 # CMP & jack rafters

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Hello all,
Long time lurker, first time poster. A little background, Im a second generation builder from MS. I've learned a lot, sweated even more and sometimes even made some money! I was taught to cut roofs with a framing square and rafter charts. Its archaic yet efficient but some what limited....so I bought a construction master pro at the lumber yard. My question pertains to the calculated jack length measurements. Following is the way I have figured a simple stick framed 2x hip roof 24"o.c. with a 36' length and a 24' width with a 6/12 pitch.

length-width+thickness of ridge= actual ridge length
36'-24'=12' +1"1/2=12'-1"1/2 ridge length

total span-ridge divided by 2 = adjusted run
24'-1"1/2=23'-10"1/2 divided by 2 = 11'-11"1/4 run

11'-11"1/4 run, 6" pitch, diagonal = 13'-4"3/16 common
H/V = 17'-10"7/8
jack rafter 1 length 11'-1"15/16

OK, here's the question. Is that jack length to short or long point? Do I need to subtract 1/2 the 45* thickness of the hip(1"1/16) for this length on each jack to make layout work for 2' o.c. Or do I just cut the thing and drop it in place? the reason i ask is a little "magic" has to be worked on the hip with the calculated measurement, but there is no need in dropping or backing the hip if done correctly also those lengths don't jive with my rafter tables.

I know its long winded, but this is nothing they cover in the manual. I know this also borders on info that framers might not want to give out, but I would love to know. Any help would be great.

Thanks fellas
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KC,

As you suspect, the CMPs do not make automatic Jack Length adjustments for you, so pressing the [Jack] key in sequence from the Common Length will not produce the correct lengths. Also, as you guess, Subtracting half the H/V (@ 45*) will give you correct lengths for Regular Pitch Jacks spaced o.c. from the end of the ridge, which will be the short-points of the cheek-cut bevel. (add the diff *(1-1/2" - 1-1/16" = 0-7/16") to the Common Run for long points.)

Same basic idea for Irregulars except the proportions are not equal ( 1:1 ), they are Pitch/Pitch. Irreg Roof Jack lengths can be calculated from layouts taken from plates and ridges using the Plan-View ratios as adjusted o.c. settings. I find this more reliable than using the standard o.c. spacing's that the CMs use with Irregular calculations. However, the sequence for Irregs will solve the Irreg H/V lengths from the same effective Run, then clear it and solve the jacks for each pitch separately.

Once you figure out how the CMP works, manipulate it to find the lengths/miter-angles that you are after. Take notes, make lists too.
• King Common
• King Common and CATSKINER
Thanks guys. so let's see if I understand, subtracting half the 45* thickness of the hip (1"1/16 in this instance) from the calculated distance will give me the actual cut length for the jacks from the wall building line.

Jack length from CMP 11'-1"15/16 minus 1"1/16 = 11'-0"7/8

11'-0"7/8 is the length from the H.A.P. line to the short point of the cheek cut?
:no:

The 1-1/16" dimension is found in Plan-view, so it is a "Run" deduction, not a length deduction. And subtracting this gives you the short-point of the bevel angle, (45*). *(subtract from effective run of common)

Effective = actual, ie.; effective run, effective length, etc..

Eff'n Run is in a level plane, and Eff'n Length is in the Roof Plane. Thanks guys. so let's see if I understand, subtracting half the 45* thickness of the hip (1"1/16 in this instance) from the calculated distance will give me the actual cut length for the jacks from the wall building line.

Jack length from CMP 11'-1"15/16 minus 1"1/16 = 11'-0"7/8

11'-0"7/8 is the length from the outside edge of the plate to the short point of the cheek cut.
What tables are you using? When I look in my full length roof framer, the numbers are consistent with my Construction Master. 11'-2 1/8" calculated first jack rafter.
those lengths don't jive with my rafter tables.
Is this what you were trying to put together? The cross on top would be the calculated measurement from your Construction Master, then you measure your 1 1/16" hip thickness perpendicular to the plumb line.
OK, here's the question. Is that jack length to short or long point? Do I need to subtract 1/2 the 45* thickness of the hip(1"1/16) for this length on each jack to make layout work for 2' o.c.

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Yes! The above picture is what I was looking for (that's the way I typically mark them with a framing square). I have to admit my rafter tables were right, these old eyes aren't what they use to be.
Here's the way I ended up figuring the jacks with the cmp

common length
- length of the on center spacing (24" x 1.1180)
- length of 1/2 45* thickness of hip (1"1/16 x 1.1180)

13'-4"3/16
-26"13/16
-1"3/16
=11'-0"3/16 actual length of longest jack to short point from building plumb line. Worked like a champ! Consecutive jacks were shortened by the jack step using the answer as Jack 1 instead of the CMP. Thank you gentle men for the help.
Nice job. Any good framing book will illustrate what you're asking, you might want to invest in one, I use "Carpentry" by Leonard Koel. I see you calculate the actual lengths, which isn't really necessary. Once you mark the calculated numbers on your rafters, you can make adjustments with your framing square. That way you're going from calculated to actual without ever knowing what the actual dimensions are.
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