Roofs, Sept 7, 2017

We are proceeding through the limited days of the roofs module and we are making progress.

Currently, we are working on the dormers on our little building and trying to figure out the math and techniques to install the plates, ridge and rafters:IMG_5544


We introduced the concept of using the “slope gain factor” or “SGF” to calculate the length of the rafters. Because these rafters are essentially “jack rafters” and not common rafters, they are a little more complex to mark out and cut, but no worries, we will work on some common rafters soon enough.

The steps to find the SGF are fairly simple with your calculator. In this case, the roof has a 4/12 slope, so the following process applies….

On your calculator, enter:

  1.     4, “rise”;
  2.     12, “run”;
  3.     “diagonal”;
  4.     divide by “run” and the display will give you the SGF for a 4/12 roof slope, in this       case 1.054 (round to 4 decimal places if necessary).

What this mean is that if you measure the total run of the span of the roof and multiply by this “factor”, you will find the length of the diagonal of the rafter you want to cut. (BTW, each roof slope has its own SGF and will have to be calculated separately…)

We will review this each and every day and I will attempt to clarify on this blog.

Questions? Let me know and I will answer and/or clarify…




About jhcarpentry

I am a teacher in the Construction Carpentry Program at the Chateauguay Valley Career Education Centre (CVCEC) for the New Frontiers School board. This blog is a way for me to connect with my students.
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