HIP RIDGE Rafters – Do You Know How to Calculate Them?

In this computer aided age, our brain really needs some exercise – at least every once in a while. I decided it was time for my brain’s workout. I set a goal to come up with formula to determine slope of the hip ridge rafter.


I went step by step and used these basic facts to help me:

1) Back at Hines Precision Components, we used to draw all truss layouts in CAD – by hand, we did not use TrusWal’s layout program; I knew that in order to draw a dual pitch corner, I had to use the respective (side and end) slopes as angled line’s projections to x and y axis to get initial ridge rafter angle. For example, if side pitch was 4/12 and end pitch was 8/12, I would have to draw a ridge line with horizontal projection of 8 ft and vertical projection 4 ft – that would give me correct angle. Like this:


2) Then I realized that instead of dealing with side and end slope values, I can assume the horizontal projection being “1” and use SIDE-TO-END SLOPE ratio to draw the vertical projection and get the correct angle – in the example above, I would be drawing 1 horizontally and ½ (= 4/8) vertically. A generic figure would look like this:


3) According to Pythagoras’ theorem, I can now determine the unit length of ridge line’s projection to horizontal plane:


4) Knowing the length of ridge line’s projection to horizontal plane, all I had to figure out was the height at end of ridge line – again, using basic trigonometry, I was able to do that. Height at end of ridge line H=b/12 – see figure below:


5) My last step was to determine the hip ridge rafter slope: by definition, the slope is a ratio of vertical raise over 12 inches of horizontal length – using picture above, this should be directly proportionate to HEIGHT over rafter’s projection “c”. Like this:


And that’s it! The formula gets really easy for hip roofs where side slope is the same as end slope… i.e. b/a = 1

Well, I can go back to my computer aided design, knowing that “I know” how they do it now…

Martin Horak – Design Professional

Gould Design, Inc.