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Fixed Ridge Height and Rafter Pitch

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  • Fixed Ridge Height and Rafter Pitch

    There's been discussions before about when doing an addition on a house and your new ridge has to match the height of the existing ridge and the new addition has wall heights given.

    There's many different situations that we run into but the big question is what is the pitch of the rafter or how do you figure out the rafter for this?

    I've tried several ways over the years like nailing the new addition ridge in place and scribing the rafters which works. I've also extended my top plates past because the ridge has a fly rafter on the gable end so when you nail the ridge in with it sticking past whatever the overhang is so now scribing it on extended top plate makes it easy. I've aslo nailed a 2x4 up plumb and braced in the center of the gable cut to the exact height of the fixed ridge and then scribe the rafter and that works.

    The way I do it now doesn't matter what size rafter you use or what size walls you use whether the walls are 2x4, 2x6, 2x8 or the rafters are 2x6, 2x8, 2x10, 2x12 it doesn't matter.

    What I do is just take the ridge height that is already there and use that as my rise from the top of the plate and then figure the run from the INSIDE of the top plate and to the back of the ridge which is already deducting half the thickness of the ridge.

    Doing it this way doesn't matter what size rafter, ridge you use or what size walls you use whether the walls are 2x4, 2x6, 2x8 or the rafters are 2x6, 2x8, 2x10, 2x12 it doesn't matter.

    Once I have the run and rise I figure the Hypotenuse and pitch and do all this from the bench not even going near the roof but that wont be the end result pitch. There's a thread at Joe Fusco's forum where a guy had a 16' span with 2x6 walls with 2x12 rafters and a 1-3/4" lvl ridge with a fixed ridge height of 145-7/8" from the deck but you want the ridge height from the top plate.

    This is what I posted to him using his numbers and my drawings are from his numbers.

    "You gave a number of 145-7/8" from the deck to the fixed ridge height but not the number from the top plate to the fixed ridge height which is the one you need but I'll just give an example as if you had 8' wall using 92-5/8" precuts which gives you 97-1/8" with your 2 top plates and your bottom plate and a 16' span with 1-3/4" lvl ridge. Also you said that the rafter was higher that the ridge and lower than the ridge which still doesn't matter because the TOP of the rafter still shouldn't be higher than the other fixed ridge that your following."

    16 - 1-3/4" = 15'10-1/4" / 2 = 7'11-1/8"

    7'11-1/8" - 5-1/2" (2x6 wall) = 7'5-5/8" (run from inside plate to back of ridge)

    145-7/8" - 97-1/8" = 48-3/4" (Rise)

    7'5-5/8" [run]

    48-3/4" [rise]

    [diag] = 102"

    [pitch] 28.54°


    1) Now lay your 2x12 dowm flat on the top of your horses crown facing up and start from the right side and hook your tape at the TOP of the rafter and measure down to the BOTTOM of the rafter and mark 102".

    2) Take your speedsquare and hook it at the TOP of the rafter where you hooked your tape and now slide your speedsquare until it reads 28.54° at the chalk line and hold it there and mark the right side of the speedsquare and that's your plumbcut and look at the speedsquare where it hits the top of the 2x12 and whatever that ANGLE says, that's your PITCH.

    3) Go down to the bottom of the rafter where the 102" chalk line hits and lay out your birdsmouth and overhang with seatcut of 5-1/2" using a speedsquare or framing square with the pitch that you were given and your done.

    I hope this is clear because this takes about 2 minutes to do and is very accurate.

    The bigger the rafter the smaller the pitch is. Sometimes you don't have a fixed wall height but you have a fixed ridge height and the rafters are 2x12 on the new addition and the rafters on existing are 2x6 but you have to have the same fascia height and overhang so therefore your walls are shortened.

    The way I do it works for that to but you use the existing overhang measurement from the top of the rafter at the back of the fascia which is the end of the overhang and get that measurement to the plate line whether it's below or above that's all right because your just working backwards now no matter what size rafter you'll use and lay it out from the inside of the plate.

    Here's a couple of drawings for the measurements above.
    Attached Files
    Joe Carola

  • #2
    Re: Fixed Ridge Height and Rafter Pitch

    Good post and pics Joe. I'm glad I dropped by here to read this because your pictures have given me an idea. Is it O.K. with you if I make a copy of them?
    "I cannot teach anyone anything; I can only make them think." - Socrates

    Comment


    • #3
      Re: Fixed Ridge Height and Rafter Pitch

      Originally posted by Joe Bartok
      Good post and pics Joe. I'm glad I dropped by here to read this because your pictures have given me an idea. Is it O.K. with you if I make a copy of them?
      Joe,

      Go for it. I hope this is clear because I've explained this technique before and it wasn't undestood until you see it laid out on a rafter on the bench or the drawing as if the rafter was up and where the triangle is Constant and will never change no matter what size rafter you use or wall thickness because it's all figured from the inside of the wall and the ridge height is fixed so the traingle id the same.

      If someone chooses to make the seatcut bigger then the wall thickness then your traingle will change for example if you have a 3-1/2" wall and the run to the back of the ridge is 7' and they make the seatcut 5" so you just deduct 1-1/2" off the 7' run and have a 6'10-1/2" run with the same fixed ridge height with a different hypotenuse but you would have a 5" seatcut.

      The key thing to this is once you snap your hypotenuse measurement line from the top of the rafter down to the bottom of the rafter and then slide your speedsquare at the top until it reads the pitch you've cone up with when it hits the chalk line the CORRECT pitch will read at the top of the rafter and then you just scribe the speedsquare without moving it.

      The angle will you come up with when you have your run and rise will always stay the same but will change at the top of the rafter when using different size rafters. The bigger the rafter the smaller the pitch.
      Joe Carola

      Comment


      • #4
        Re: Fixed Ridge Height and Rafter Pitch

        Thanks Joe.
        Your method is clear and I understand what you are doing.
        I'm still trying to see if I can come up with a way to do this by calculation and not have to take a measurement, layout or scribing. I haven't had any luck so far but I looked at your drawings today and thought of something I didn't see last night.
        "I cannot teach anyone anything; I can only make them think." - Socrates

        Comment


        • #5
          Re: Fixed Ridge Height and Rafter Pitch

          Joe,

          I'm thinking that you take rafter width which in my drawing is a 2x12 and when you look at my drawing "laying on the bench" you will see that you have a triangle that was created by the 102" diagonal measurement and if you take the 11-1/2" thick rafter you have a rise and diagonal and now you can get the run and the pitch angle and subtract it from the first angle.

          See the triangle there with the 11-1/2" 2x12 as the rise and the 102" as the diagonal

          11-1/2" [rise]
          102" [diag]
          [pitch] 6.47° or 1-3/8"/12
          28.54° - 6.47° = 22.066642°

          What do you think?
          Attached Files
          Joe Carola

          Comment


          • #6
            Re: Fixed Ridge Height and Rafter Pitch

            That's what I was looking at. When I tried this last night the measurements and known values I tried to work from (like the H.A.P. or the plumb cut) weren't helping any because I always needed the pitch, which I didn't know yet, to find the H.A.P. or plumb cut.
            I'm going to think about this over the weekend ... maybe we're onto something here. There are lots of triangles on those rafters to play around with.
            "I cannot teach anyone anything; I can only make them think." - Socrates

            Comment


            • #7
              Re: Fixed Ridge Height and Rafter Pitch

              Joe,

              I just did it on drawing paper and it worked.

              I drew an 11-1/2" x 2' rectangle and at the top right corner using these measurements from before.

              11-1/2" [rise]
              102" [diag]
              [pitch] 6.47° or 1-3/8"/12
              28.54° - 6.47° = 22.066642°

              I subtracted 6.47° from 90 and got 83.53° and hook my speedsquare on the top right corner of the paper as if it was the top of the rafter and slid the square until it read 83.5° on the top of the line which would represent the top of the rafter and scribed that line.

              That represents my 102" diagonal line that I would snap in my drawings above or on the rafters itself. I then put the speedsquare back in the same spot and slid it until it hit the diagonal line until it read 28.54° and at the top of my rectangle that would represent the top of the rafter it read 22°.
              Joe Carola

              Comment


              • #8
                Re: Fixed Ridge Height and Rafter Pitch

                I haven't had a chance to try it yet but there's no reason why this shouldn't work on paper. I wish I'd had your drawings to study last night. (I only got a quick look at them in Joe Fusco's forum and I don't have the Internet on my computer at home).
                "I cannot teach anyone anything; I can only make them think." - Socrates

                Comment


                • #9
                  Re: Fixed Ridge Height and Rafter Pitch

                  As best I can tell, you're not trying to line up the new fascia line to the old. If you were, then all this work would be unnecessary. New additions' fascia lines and overhangs should match the existing in most cases. And why not? Just so you can use 2x12s on an 8' run? Why?

                  Find the overall rise from top of existing roof to top of fascia. Subtract a couple of inches so the shingle caps don't show a bulge from the front of the house. (Amatuers don't think about this step) Divide that rise by the run (including overhang width), and multiply by 12. That's your pitch. If you need a certain size rafter for a vaulted ceiling, the walls may HAVE to be lower than existing walls. Measure the height from top of fascia to top of wall. Using the roof pitch that you just found, figure how much the rafter will rise just from the fascia to the wall. Subtract from that number the fascia-to-wall rise, and you've got your HAP. If that leaves too much seat cut past the inside of the wall, then I guess your fascias and/or overhangs will just not match, or the new walls need to be lower.
                  1) Unconsciously Incompetent: He knows not, and knows not that he knows not. He is a fool. Shun him.
                  2) Consciously Incompetent: He knows not, and knows that he knows not. He is simple. Teach him.
                  3) Unconsciously Competent: He knows, and knows not that he knows. He is asleep. Wake him.
                  4) Consciously Competent: He knows, and knows that he knows. He is wise. Follow him.

                  May we all endeavor to progress from not knowing that we know not, to knowing that we know.

                  Comment


                  • #10
                    Re: Fixed Ridge Height and Rafter Pitch

                    Originally posted by TSJHD1
                    As best I can tell, you're not trying to line up the new fascia line to the old. If you were, then all this work would be unnecessary. New additions' fascia lines and overhangs should match the existing in most cases. And why not? Just so you can use 2x12s on an 8' run? Why?

                    Find the overall rise from top of existing roof to top of fascia. Subtract a couple of inches so the shingle caps don't show a bulge from the front of the house. (Amatuers don't think about this step) Divide that rise by the run (including overhang width), and multiply by 12. That's your pitch. If you need a certain size rafter for a vaulted ceiling, the walls may HAVE to be lower than existing walls. Measure the height from top of fascia to top of wall. Using the roof pitch that you just found, figure how much the rafter will rise just from the fascia to the wall. Subtract from that number the fascia-to-wall rise, and you've got your HAP. If that leaves too much seat cut past the inside of the wall, then I guess your fascias and/or overhangs will just not match, or the new walls need to be lower.

                    TSJHD1,

                    If you read my first post these measurements and rafter sizes were from someone esle. What I descirbed from above was with a fixed wall height and the guy didn't mention anything about fascia lines or soffit.

                    This is what I said in my post also.

                    "Sometimes you don't have a fixed wall height but you have a fixed ridge height and the rafters are 2x12 on the new addition and the rafters on existing are 2x6 but you have to have the same fascia height and overhang so therefore your walls are shortened.

                    The way I do it works for that to but you use the existing overhang measurement from the top of the rafter at the back of the fascia which is the end of the overhang and get that measurement to the plate line whether it's below or above that's all right because your just working backwards now no matter what size rafter you'll use and lay it out from the inside of the plate."

                    So I always work from the back of the fascia line at the top and back including my overhang in with the run and use the measuremnet from the top of the fascia below the plate and sometimes above the plate for the rise and then use that measurement to layout the birdsmouth. Keeping the seatcut the same as the thickness of the wall willo make you shorten the wall.

                    This is a picture I drew years ago. It's not a good one.

                    The second one is better.
                    Attached Files
                    Last edited by Joe Carola; 10-08-2005, 04:45 PM.
                    Joe Carola

                    Comment


                    • #11
                      Re: Fixed Ridge Height and Rafter Pitch

                      Originally posted by Joe Carola
                      There's been discussions before about when doing an addition on a house and your new ridge has to match the height of the existing ridge and the new addition has wall heights given.

                      There's many different situations that we run into but the big question is what is the pitch of the rafter or how do you figure out the rafter for this?

                      I've tried several ways over the years like nailing the new addition ridge in place and scribing the rafters which works. I've also extended my top plates past because the ridge has a fly rafter on the gable end so when you nail the ridge in with it sticking past whatever the overhang is so now scribing it on extended top plate makes it easy. I've aslo nailed a 2x4 up plumb and braced in the center of the gable cut to the exact height of the fixed ridge and then scribe the rafter and that works.

                      The way I do it now doesn't matter what size rafter you use or what size walls you use whether the walls are 2x4, 2x6, 2x8 or the rafters are 2x6, 2x8, 2x10, 2x12 it doesn't matter.

                      What I do is just take the ridge height that is already there and use that as my rise from the top of the plate and then figure the run from the INSIDE of the top plate and to the back of the ridge which is already deducting half the thickness of the ridge.

                      Doing it this way doesn't matter what size rafter, ridge you use or what size walls you use whether the walls are 2x4, 2x6, 2x8 or the rafters are 2x6, 2x8, 2x10, 2x12 it doesn't matter.

                      Once I have the run and rise I figure the Hypotenuse and pitch and do all this from the bench not even going near the roof but that wont be the end result pitch. There's a thread at Joe Fusco's forum where a guy had a 16' span with 2x6 walls with 2x12 rafters and a 1-3/4" lvl ridge with a fixed ridge height of 145-7/8" from the deck but you want the ridge height from the top plate.

                      This is what I posted to him using his numbers and my drawings are from his numbers.

                      "You gave a number of 145-7/8" from the deck to the fixed ridge height but not the number from the top plate to the fixed ridge height which is the one you need but I'll just give an example as if you had 8' wall using 92-5/8" precuts which gives you 97-1/8" with your 2 top plates and your bottom plate and a 16' span with 1-3/4" lvl ridge. Also you said that the rafter was higher that the ridge and lower than the ridge which still doesn't matter because the TOP of the rafter still shouldn't be higher than the other fixed ridge that your following."

                      16 - 1-3/4" = 15'10-1/4" / 2 = 7'11-1/8"

                      7'11-1/8" - 5-1/2" (2x6 wall) = 7'5-5/8" (run from inside plate to back of ridge)

                      145-7/8" - 97-1/8" = 48-3/4" (Rise)

                      7'5-5/8" [run]

                      48-3/4" [rise]

                      [diag] = 102"

                      [pitch] 28.54°


                      1) Now lay your 2x12 dowm flat on the top of your horses crown facing up and start from the right side and hook your tape at the TOP of the rafter and measure down to the BOTTOM of the rafter and mark 102".

                      2) Take your speedsquare and hook it at the TOP of the rafter where you hooked your tape and now slide your speedsquare until it reads 28.54° at the chalk line and hold it there and mark the right side of the speedsquare and that's your plumbcut and look at the speedsquare where it hits the top of the 2x12 and whatever that ANGLE says, that's your PITCH.

                      3) Go down to the bottom of the rafter where the 102" chalk line hits and lay out your birdsmouth and overhang with seatcut of 5-1/2" using a speedsquare or framing square with the pitch that you were given and your done.

                      I hope this is clear because this takes about 2 minutes to do and is very accurate.

                      The bigger the rafter the smaller the pitch is. Sometimes you don't have a fixed wall height but you have a fixed ridge height and the rafters are 2x12 on the new addition and the rafters on existing are 2x6 but you have to have the same fascia height and overhang so therefore your walls are shortened.

                      The way I do it works for that to but you use the existing overhang measurement from the top of the rafter at the back of the fascia which is the end of the overhang and get that measurement to the plate line whether it's below or above that's all right because your just working backwards now no matter what size rafter you'll use and lay it out from the inside of the plate.

                      Here's a couple of drawings for the measurements above.
                      Joe,

                      We don't do any remodeling, but that is a slick method. Nice work and I'm going to steal it like I've stolen every other cool trick or formula you've posted on this forum :-)
                      www.Pioneerbuildersonline.com
                      http://instagram.com/awesomeframers
                      http://www.youtube.com/user/Raftercutter

                      Comment


                      • #12
                        Re: Fixed Ridge Height and Rafter Pitch

                        I usually just chalk a line on a floor or driveway. get the ridge center using "bisecting lines make perpendicular angles" for the 90 degree. Mark the o.h. fascia top,ridge top, chalk lines and cut my rafter patterns right on top of the lines. I like Joes way too
                        Last edited by Andrew R.; 10-08-2005, 05:58 PM.

                        Comment


                        • #13
                          Re: Fixed Ridge Height and Rafter Pitch

                          Originally posted by Andrew R.
                          I usually just chalk a line on a floor or driveway. get the ridge center using "bisecting lines make perpendicular angles" for the 90 degree. Mark the o.h. fascia top,ridge top, chalk lines and cut my rafter patterns right on top of the lines. I like Joes way too
                          Andrew,

                          There's many options that you have here.

                          The whole thing here is whether or not you have plate heights that are given on the plans or if they say "VIF" (VERIFY IN FIELD) and fascia lines and overhangs have to match. If it's cathedral ceiling or you have ceiling joists. Your options are limited with cathedral ceiling.

                          If you have a plate height that is given and bigger rafters then the existing and a shorter span with cathedral ceilings. Then there's is no way your overhangs will match if you keep your seatcut at the wall width. Your fascia can always match but you will either have a smaller overhang because the pitch is steeper on the new addition.

                          If you have the plate height given and you have ceiling joist and your new roof pirch is steeper we always put blocks in between the joists whatever height we need get the fascia and overhang to line up.

                          If you don't have a wall height and it says "VIF" with cathedral ceilings then you will set the wall height with the measurement and overhang given from the top of the existing facsia to the deck and let's say that's 96", now lay out your birdsmouth and whatever your measurement from your seatcut is if it's 4" higher then the fascia line then you add that to the 96" wall height measurement you already have which would be 100" wall height.

                          Most of the time on an addition off the back of a house your cutting away the old fascia and overhangs to frame up your walls so there's always a existing rafter sitting close to the new wall so you can just measure from the top of the plumbcut at the end of the overhang which is the back of the fascia up of down to the top plate and that's your mark.

                          So for example if you have a 2x4 wall and that mark is 4" lower than the existing plate line and your height from the plate to the top of the existing ridge is 6' then you just add the 4" to that and 6'4" is your rise and your run will be figured from the back of the ridge to the back of the fascia with a 1' overhang and that for example would be 8'. That's all you need and you have your diagonal (rafter length) and pitch from there.

                          6'4" [rise]

                          8' [run]

                          [pitch] 38.37° or 9-1/2"/12

                          [diag] 10'2-7/16" (rafter length)


                          Now hook your tape at the top of the rafter let's say from the right to left and mark 10'2-7/16" that's top of plumbcut to the end of the overhang (back of fascia) and now hold your framing square with the 12" mark on that mark and 9-1/2" going up to the plumbcut and mark the bottom level mark and the 9-1/2" side plumb and then mark 4" up. Now just slide the square on the level mark 3-1/2" which is the inside of your wall anthen scribe the 9-1/2" side plumbcut. Off that 9-1/2" plumbcut mark layout your 3-1/2" seatcut starting flush with the inside of the wall.

                          Now measure from that seatcut line to your 4" mark and that's your wall height.

                          It's very hard to explain this here. I wish I could show this in person or in the field. This all takes a couple minutes to do. The most important and easiest way is figuring from the overhangs. I can't stress that enough how it makes everything easier even on bastard roofs..

                          All you need is:

                          1) Ridge height

                          2) Run (back of fascia)

                          3) Height from top of existing fascia plumbcut on rafter.

                          Do this all on the bench snapping lines on the deck and no stringing lines or holding up rafters. All this takes about 2 minutes.
                          Joe Carola

                          Comment


                          • #14
                            Re: Fixed Ridge Height and Rafter Pitch

                            Originally posted by Tim Uhler
                            Joe,

                            We don't do any remodeling, but that is a slick method. Nice work and I'm going to steal it like I've stolen every other cool trick or formula you've posted on this forum :-)
                            Tim,

                            A lot of times we get this on custom homes that we frame and I do the same thing. It's always big to keep fascia lines and overhangs the same around here so I've learned to do thinngs many different ways that are productive and some not so productive but this way is the fastest and easiest way to do it.

                            The only problem that you have to watch out for when you see "VIF" is that when you start lowering wall heights and the bottom of the headers have to match and be at the same height because all the windows are set at the same height and we have frieze board to put on top it can cause problems with the size headers you use.

                            Sometimes we cut one plate out or there's been times where we cut both plates out. There's also been times where we put the header on the top plate.

                            The house I just framed was shaped like an H the two side rafters were set on top of the ceilings joist and the pitch was 7/12 and the main pitch was 5-11/16"/12 because the main roof sat on two 30' trusses that were made at 5-11/16". I have pictures if you want to see them.

                            This house has two layers of frieze board one layer of dentil molding and one layer of crown molding.

                            I see you cheated with those cedar shake panels.......;-)
                            Joe Carola

                            Comment


                            • #15
                              Re: Fixed Ridge Height and Rafter Pitch

                              Joe,
                              Your'e right about the wall and rafter sizes. I can still use existing or main as my initial chalk line and figure my plate height from there and get whatever seat cut I want to make. Mainly I am trying to get the eave and fascia to line up. After I get my "top" line I just lay the rafter on it I am using on it and start squaring off the first line for plumb cuts, tails, birdsmouths, ridge cuts. Essentially I am laying out a truss in a way on a big work table. I can layout ridge beam notches this way and check my rafter cut fits on the ground. Guess I'm screwed if the driveway is gravel or there are too many inside walls and braces to get the chalk line down..
                              Edit,,and yes the headers lining up low to high pitches can be a pita, as I said in an earlier thread , my headers on the last job were above the plates so high I ended up notching rafters around them and toenailing tails on.
                              Last edited by Andrew R.; 10-08-2005, 07:20 PM.

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