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Construction master + rafters = ???

danno7x | Posted in Construction Techniques on July 2, 2009 01:41am

What am I missing here? Using the construction master pro and trying to use it to figure rafter length.  Usually have a specific ridge height and span to go by so I was trying to use it to tell me the pitch and length of the rafters.  I want to be real cool and cut em all on the ground correctly.

Put it all in and get a number thats not right.  How do I figure in my HAP cut into the number it gives me?  What exactly is the “point to point” the manual talks about.  After trying to put a birdsmouth (HAP cut) on it would change the angle that the calculator gave me. 

I learned to use a rafter book measure down the top of the rafter get a plum line and put birdsmouth on with speed square.  It dosent apear thats how its done with the calculator. I dont often have even runs or pitches so I thought the calc. would work best.  The rafter length number it gives me appears to be short anyways and then the angles are off after the birds mouth is added to the mess. 

I dont get to do these very ofthen, 3 times a year or so but Im now more confused than ever.  I dont have any example to try now I just dont like to be screwed up like this.  No one around me seems to have a friggin clue.  Ive got a feeling its something simple Im overlooking. THANKS

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Replies

  1. jimAKAblue | Jul 02, 2009 02:27pm | #1

    The calculator is giving your the theoretical length, which can be  measured at the top of the rafter. Depending on how you input your data, you'll probably have to shorten for the ridge and add your heel height in any way that makes sense to you.

    1. danno7x | Jul 02, 2009 11:34pm | #8

      That answer is lacking all kinds of detail,  I cant add anything in a way that makes sense to me because none of it was making sense.  I know its a theoretical length but I couldnt figure out how to apply it.

  2. Piffin | Jul 02, 2009 02:48pm | #2

    What you describe sounds about backwards from way I do it. How was your ridge height calculated?

    I figure my HAP based on rafter size and the pitch and I have to know those two before I can know where to elevate the ridge to.

    are you remembering to subtract half the width of the ridge beam/board?

     

     

    Welcome to the
    Taunton University of
    Knowledge FHB Campus at Breaktime.
     where ...
    Excellence is its own reward!

    1. jimAKAblue | Jul 02, 2009 03:29pm | #3

      I agree. The height of the ridge is a sum of the HAP plus the total Rise of the rafter.

      I remember having this discussion with Joe F. His insistence on having a fixed ridge height resulted in a dogfight between us that was never resolved, as far as I can best remember LOL.

      There are occasions when fixing the ridge height supercedes the need to maintain a fixed slope. If the builder is up against height restrictions, then it might become necessary to fix the ridge and change the slope but I've never encountered that situation myself. I have had to fix a ridge, under a window to ensure proper flashing but that is another thread....

    2. danno7x | Jul 02, 2009 11:30pm | #7

      Yes I remember to subtract half the ridge.  Fixed ridge height because of tying into something exisiting most of the time.  So your using the pitch you desire to dictate where you locate the ridge height? 

       Im not sure I get that totally, then based on the rafter size you come up with a HAP and subtract that from the rise to get your length?  The way joe bartok described it to me seems a little more straight forward.  Educate me.

      1. Piffin | Jul 02, 2009 11:53pm | #9

        If you have a method that makes sense to you now, don't let me confuse you. There are a few ways to do this and it is not one of those things I explain well without a framing square in my hand, so I usually stay out of this topic.Hard to do on a forum. I can't say see this mark right here - pointing with my pencil...and then you do like this ...Joe Bartok and Joe Carola and Joe Fusco all do real well explaining rafters, so I guess it's a good thing my first name is not Joe 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

        1. doodabug | Jul 03, 2009 12:36am | #10

          That's what I wondered. Why are all the math guys named Joe?

  3. JoeBartok | Jul 02, 2009 04:42pm | #4

    I don't know anything about CM calculators (sorry!) but here is a link to rafter dimensioning using trigonometry ... Common Rafter Dimensions, Adjustments and Heel Height

     

    If you are talking about working to a fixed ridge height here are a couple of links to some JLC threads. This one was started by Joe Carola ... Fixed Ridge Height and Rafter Pitch

    This calculator is a result of that thread and his solution ... Fixed Ridge Height Pitch and Rafter Calculator

    Heel height and remaining rafter depth in conjunction with a fixed ridge height were discussed in this thread ... The math, please

    Links to calculator, diagrams and theory ... Rafter to Fixed Ridge Height Analysis

    Joe Bartok



    Edited 7/2/2009 9:43 am ET by JoeBartok

    1. danno7x | Jul 02, 2009 11:23pm | #6

      I appriciate you taking the time to post those links.  The one where it talks about measuring to the INSIDE of the top plate makes so much sense.  That would be the "point to point" number the calc. gives me.

      That is good way to do it and then get a full seat on the wall.  I think its saying the seat cut would be from that point inverse of the ridge cut angle.  Like if ridge cut was 40* and I measured along the rafter from the top edge to the bottom marked it the number the calc, gave me.  That should be my point then go forward of that with a 50* level or seat cut and go plumb down at say 6" on a 2x6 wall. 

      I guess if I didnt want a full seat cut because it would be taking out too much of the rafter I would just move the measurment towards the outside of the wall some to compensate.  I think its making sense to me now.

      I end up having fixed ridge height because I do mostly additions and remodels so its dictated by the original height.  On something new it will most likely get trusses around here ( bklah)  While I understand the advantages of trusses they arent any challenge or fun.

  4. Doobz26 | Jul 02, 2009 07:44pm | #5

    I'm guessing your ridge height was measured from the top plate, which you entered as your "rise."  What you need to do is subtract your HAP from that number and use that as your "rise."   Your "Run" should be measured from the face of the ridge, not the center. 

  5. Framer | Jul 03, 2009 03:43am | #11

    I started the same thread here as Joe Bartok linked you to.

    You can't go wrong doing it this way no matter what size rafter you have going from the inside of the plate. It always works. The other way of course of doing it is just setting the addition ridge height matching the existing nailing it level and plumb and braced and then just scribing and that works perfect. By the time my guys do that I have the rafters cut already and they always fit.

    http://forums.taunton.com/tp-breaktime/messages?msg=64231.1

    Joe Carola
    1. danno7x | Jul 03, 2009 01:17pm | #12

      Yea the set and scribe is what usuallly ends up happening just so everything will work out but I want to know everything I can and do  better than I was taught. 

      Was I right about the inverse to get the seat cut?  with out that angle it wouldnt work but the wall makes a 90* so it made sense to me that if the plumb cut was 35* the seat cut would be 55*.  Thanks for the help

      1. User avater
        Joe | Jul 03, 2009 02:17pm | #13

        Dan,The best way to describe what the CMP is doing and what the manual means by "point to point" is to understand that it just calculates the three legs of a right triangle. One being the "rise" [vertical leg] one being the "run" [horizontal leg] and the hypotenuse (rafter length) [diagonal leg]. Where you locate those points is your own concern. If you just enter in a straight run and rise (run being the measurement from the outside of the rafter plate to the center of the ridge), the rise being either some arbitrary height (entered with the [Rise] key) or a unit rise like 6 (entered with the [Pitch] key) the value or "length" you get back is that which goes from a "point" that's located at the top of the outside plate and the a "point" located at the top center of the ridge.To get a meaningful "length" you have to "move" the points around a bit ;-). Like moving the point on the outside of the rafter plate up to allot for the H.A.P. (subtracting its value from the rise) and moving the point at the ridge from its center to its face (subtracting 1/2 of its thickness from the run). These will require just a little bit of addition and subtraction on your part and there are a few different method of doing so.

        View Image

        http://www.josephfusco.org

        http://www.constructionforumsonline.com

        1. User avater
          Joe | Jul 03, 2009 02:24pm | #14

          If you take a look that the calculator I have on my site: http://www.josephfusco.org/Calculators/Simple%20Roof%20Calculator.htmlYou can see where it makes allotments for the various elements to adjust to get the correct rafter lengths. You have to make the same adjustments using the CMP to get the correct lengths as well.

          View Image

          http://www.josephfusco.org

          http://www.constructionforumsonline.com

          1. Snort | Jul 03, 2009 10:58pm | #15

            You can also go mathless... lay it out on the deckhttp://www.tvwsolar.com

            We'll have a kid

            Or maybe we'll rent one

            He's got to be straight

            We don't want a bent one

            He'll drink his baby brew

            From a big brass cup

            Someday he may be president

            If things loosen up

          2. User avater
            Joe | Jul 04, 2009 01:42am | #16

            Or use a block and String: http://www.josephfusco.org/Articles/Roof_Cutting/Basic_Roof_Framing/Documents/simplemethod.html

            View Image

            http://www.josephfusco.org

            http://www.constructionforumsonline.com

      2. Framer | Jul 05, 2009 07:02pm | #17

        Was I right about the inverse to get the seat cut?  with out that angle it wouldnt work but the wall makes a 90* so it made sense to me that if the plumb cut was 35* the seat cut would be 55*.  Thanks for the help

        If the new plumbcut was 35° at the top of the speedsquare where it hits the top of the rafter, the seatcut would be 55°from the bottom of the rafter, not the red line.. Joe Carola

      3. Framer | Jul 05, 2009 07:03pm | #18

        Don't know what happened to my drawing I posted. Here it goes again.Joe Carola

        1. danno7x | Jul 05, 2009 09:33pm | #19

          Thanks Joe, that is exactly what I had pictured in my head weather I conveyed that in what I wrote down who knows. 

          I was just missing a small piece of the puzzle,  I kinda learned some for one guy and some from another guy while never doing a bunch of it for myself and thats not a good way to learn and retain anything.

          So without screwing  myself up how does the point to point number work for jack rafters, where do the angles it gives you fit in and all that?  You don't need to explain it in detail just shoot me a good link to read.  I like to be educated.  THANKS

           

          1. Framer | Jul 06, 2009 04:53am | #20

            So without screwing  myself up how does the point to point number work for jack rafters, where do the angles it gives you fit in and all that?  You don't need to explain it in detail just shoot me a good link to read.  I like to be educated.  THANKS

            Once you've established that the pitch of the common rafter is 35° which is 8-3/8:12 pitch. The jack rafters are laid out the same using 35° on a speedsquare or 8-3/8:12 with a framing square. How you measure your jack rafters is up to you, but the 35° or 8-3/8:12 is the same as the common rafter. 

            You can forget about how you figured the real 35° pitch out with the 7'5-5/8" run and 48-3/4" rise and 102" diagonal because that 102" diagonal chalkine mark is the key to getting the pitch and that's all you need it for. After that you don't need it anymore.

            Even when your laying out the valley jacks on the false valley going up the roof, you set your plumbcut at 35° and the bottom level cut is 55° and set the saw tilt to whatever the pitch of the existing roof is.

            I hope this makes sense.

             Joe Carola

          2. danno7x | Jul 06, 2009 01:22pm | #21

            I think I got it all, it would be nice to put it into practice but could not even happen this year.  If we frame any roofs it'll most likely get trusses.  People seem convinced they are more cost effective.

            I'm anxious to give it a try with the calc.  Thanks for the help.

          3. jimAKAblue | Jul 06, 2009 04:52pm | #22

            If you want to learn about rafters, and you are getting trusses shipped, just apply your knowledge to the trusses and check to see how the numbers all come together.

            The trusses will have use the same principles as conventional rafters.

  6. MrJalapeno | Jul 06, 2009 07:51pm | #23

    Hey Danno7x

     

    Simply put…

     

    The length of a rafter is the hypotenuse of a right triangle. 

     

    The intricacies of pre-cutting complicated roof frames are more easily solved from understanding, or dissecting, the geometry that creates the right triangles found throughout a roof’s design scenarios.  Once the simple geometric concepts of the roof’s designs are understood, the plan’s given Pitches, Spans, Runs, or Height dimensions can be used to solve the additional information that the sawyer needs to cut the rafters. 

     

    Roof cutting is about understanding the geometry so you know how to apply the mathematical solutions that you calculate.  When it comes to how to calculate and which tool is preferred to use, that is up to each individual to decide.  They are all similar in function in that they will solve the math if the correct inputs are used.  The CM calculators are my onsite favorites for speed, accuracy, and convenience, but understanding how to use a steel framing square will further your geometric conceptual understanding behind the math and also provided you with an additional resource that doesn’t require batteries to operate.

     

    Good luck, and keep reading, learning, and practicing.

     

     

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