Sunday, October 20, 2013

Roof Framing Base Knowledge #1

I finally had the time to have Brian and Erik draw a Jack Rafter Layout Board and show them how to layout a hip rafter correctly. Also, I had them cut the jack rafter back bevels (side cuts) with a hand saw. It might be the first time in 60-80 years that a stick framer carpenter in California has cut any rafter bevel with a hand saw.

Jack Rafter Layout Board



The jack rafter layout board allows you to draw out the real roof surface of the roof. On this house we had to align our rafters with the open web floor joist and it resulted in 4 different lengths for the first jack rafter length at each of the 3 hip rafters. Once you draw the jack rafter layout board you use the run of the jack rafter in plan view to draw out the jack rafter in the roof surface view to find the dimension of the jack rafter.












Drawings of jack rafter back bevel layout.





Here's Brian and Erik cutting the jack back bevels with a handsaw. It's important to know how to layout a jack rafter head cut. Once you layout and cut a jack rafter back bevel angle you'll have a better understanding of how the compound angles are developed using a skill saw.

















The Seven simple steps to layout the hip rafter correctly.
  1. Hip Rafter Plumb Line
  2. Hip Rafter Level Line
  3. Hip Rafter Shifted Plumb Line
  4. Hip Rafter Backing Depth Line
  5. Roof Plane Alignment  Point
  6. Hip Rafter Seat Cut Line
  7. One Length Method for Hip Rafter Head Cut Line.



One Length Method for Hip Rafter Head Cut Line.
Mark off the hip rafter head cut plumb line from the Roof Plane Alignment Line. 

The hip rafter run to the ridge was 71 1/2". With a 5 1/2" - 12 pitch the hip rafter length was 106 5/16" . Mark off the  106 5/16" from the Roof Plane Alignment Line to mark the hip rafter head cut line. 


Example of the One Length Method with only the necessary lines to cut the hip rafter. The skill saw cuts at the seat of the hip rafter develop the correct angles to locate the theoretical hip rafter plumb line at the corner of the plate line. 












9 comments:

  1. This should be a good article!


    Don't forget your paste wax . . .

    ReplyDelete
  2. I'm sure Erik and Brian are glad this lesson is over! Lol! Could you imagine cutting an entire roof with those saws? (Time lapse on that . . . =)

    If I had to cut Hip Rafters with a handsaw I'd be using the "Swanson's" method (notching the corner to accept the square cut heel). But with a couple of opposite tilting circs, the reverse diamond is available. (Single Cheek at the head to meet extended ridge band?)

    The more familiar you become with the "One Length Method", the less plumb lines you'll find necessary.

    ReplyDelete
  3. the older hand saws used for jack cutting were super sharp and much longer that the ones your boys used ,so there would have been fewer strokes . The jacks were also laid out on the stock so that only one cut was needed for both jacks the longest jacks cutoff was the short one for the other side of the hip . I was taught that method . We don't stack cut like you guys out west ,we had big long horses which we could layout 16 or more rafters then use a template to mark the rafters

    ReplyDelete
  4. Yea, I wish I still had my handsaws from the 70-80's, but they only had to cut one jacker rafter each. The main purpose of the lesson was to show them how to layout the jack rafter cheek cut using a scrap of lumber the same thickness as the jack rafter to mark the second plumb line and draw the back bevel across the top of the jack rafter from plumb line to plumb.

    ReplyDelete
  5. Richard,
    I have a complete under standing of the One Length method, but I wanted to show the boys the correct way to layout out the hip rafter. So, all of the lines in the pictures are necessary. However, I did use the one length method to layout the the hip rafter plumb line at the peak of the rafter, but added another plumb line at the the peak of the hip rafter on the other side of the hip rafter so I could cut the hip rafter with a handsaw. Also, I cut another cripple hip and the fastest way to layout and cut the cripple hip is to mark off the length of the cripple hip, draw the plumb lines, then use a scrap of lumber the same thickness of the hip rafter and mark off the second set of plumb lines. Cut the outside plumb lines on a 45 and then cut the inside plumb lines on a 45.

    Also, you should say that the One Length Method is used from the hip rafter roof plane alignment point. Mark the plumb line for the hip rafter roof plane alignment and measure from that point to your hip rafter head cut plumb line.

    ReplyDelete
    Replies
    1. Sim,

      Very well presented.

      The significance of this exercise is much more important than most conventional roof framers would realize. And the only reason that it can be accomplished with less plumb lines is because of the use of the Modern circular saws. It virtually over simplifies the process.

      It would be interesting to see a similar exercise for an Irregular Hip and Jacks. The proportions of the stock can be used for many bevel cut layouts and proper layout placements. (I.e.; Theoretical ridge space between the Valleys and Cripple Hips, at the lower ridge. And the long-point to short-point offsets for H/Vs and Jacks.)

      I think that the easiest way to see how the geometry of H/V Rafters work is to visualize the theoretical Cripple Hip that is present in every H/V rafter.

      Delete
  6. Billy,

    After the boys cut the jack rafter with a hand saw I now think that no one should be allowed to cut a roof until they've cut a jack rafter with a handsaw.

    ReplyDelete
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    ReplyDelete