If you know the per metre run for the commons you can also use this times by the jack centres. i.e roof pitch of 40 degrees gives a per metre run of 1.305, jack centres = 400, diminish is 1.305 x 400 = 522mm
....and also if dont know the rafter length per metre run for commons you can get it by using Cos as follows.... take 1 divided by Cos pitch°= length per metre run. i.e. 1 divided by Cos pitch 40°=1.305
@@michaeldownes516 Excellent, thanks for that, i very much enjoy your animated videos, i love pitching roofs but also love to to understand the geometry of how and why everything works.
The Jack diminish is the distance traveled along the roof slope...which relates directly to the run or horizontal distance traveled from one jack edge cut to the same point on the next jack. So for or every 16 inch traveled horizontally it will be matched by a longer distance along the slope/pitch (to get to the same point).....the steeper the slope/pitch, the greater the distance along the roof pitch line. The Cos formula for jack diminish just uses the known diminish run distance combined with known pitch to then determine the distance traveled along the pitch line. The jack diminish run distance = rafter centres ,but this is only the case if the hip/valley is at 45 degrees to the jacks on plan.
Your graphics are extremely helpful in helping to visualise and understand joinery concepts. In this instance too, your additional remarks in the comment section relating to distances along the roof inclination and their relationship with the roof slope and additional Jack rafter length is important too. Many thanks for sharing your knowledge and for providing great graphical content!
