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  1. #1
    Join Date
    Dec 2009
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    Sterling Heights, MI
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    Default Dropping the hip....

    Buildcalc is very kind to me and gives me the bevels for each side of the hip, when I'm building an irregular roof. However, in practice, I only rarely bevel the hip. Most times, I'm filling in a hip on the bottom section of a trussed hip set. I don't bevel the hip: I drop it. And, when I drop it, there's usually no particular reason why I need to place the top of the hip in a highly accurate position because of the normal wonkiness of the prefabbed hipset package.

    Anyhows, I was hoping someone could give me some guidance on how to figure the drop using geometry. Also, I'd like to know precisely how to calculate how off center that a dropped hip should be placed, relative to the theoretical plane of the hip.

    The house I'm framing right now has a 7/12 major roof and a 14/12 minor. Build calc gives me all the bevels, etc, but it doesn't give me the drop or offsets. This isn't a post and beam hip, so I'm just going to drop it as usual but I'd really like to be able to explain the math/angles to my rookie help.

  2. #2
    Join Date
    Jun 2004
    Location
    Georgia
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    1,824

    Default Re: Dropping the hip....

    On my favorite roof pitch of 6 in 12, the formula I use is Drop = 6 x (½ thickness of hip stock)/17. I don't remember where it comes from.

  3. #3
    Join Date
    Oct 2005
    Location
    off the coast of new england
    Posts
    515

    Default Re: Dropping the hip....

    Its very easy and we have gone over this many times . Beveling the hip is actually called a backing .with an irregular hip-valley there is a very easy geometric shift . Look at sim Ayers page ,or go to Joe Bartocks web page or look at the artical I did several years ago . the Timber Framers Guild has what is called the Hawkingdale angles ,which is a free download
    All you do is insert your pitches and deck angles and it gives you all you need . You may actually need to do some home work to really understand the hawks

  4. #4
    Join Date
    Jun 2004
    Location
    San Antonio, Texas
    Posts
    753

    Default Re: Dropping the hip....

    Jim,

    You can layout the H/V rafters on the corner using your framing square. The plan angle using the Framing Square is 7/14. If the Hip board is 1-1/2" thick then use the inside scales for setting your angles and use the tongue of the square as the rafter thickness to place the bird's mouth layout at the corner. (This layout can be mocked up at the sawhorses on a sheet of decking) The 9 inch mark (Square/Opposite the 7" mark on the tongue) will intersect the adjacent plate, this places the HAP equally at both plate lines for the Hips heel cut. Mark both sides of the tongue, that's the hip placement at the corner. ( You can simply measure your offsets, they become the "Drop" adjustments made to the effective run that you calculate the Hip's length with.)

    *As far as how to cut the Hip rafters, if you calculate the length at the plan-view line(s) you just laid out, and apply the standard HAP, you will not need to know what the "Drop" is, nor how to calculate it. The length generated by your calculator will be the length marked at the side, and will represent short-to-short of the plan-view angles, or cheek bevels.

    For Hip trusses I don't even attempt to cut the sharp angled bevels if that's the one I'm faced with, (typical for irregular Hip truss sets). I just cut the H/V pitch with a square cheek bevel and nail the snot out of it to the truss/girder. An example length calc would go something like this; Lets say the girder for the hip system is set back 6' from the wall/corner. Deduct the dim taken from the mocked-up layout, 6' minus 1-5/16" equals 5' 10-11/16", this is the effective run as taken for the minor pitch, (14/12) Process the Hip length using this as the Run.

    I get 14' 10-5/16" length w/ 27.55 degs plumb and 62.45 degs level angles.

    Btw, the other side of the hip layout is 11/16", as taken from the corner on the plate bearing the minor pitch, which is the plate that has the major effective run contained on it, minor jacks (Mono jacks) are laid out on it too.
    Last edited by Richard Birch; 04-10-2013 at 09:02 PM.

  5. #5
    Join Date
    Dec 2009
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    Default Re: Dropping the hip....

    Quote Originally Posted by charles View Post
    On my favorite roof pitch of 6 in 12, the formula I use is Drop = 6 x (½ thickness of hip stock)/17. I don't remember where it comes from.
    That's an interesting formula and I wonder how close it really is.

    I think we were taught to multiply 1/16 x the pitch.

    I'm just trying to figure out if there is an easy answer to explain to a novice how much to offset the centerline of the hip, when confronted with something like a 7/12 and 14/12 situation. Obviously, the answer is somewhere between 3/4" and 0" so it's really not that big of a deal. If I offset the hip toward the high slope 1/4", how much could I be wrong LOL?

  6. #6
    Join Date
    Jun 2004
    Location
    San Antonio, Texas
    Posts
    753

    Default Re: Dropping the hip....

    Quote Originally Posted by jimAKAblue View Post
    That's an interesting formula and I wonder how close it really is.

    I think we were taught to multiply 1/16 x the pitch.

    I'm just trying to figure out if there is an easy answer to explain to a novice how much to offset the centerline of the hip, when confronted with something like a 7/12 and 14/12 situation. Obviously, the answer is somewhere between 3/4" and 0" so it's really not that big of a deal. If I offset the hip toward the high slope 1/4", how much could I be wrong LOL?

    Funny Stuff!

    Chas's formula, 6/17 x 3/4", is only applicable to a regular 6/12 Hip.

    Your guess of 1/4" is 3/16" less than perfect for the 7 and 14 irregular pitch combination of your roof. The perfect offset, or shift, for your hip is 7/16". But who cares what it is as long as the shoulders are in plane.

    With a CMC, once you have the plane view angle of your hip entered into the calculator, just enter the Hip's thickness as the [Diag] and press [Rise],(=11/16"), the [Run], (=1-5/16") to get your offsets as measured along the plate lines. *(These dims are again Hypotenuses as seen in plan-view relative to the adjusted, or shifted, center-line of the Hip. The shift is the [Rise], (5/16") on the "lesser" shifted side, (Minor Pitch, 14/12), and the [Run], (1-3/16"), or the "greater" shifted side, (Major Pitch, 6/12).)

    If BuildCalc will let you enter the two pitches as a Rise and Run, you should be able to calc the shift(s). Once you have the shift(s) *(Along the plate lines!) use them as your effective run adjustments and calculate the Hip's length w/o the need to make "Drop" adjustments, "After the Fact."

    If you make the math adjustment (find the effective Run) for the H/V prior to calculating it's length, you don't have to adjust it (or "Drop") later. The lines made to mark the length are used to properly place the rafter. *(Some extra Tail clearance at the heel may be necessary, typical, or use the proper cheek bevel angle. *square cut birds-mouths, H/V, won't fit without clearance. Ripping the tail width works too.)

    Also, the "shifts" as found on the plates are more useful for pre calculating and cutting the remaining jacks, using the sheathing cut formulas. (We can rehash that next.)

  7. #7
    Join Date
    Dec 2009
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    Default Re: Dropping the hip....

    Quote Originally Posted by Richard Birch View Post
    Funny Stuff!

    Chas's formula, 6/17 x 3/4", is only applicable to a regular 6/12 Hip.

    Your guess of 1/4" is 3/16" less than perfect for the 7 and 14 irregular pitch combination of your roof. The perfect offset, or shift, for your hip is 7/16". But who cares what it is as long as the shoulders are in plane.
    It is funny, isn't it?!

    I've never really had to think too hard about any of this because the irregular hips that I'll be cutting will only be the filler for the hipsets. Furthermore, I always have my overhangs completed before I set the roof. So, to fill in that theoretical 6' hipset, I'd hook my tape on a nail at the eave, where the hip would intersect it (the effective run) and measure it to the apex. I make a square plumb cut there and then make the cheek cuts. My saw can easily make a 60 degree cheek cut, so I like to do them, even though it's not necessary.

    And now, thanks to you, I know how much to offset the centerline of the cheek cut: on a 7 and 14, I offset it 7/16. I have a 12/12 and 14/12 hip to cut. Using your plan view instructions, I see that I need a 1/8 adjustment for the centerline. So, I now have a new rule of thumb: for every inch difference in pitch, I'm offsetting the hip centerline by 1/16th!

    Now I need to figure out the math on this. I'm almost there.

  8. #8
    Join Date
    Jun 2004
    Location
    San Antonio, Texas
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    753

    Default Re: Dropping the hip....

    Quote Originally Posted by jimAKAblue View Post
    It is funny, isn't it?!

    Now I need to figure out the math on this. I'm almost there.

    Lol, I hope you get there.

  9. #9
    Join Date
    Jan 2005
    Posts
    65

    Default Re: Dropping the hip....

    If you mark the plumb cut on a piece of scrap and cut the bevels on the top edge as provided by the program, maybe you can just measure the drop. Close enough?

    West coast, home dogs.

    Ted

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