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Discussion Starter · #1 ·
I recently purchased a Construction Master Pro Calculator and sent their support team an e-mail, but have gotten no response, so I thought the community here might be able to help.



My questions are:

1) When solving for Jack lengths is the answer given to the long point or the short point of the bevel cut?

2) It says Jacks are set at a default 16" O.C. Is that pulling from the first common on a side or the corner? If from the corner is it 15-1/4" and go?

3) Bear with me this will take some explaining. I do a lot of arches at work. My standard method is to establish the starting height (6'-8" off the floor), my Apex (1-1/2" down from the header, centered in the opening), and my opening width (say 8' for example).

Using these 3 numbers I rip plywood, lay it on the floor and mark all 3 locations. I then swing my tape half the distance of the opening from all 3 locations until I have 4 points of intersection. 2 above and 2 below the plywood. Then I set a nail at the top location, hook a snap line on it and plane it through the bottom point on the same side and snap. Repeat for the other side.

Where the 2 snapped lines intersect is my swing point to mark the arch. I set a nail and swing my tape. If done correctly I have the same number to all 3 marked locations on the plywood (my Apex and my left and right side starting heights).

To do this takes quite a bit of open floor space and its time consuming.

I'm wondering if I could just lay my plywood down and snap a long center line. Then using the Calculator and the 3 numbers I have (opening width, apex, and starting height) solve for the number I need to pull down the center line to find my Arch swing point?

Thanks.
 

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Discussion Starter · #2 ·
Also we have some second floor decks that overlook the foyer and great room that have a radius to them. The plans give me some information, but I'm wondering what the best method is, since I don't have enough deck space to do the above method.

Is there a way the calculator could work here as well?
 

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To the best of my knowledge:

1) long to short, center to center, or short to long.

2) From your first jack, which ever end you choose to start from.

3) width squared/2 + height squared / 2 h if I remember correctly.
 

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Discussion Starter · #4 · (Edited)
To the best of my knowledge:

1) long to short, center to center, or short to long.

2) From your first jack, which ever end you choose to start from.

3) width squared/2 + height squared / 2 h if I remember correctly.
1) There is no long to short or short to long in a jack that I am aware of. The measurement is from the birds mouth (which is square) to the short or long of a bevel. Here is a video (click) of the How To for the rafter section. When pressing the Jack button over and over it gives you all your Jack lengths down the hip or valley, however the part I am confused about is if the answers are to the long or short point of the bevel?

2) Got it thanks.

3) Could you elaborate more? For example you have an opening width of 8' and a starting height of 6'-8". The Apex would be 1-1/2" down from the header. Lets say the difference is 14-1/2". So a total height off the floor to the Apex is 7' -10-1/2".

So based on your equation.

Width = 8'
Squared = 64' SQ FT.
Divided by 2 = 32' SQ FT.

Height = 7' - 10-1/2"
Squared = 62.01563' SQ FT.
Divided by 2 = 31.00781' SQ FT.

Width 32' SQ FT. + Height 31.00781' SQ FT. = 63.007781' SQ FT

There is no way I pull 63 feet down the center line to swing an 8' wide arch.

Am I missing something?

Thanks.
 

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Sorry I was doing some of that from memory. So it's (w /2 ) squared + h squared/ 2h. Disregard the 6' 8" starting height. 96/2=48 squared + 14. 5 squared / 29 = 86.7"
 

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Discussion Starter · #6 ·
Sorry I was doing some of that from memory. So it's (w /2 ) squared + h squared/ 2h. Disregard the 6' 8" starting height. 96/2=48 squared + 14. 5 squared / 29 = 86.7"
Sorry I just want to get this correct cause I'm having trouble reading your formula.

Its width divided by two = 48" then I square it = 2304 SQ IN.

Then 14 1/2" squared = 210.25 SQ IN.

Then I divide by 29? 29 inches?, why 29?

I'm inputting this stuff in feet or inches using a construction calculator and this is what I'm getting, what am I doing wrong.

Thanks for your patience, :clap:
 

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Just do it in decimal form with no inches, so it's purely a math formula. 29 is h x 2 or 14.5 x 2=29.
 

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Discussion Starter · #8 · (Edited)
Hopefully the last time, sorry.

Your Formula is: (w /2 ) squared + h squared/ 2h

1) Width divided by 2.
96/2 = 48

2) 48 squared = 2304

3) Height = 14.5, squared = 210.25

4) 2304 + 210.25 = 2514.25

5) Divided by the Height x 2 (29) = 86.69828

6) Convert to inches = 86 11/16"

So If I snap a center line and measure down 86 11/16" from the Apex , I should have the same number to my 2 starting points on either side, correct?
 

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Yes, 14 1/2" lower than the apex. So, given your original, 6' 8" on each side, with the apex at 94 1/2", 96" wide.
 
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Discussion Starter · #10 ·
Thanks for all your help.

One question that still remains, more on the calculator side is:

When pressing the Jack button over and over it gives you all your Jack lengths down the hip or valley, however the part I am confused about is if the answers are to the long or short point of the bevel?

Anyone know that.
 

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Thanks for all your help.

One question that still remains, more on the calculator side is:

When pressing the Jack button over and over it gives you all your Jack lengths down the hip or valley, however the part I am confused about is if the answers are to the long or short point of the bevel?

Anyone know that.
Don't know if I can explain, but it depends how you measure your run. Say your longest rafter is setting 115 inches down the wall from the hip, to the outside (long point) and you enter 115 " as your run, the diagonal will be to long point. Enter the measurement to the inside (short point) in this case 113½" as your run, diagonal will be to short point.
 

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We'll, doesn't that depend on where your first common is in relation to the end of the ridge? Unless you want to start from the outside corner like you said, at 15 1/4 go. That makes the one on the corner effectively zero at the long point with each one a 16" jack length longer to the long point.

But the CM gives you jacks in DESCENDING order. So I, being kind of old school, field measure my first jack, usually to a long point, and enter that as my diagonal measurement. Then push jack to get the jack lengths.

Smarter guys than me can probly figure out if it's 6" hooking the common to the hip at the ridge,the first jack has an 10" shorter run to the long point than the common.

I think what you 're asking depends where you're measuring from.
 

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Thanks for all your help.

One question that still remains, more on the calculator side is:

When pressing the Jack button over and over it gives you all your Jack lengths down the hip or valley, however the part I am confused about is if the answers are to the long or short point of the bevel?

Anyone know that.

Neither! The CM does not perform any jack rafter length adjustments for you. You need to know how to find the Jack Rafter's "Effective Run".

The "Effective Run" of the jack is equal to the jack rafter lay-out Dimension found on the plate for Regular Pitch Hips. (Out-to-In, or from the long side lay-out of the jack to the inside lay-out of the Hip. For VJ's, use lay-out on Ridge)

For jacks laid out 16" o.c. from the corner, the Long Point ER for the shortest jack is 16-3/4" minus half the Hip thickness at the plan-view angle. (For a typical 1-1/2" thick Hip rafter this is 1-1/16")

16-3/4" minus 1-1/16" = 15-11/16", [Run], [Diag] = JR length
 

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Discussion Starter · #14 · (Edited)
What you guys are saying makes sense.

I just assumed based on this video I could enter the information and get all my jacks, but what they don't explain is where that measurement is too.

Watch this and let me know what your seeing.

https://www.youtube.com/watch?v=F4Q4XZ8xGZI

I guess in the field I could just cut a test one based on the answers and figure it out.

I'm just trying to get used to this calculator and figure out all the things I can do with it so I can work smarter and faster.
 

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"If you can determine a line length found in Plan-view, you can calculate a Rafter Length from it."

Span: Generally, the width of a structure.

Run: For equal pitches, half the Span.

Effective Run: The dimension used to calculate Rafter lengths from. Generally the adjusted Run, typically the Run with half the ridge deducted from it.

The Terms/definitions above are true for simple equal pitch roofs (aka; Regular Pitch) and Concepts applied to Irregulars.

"A Rafter Length is simply the Hypotenuse of a Right Triangle."

Roof Cutter's Basics (This is a SketchUp presentation demonstrating how to cut Irregular Hip an Valley Rafters. The same concepts apply to Regular Hip/Val Rafters. To get the Jacks to fit perfectly, you'll need to get the H/V rafters perfect first.)

The YouTube video is a conceptual guide to calculating Rafter Lengths and bevels/pitch angles with CM Calculators, not a complete authority "How-to-do-it" guide. There is tons of info on this site as well as many library books about Roof Cutting/Framing available.

"Concepts" are "General Ideas."

;)
 

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Great calculator, and I actually took a class through the carpenters union on how to use this particular calculator. It is an extremely helpful tool. They taught us everything from rafters, to yards of concrete in caissons. Of course I forget half lol but I do use it all the time
 

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My questions are:

. . . .

3) Bear with me this will take some explaining. I do a lot of arches at work. My standard method is to establish the starting height (6'-8" off the floor), my Apex (1-1/2" down from the header, centered in the opening), and my opening width (say 8' for example).

Using these 3 numbers I rip plywood, lay it on the floor and mark all 3 locations. I then swing my tape half the distance of the opening from all 3 locations until I have 4 points of intersection. 2 above and 2 below the plywood. Then I set a nail at the top location, hook a snap line on it and plane it through the bottom point on the same side and snap. Repeat for the other side.

Where the 2 snapped lines intersect is my swing point to mark the arch. I set a nail and swing my tape. If done correctly I have the same number to all 3 marked locations on the plywood (my Apex and my left and right side starting heights).

To do this takes quite a bit of open floor space and its time consuming.

I'm wondering if I could just lay my plywood down and snap a long center line. Then using the Calculator and the 3 numbers I have (opening width, apex, and starting height) solve for the number I need to pull down the center line to find my Arch swing point?

Thanks.

I know Kiteman already gave you a formula for solving arch radii, here is another interesting (geometric?) solution.

When I first figured how to create equal drops for all the arches in a house I would draw the initial triangle and measure the hypotenuse length, then divide it in half, and multiply by the (Hypotenuse/Rise) of the original triangle. The Illustration below is how I worked them out with the equal geometric proportions and the CM Calculator.
 

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Discussion Starter · #20 ·
Thanks!

Yeah my boss does this method but snaps all the lines and does not use a calculator.

Basically snapping a center line, a connection line from apex to one sides starting height. Then finding half the distance on that line, uses the 3/4/5 squaring method to find the swing point on the center line.

I do like having another way of solving for it though. Thank you.
 
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