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Roof framing 101

dug | Posted in Construction Techniques on September 13, 2006 01:26am

     Framing question…. If I’m building a screen porch with a 4-12 pitch gable roof and tying in 90 degrees to an existing 10-12 pitch roof, how do I get the soffits to come out level. Do I have to drop the plates (walls) on the 4-12? Is their a formula for this or do I step my rafter tails off with a square and adjust thru trial and error. I’d like my soffits to be the same width if possible.

    This will be a farmers valley or California style valley tie in so is it absolutely neccessary to remove the shingles before I lay down my valley boards.

     Thanks

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  1. Framer | Sep 13, 2006 02:30am | #1

    It all depends on what size the existing rafters are and what size the new rafters are and also if you want equal overhangs.

    If you use the same size rafter as the existing rafter and you have a 12" overhang, then your plate height difference would be 6"

    10 - 4 = 6" (Plate Height Difference)

    10/12 roof would have a 10" drop for 12" overhang.

    4/12 roof would have a 4" drop for a 12" overhang.

    Joe Carola
    1. dug | Sep 13, 2006 03:49am | #5

      10 - 4 = 6" (Plate Height Difference)

            Thanks Joe, this is what I could'nt get in my head before. Now I'm thinking well, duh...

            Now, if I adjust my plate height to account for this, will I be able to make my soffits level and the same width also.

           This is why I like this site, thanks again!

          

  2. Schelling | Sep 13, 2006 03:00am | #2

    Joe has given you a good answer. I would only add that you need to be careful that the finish edges line up. If you are adding on a sub fascia and a fascia and an additional piece of cornice moulding, these will all change the finished heights on bastard roofs. I like to lay all this out on a test rafter or two so I can see all the pieces drawn out in place.

    It makes a good homework assignment the night before you cut your rafters.

    1. dug | Sep 13, 2006 04:00am | #6

      It makes a good homework assignment the night before you cut your rafters.

            Wow, I figured this was second nature to you guys that frame everyday. Glad to know I'm not the only one who has to "think" about this.

           

      1. Schelling | Sep 13, 2006 04:10am | #8

        I don't frame every day and that is why, like you, I have to think about this and consider all the angles. Just be glad that you only have to worry about the overhang and the height at the eaves. Cutting the bastard valleys is the next lesson.

  3. User avater
    Huck | Sep 13, 2006 03:18am | #3

    If your soffits are the same width, then your plate heights will have to vary to make it work.  I think that the best and easiest way is to lay out the soffit detail full-size on a the slab or a piece of plywood.

    "...never charged nothing for his preaching, and it was worth it, too" - Mark Twain



    Edited 9/12/2006 8:29 pm by Huck

  4. User avater
    Huck | Sep 13, 2006 03:27am | #4

    Something like this...

    View Image

    "...never charged nothing for his preaching, and it was worth it, too" - Mark Twain

    1. dug | Sep 13, 2006 04:02am | #7

            I'm following ya, but it was a little foggy to me since I don't frame everyday and I gona be tying in at 90degrees. Thanks for the reply.

  5. RoofFraming101 | Sep 20, 2011 03:22am | #9

    Rafter angles

    Some additional info on rafter angles for 4/12-10/12 bastard valley:

    • If your 4/12 rafters are landing on top of your 10/12 roof then the rafter angle for the level cut (even tho its not technically level) of your 4/12 rafter will be 68.62 degrees.

    • Your valley rafter will run at a 21.8 degree angle from your 4/12 side and 68.2 from your 10/12 side.

    • Your jack rafter cheek cut angles will be 21.8 on 10/12 side and 68.2 on your 4/12 side.

    • Valley/Hip 4/12 run secant  (4/12 common rafter run x 1.127 = Valley length)

    • Valley/Hip 4/12 diag secant (4/12 common rafter diag/length x 1.070 = valley length

    • Valley offset 12" overhang 7 13/16" toward 4/12 side.

    • Frieze block/square cut facia angles: 4/12      miter=7.3       bevel=20.23

    • Frieze block/square cut facia angles: 10/12    miter=44.6     bevel=45.51

    Hope these help.  Good luck!

    For additional tips and info on Roof Framing visit http://www.roofframing101.com

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