
  11. yozman

    yozman New Member

    dirtydeeds just realised you have worked on wrong measurement the span is 2990 you have put 2290 thanks yozman
     
  12. chappers

    chappers Member

    if you are going to birdsmouth your rafters over the ridge then the measurements given for the rafter run should be made using the 2990 and not subtracting the thicknes of the ridge plate, your plumb cut at the top should then be 50mm back.
     
  13. dirtydeeds

    dirtydeeds New Member

    thats why i asked handyandy (another person) to check my calculations

    2990-50 = 2940

    1000 height

    using tangent opposite / adjacent

    1000/2940=.340136

    inverse tan = 18.78 degrees = roof slope = plumb cut

    seat cut = 90 - 18.78 = 71.22degrees

    you can use pythogoras therom to get the length of the rafter (plumb cut at ridge to plumb cut at birdsmouth) as i said last night dont cut the rafter this length because you need to allow for the fascia and soffit

    you can also use the calculation 1/cos 18.78 degrees x length to get the length between plumb cuts
     
  14. dirtydeeds

    dirtydeeds New Member

    thanks chappers, didnt spot the birdsmouth over the ridge.

    yoz, redo the calcs as chappers says with no deduction for the ridge
     
  15. panlid

    panlid New Member

  16. dirtydeeds

    dirtydeeds New Member

    sid, morning

    its no good grinning, im not perfect like mr MDF

    im still waiting for him to put his head above the parapet (sorry door)
     
  17. yozman

    yozman New Member

    sorry to be a pain dd but could you give me the calc
    based on 2990

    cheers
     
  18. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    because otherwise the length of the rafter will be
    wrong and as a consequence the angle of the roof
    will
    alter.

    more on the angles to follow

    How come ?

    Handyandy - really

    How come ? Top of plat to ridge is top of plate to ridge whatever height of plat, so same angle, if 1000mm higher than plate. Just curious.

    Oh, and ;) on the bater couple of posts back(knew I'd come in useful for something, even if it was as an excuse)


    Handyandy - really
     
  19. dirtydeeds

    dirtydeeds New Member

    andy,

    first i notice that you did not check the calculations for me and went for a p* take instead. it seems to me that possibly you cant do the maths???????????

    second reason why i think you cant do the maths is because you didnt help the guy with his problem

    There is a third possibility. you can do the maths and you cannot find nothing wrong with the basic principals and have no interest in helping the guy because your raison detre (my french spelling is useless) is to have arguements with people.

    anyway i will now rise to the bait. you question the fact that if the rafter length is not correct (longer or shorter) the angles will change. Please STOP and THINK.

    use a 3, 4, 5 triangle, the roof pitch would be 36.87degrees approx . Now if you cut the rafter longer at say 5.1 the pitch alters to 38.34 degrees approx

    ( i have put approximatly so you dont argue the point needlessly)

    I rose to your bait BUT ive snapped your line. weep celebrity mdf joiner
     
  20. dirtydeeds

    dirtydeeds New Member

    PS andy do YOU use mdf for rafters as well.

    Woops! now that was deliberatly provocative.

    That was very bad dd. Shush youre lying mouth



    DAM andy has got me at his arguement game AND im talking to myself
     

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