# Cutting sheathing for compound roof angles...



## avasa (May 6, 2011)

Lets say you have a 7/12 roof with a 10/12 dormer extending from it. How do you calculate the angle to cut the sheathing at 
To make the valley? I know you can measure as you go, but what is the mathematical formula (or can it be figured out accurately with a speed square or framing square?


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## kyle_dmr (Mar 17, 2009)

Honestly, don't know the formula. It's probably listen in my go to book though. Roof Framers Bible. Lists all single, and dual pitch sheet cut angles for both pitches. About 10$ on amazon. Well worth it


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## ryanshull (Nov 1, 2012)

avasa said:


> Lets say you have a 7/12 roof with a 10/12 dormer extending from it. How do you calculate the angle to cut the sheathing at
> To make the valley? I know you can measure as you go, but what is the mathematical formula (or can it be figured out accurately with a speed square or framing square?


You are going to have go take a measurement anyway, why do you want to overthink it?


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## Ninjaframer (May 28, 2011)

Here ya go, here's the key strokes on a construction master- any trig calc will work though.


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## Ninjaframer (May 28, 2011)

Sorry- those are regular sheathing cuts- what your after is the valley sheathing cut of a bastard- id have to sit down and think about it and right now I'm off to a fire meeting, if someone doesn't get it before I'm done I'll figure it out- its been awhile since I've used that kind of math and I could do with a refresher


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## FramingPro (Jan 31, 2010)

just drop your sheet in the valley, on whichever roof, make sure it is square to the roof, and then move it until the bottom corner sits in the valley, measure the distance at the top, transfer that distance to the bottom and snap a line from there to the corner, perfect cut every time
or mark a point 4' up from the bottom of the sheet and measure that distance, to the valley, once you have that scrap, use it as a pattern for the next pieces


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## Kent Whitten (Mar 24, 2006)

FramingPro said:


> just drop your sheet in the valley, on whichever roof, make sure it is square to the roof, and then move it until the bottom corner sits in the valley, measure the distance at the top, transfer that distance to the bottom and snap a line from there to the corner, perfect cut every time


You had better be perfectly parallel to get that perfect cut though. You can miss by 1/2" easy and then be called a hack.


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## jkfox624 (Jun 20, 2009)

Just measure your first one and from there you have the number to add for each sheet. No reason to make it more difficult.


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## griz (Nov 26, 2009)

FramingPro said:


> just drop your sheet in the valley, on whichever roof, make sure it is square to the roof, and then move it until the bottom corner sits in the valley, measure the distance at the top, transfer that distance to the bottom and snap a line from there to the corner, perfect cut every time
> or mark a point 4' up from the bottom of the sheet and measure that distance, to the valley, once you have that scrap, use it as a pattern for the next pieces


That method may work on the shed in your backyard or your neighbors job...

But if you are on a production crew doing that you'll be unemployed after the first coffee break....if it takes that long...


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## asevereid (Jan 30, 2012)

It can be laid out with a framing square...but the following info is probably veeery wrong.
I think the 10/12 going in to the 7/12 would be face cut at 7/12: cut on the run, and then the bevel would be laid out with 10/12, cut on the rise. Been over a year since I laid out a valley cut, but I know the compound cuts CAN be laid out with the framing square, and the rise/run. If anyone can clarify this method further, it would be appreciated: I obviously need a refresher.

I just re-read what I wrote...and already the first cut is wrong...damnit..time to hit the books again.


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## griz (Nov 26, 2009)

In my production days I used to know how to layout sheathing cuts with the venerable frame square....:thumbsup:

I can't remember what I had for dinner last night....:whistling

so the square trick is probably out of the question....:laughing:


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## Ninjaframer (May 28, 2011)

It's something like the cos of the main pitch divided by the tan of the dormer pitch and then the inverse gives the angle- eventually this B.S fest- I mean fire Mtng will be over and I'll get to figuring it out.


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## Ninjaframer (May 28, 2011)

I was right the first time- cos of main divided by tan of dormer than inverse .


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## jlsconstruction (Apr 26, 2011)

1. W ÷ LL factor = horizontal inches to add from short point to long point of the cut angle from the bottom to the top of the sheathing.
or
2. Find the common rafter incline degree of the unit rise to unit run.
Formula: W x cosine° = horizontal inches to add from short point to long point of the cut angle from the bottom to the top of the sheathing.


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## Kent Whitten (Mar 24, 2006)

griz said:


> In my production days I used to know how to layout sheathing cuts with the venerable frame square....:thumbsup:
> 
> I can't remember what I had for dinner last night....:whistling
> 
> so the square trick is probably out of the question....:laughing:


On a regular pitched roof using a cm I usually punched in 

4' diagonal
7" pitch 
Run

That gave me the amount to measure over. 

I believe that number is on the framing square, you just have to multiply by 4. From my recollection that is. I can't remember what I had for breakfast. :laughing:


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## loneframer (Feb 13, 2009)

griz said:


> I can't remember what I had for dinner last night....:whistling





Kent Whitten said:


> I can't remember what I had for breakfast. :laughing:


I can't remember what it's like to frame a house...:blink:.........


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## Stephen H (Feb 18, 2011)

Should I be embarrassed to admit----? the dumb roofer method we have always used------ guy on the roof calls down " short point 53-1/4, long point 75-1/2" and the guy on the ground cuts it and hands it up ?

while the cut man is cutting it the guy on the roof is usually flopping a full panel on the next course up-or nailing off another panel

probably a hillbilly way to operate- but we have done it that way for 25 plus years- including the winter I worked for a formerly mennonite framing contractor. Al Miller had a Framing square- but he never used a calculator that I ever saw.........
stephen

As a practical matter- I am more typically working with 1x8's and I set the valley angle with a bevel gauge-which I doubt is practical for working with 4x8 panels on a regular basis


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## Kent Whitten (Mar 24, 2006)

Stephen H said:


> Should I be embarrassed to admit----? the dumb roofer method we have always used------ guy on the roof calls down " short point 53-1/4, long point 75-1/2" and the guy on the ground cuts it and hands it up ?
> 
> while the cut man is cutting it the guy on the roof is usually flopping a full panel on the next course up-or nailing off another panel
> 
> ...


Calculating it out keeps the guy on the roof working sheets rather than measuring. All you need is to figure it once, write the number down on a stud near your cutting area, and the roof guy yells down the short point measurement. Or use a valley jig for ply. 

Measuring the long point requires one to measure the 4' way, then eyeball straight into a rough measurement into the valley. Does it get the job done? Sure. 

At my prime, I wouldn't even need that short point measurement. I already had it, cutting the next one. I did a whole hip roof once purely without any called measurements. I knew all of them.


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## Stephen H (Feb 18, 2011)

Kent Whitten said:


> Calculating it out keeps the guy on the roof working sheets rather than measuring. All you need is to figure it once, write the number down on a stud near your cutting area, and the roof guy yells down the short point measurement. Or use a valley jig for ply.
> 
> Measuring the long point requires one to measure the 4' way, then eyeball straight into a rough measurement into the valley. Does it get the job done? Sure.
> 
> At my prime, I wouldn't even need that short point measurement. I already had it, cutting the next one. I did a whole hip roof once purely without any called measurements. I knew all of them.


 Yes- but you are a carpenter and undoubtedly WAAAAAY smarter than me-certainley better at math than me !
I am just a dumb roofer- it's all I can do to remember 11,22,33,44,55.... for my chalk line snaps, LOL
Stephen


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## PrestigeR&D (Jan 6, 2010)

FramingPro said:


> just drop your sheet in the valley, on whichever roof, make sure it is square to the roof, and then move it until the bottom corner sits in the valley, measure the distance at the top, transfer that distance to the bottom and snap a line from there to the corner, perfect cut every time
> or mark a point 4' up from the bottom of the sheet and measure that distance, to the valley, once you have that scrap, use it as a pattern for the next pieces



BINGO!:clap:


But if you have a good contractors calculated you can do it that way, but the above is the simplest, and fastest way IMO...


B,


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## Kent Whitten (Mar 24, 2006)

Stephen H said:


> Yes- but you are a carpenter and undoubtedly WAAAAAY smarter than me-certainley better at math than me !
> I am just a dumb roofer- it's all I can do to remember 11,22,33,44,55.... for my chalk line snaps, LOL
> Stephen


There there....(pats Stephen on the back) it will be OK. :laughing: Sorry, I let it get away from me every now and then. 

The measure over works in theory, you just have to be careful with your measuring on the roof. It is quite easy to mess up a valley cut. It's as easy to mess it up on the ground too :whistling:


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## loneframer (Feb 13, 2009)

I don't overcomplicate it. If you know your rafter length and you know how far the valley or hip is offset, which should be known, divide the rafter length in inches by the offset in inches and you arrive at a multiplier that you can use for all cuts, even rips.

For example, a 12/12 roof with a true valley has an offset of 1" per 1" of run. On a 12 pitch, your rafter length will be 1.414" per 1" of run.

1/1.414=.707 or.707" of offset per 1" of rafter length.

Multiply the width of the rip, or 48" in the case of a full sheet by .707 and you arrive at 33.906". This is the amount removed from the end of the full sheet to fit the valley.

Again, this is math that can be done at home and used on the job site to speed production.

This math works in all instances of bastard hips or valleys and any roof pitch imaginable. All you need is two given numbers.


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## Kent Whitten (Mar 24, 2006)

Riz, I don't either. In your example, what I would punch into a CM would be 4' for the diagonal measurement. 4' being the width of the sheet going in the direction of the slope, would be the same as the diagonal of a rafter. Next if you punch in the pitch, then quickly hit the "run" button, this gives you the distance to go towards the valley on your ply, since it is a 45 and all, ending up with 33-15/16". And like you, that is what you would add to the short point measurement yelled down to you. 

Too many calculations confuse it for me. Nothing personal, but all of those numbers you just listed seemed way too much calculating for me.


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## loneframer (Feb 13, 2009)

Kent Whitten said:


> Riz, I don't either. In your example, what I would punch into a CM would be 4' for the diagonal measurement. 4' being the width of the sheet going in the direction of the slope, would be the same as the diagonal of a rafter. Next if you punch in the pitch, then quickly hit the "run" button, this gives you the distance to go towards the valley on your ply, since it is a 45 and all, ending up with 33-15/16". And like you, that is what you would add to the short point measurement yelled down to you.
> 
> Too many calculations confuse it for me. Nothing personal, but all of those numbers you just listed seemed way too much calculating for me.


The thing is, when I'm figuring ridge lengths, commons, hips/vals, jack rafters and pocket heights, all the info is on paper in front of me. I have all the necessary numbers staring at me and can figure the common difference for all instances in a matter of minutes. I've always used a TI35 and never really made the move to a CM, although I bought a CM5 a couple years ago, haven't framed a roof since.:sad:

My explanation took me longer to write than it would have taken to put to practical use.:whistling


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## Brutus (May 29, 2007)

I just measure up 4 feet, make a mark on two rafters/trusses, then measure over from the vally/hip to the rafter/truss the sheet is landing on.


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## FramingPro (Jan 31, 2010)

Brutus said:


> I just measure up 4 feet, make a mark on two rafters/trusses, then measure over from the vally/hip to the rafter/truss the sheet is landing on.


i said that already :tt2:


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## greg24k (May 19, 2007)

Is this post a joke...or what? :laughing:


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## ryanshull (Nov 1, 2012)

Kent Whitten said:


> Calculating it out keeps the guy on the roof working sheets rather than measuring. All you need is to figure it once, write the number down on a stud near your cutting area, and the roof guy yells down the short point measurement. Or use a valley jig for ply.
> 
> Measuring the long point requires one to measure the 4' way, then eyeball straight into a rough measurement into the valley..


Or use your first scrap to mark the angle on the rest


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## ryanshull (Nov 1, 2012)

ryanshull said:


> Or use your first scrap to mark the angle on the rest


Oh, and flip that same scrap backwards for other side.

How many of you have been told you don't get paid to think. 

The formula is only going to work in theory. Hard measurements don't lie.


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## Kent Whitten (Mar 24, 2006)

ryanshull said:


> Oh, and flip that same scrap backwards for other side.
> 
> How many of you have been told you don't get paid to think.
> 
> The formula is only going to work in theory. Hard measurements don't lie.


It does not just work in theory, I have done this on dozens of homes. A square, level building done right from the beginning will yield easy results when cutting ply. Now, this does not work on remodeling as nice, but new construction, there should be zero excuses. 

And a valley layout stick weighs quite a bit less than a scrap piece of ply. 

I personally feel that hard measurements can give you skewed answers sometimes. There are field conditions of where you are measuring to that will give you a much larger margin of error than a simple add on to the short point measurement yelled down to you. 

If you don't believe me, check it out for yourself. Take the hard measurement and check it with the calculated measurement. You will find that hard measurements can easily run 1/2" off. The calculated method is accurate every single time, and it is faster.


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## loneframer (Feb 13, 2009)

ryanshull said:


> How many of you have been told you don't get paid to think.
> 
> The formula is only going to work in theory. Hard measurements don't lie.


When you have 3-4 guys working under you, thinking and doing the math in advance pays much better than NOT thinking.

As far as formula only working in theory, every guy on here that understands the math involved in framing intricate roofs knows that formulas work each and every time they are applied. 

From a safety standpoint, calculating all the roof components from the desk, with a cup of coffee and no distractions is much less dangerous than measuring in each and every hip, valley and jack rafter.

Here's an example of theory put to practical use. Framed this place about 11 years ago.


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## Kent Whitten (Mar 24, 2006)

Oh chit Ryan....you made him pull out the pictures! :laughing:


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## loneframer (Feb 13, 2009)

Kent Whitten said:


> Oh chit Ryan....you made him pull out the pictures! :laughing:


Well, since you mentioned it, some more family photos....:whistling
http://www.contractortalk.com/members/loneframer-42046/albums/homes-i-have-framed/


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## ryanshull (Nov 1, 2012)

Or after your first hard measurement, do simple subtraction and you have your number to add to the short side measurements. I can think of a several ways to get it cut without a complicated formula and a trig calculator.

Has anyone figured out the formula yet anyway?

If it is complicated enough that most framers don't know or remember the formula...............hard measurement. 

My tape measure never lied by more than a foot or two.


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## Stephen H (Feb 18, 2011)

ryan,
I gotta agree with you.
Kent and loneframer have a good point- but they are envisioning a situation and working in an environment that I will NEVER face.

they are creating a building from scratch and have all sorts of info right there in front of them......

I am arriving to work on a roof on a building that has existed since,say 1914- I don't have ANY of that info available. I could easily measure the pitch for example-but I NEVER do
what I have is a chalkline and a tape-and those work regaurdless of weather I am facing a bellcast dormer intersecting a main roof or a doghouse dormer intersecting a mainroof where the main roof changes pitch mid run.

In my case- the chalk line and tape is way faster than even attempting to figure it out mathematically,rather than graphically


but I strongly suspect Kent and Loneframers' advice is more pertinent to the OPs question,
stephen


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## loneframer (Feb 13, 2009)

ryanshull said:


> Has anyone figured out the formula yet anyway?
> 
> If it is complicated enough that most framers don't know or remember the formula...............hard measurement.


I gave a basic math example of how to get the measurement. No complicated formula to remember, just simple division and multiplication.

Kent gave an example of pushing a few buttons on a CM calculator and I'm pretty sure Ninja gave the equation much earlier in the thread.

Three examples, three much different approaches and three correct answers, all done with both feet on the floor.

There are framers that have never had to hand cut a roof, let alone need the math skills to do so. Then there are framers who rarely, if ever, framed a roof with trusses.

For those of us that fall into the latter category, the math is a vital part of the project, not only for roof framing, but squaring a building that might have in excess of 30 exterior walls, for example. 

The point is, once math becomes the most important part of the job, practicing the math keeps it fresh in your mind. Maybe getting up there and measuring every cut works just fine, but I promise you, it isn't faster and it's not as accurate.


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## loneframer (Feb 13, 2009)

avasa said:


> Lets say you have a 7/12 roof with a 10/12 dormer extending from it. How do you calculate the angle to cut the sheathing at
> To make the valley? I know you can measure as you go, but what is the mathematical formula (or can it be figured out accurately with a speed square or framing square?


Referring back to the OP, 7/12 with 10/12 dormer...

Valley cut on the 7/12 side will have a long point/short point difference on a 48" sheet of 28.992", call it 29".

The 10/12 side will have a difference of 52.677", or 52 11/16"...roughly.:whistling

All with basic math, along with Pythagoreans to figure the rafter lengths, using the two known roof pitches and an assumed run of one or the other.


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## Kent Whitten (Mar 24, 2006)

loneframer said:


> Referring back to the OP, 7/12 with 10/12 dormer...
> 
> Valley cut on the 7/12 side will have a long point/short point difference on a 48" sheet of 28.992", call it 29".
> 
> ...


If you are using a full 48" wide sheet :whistling: :laughing:

You are right Riz, many ways to skin the cat. I got into the cm long ago and never looked back. Same with calc'ing it out at home. I am comfortable on the jobsite punching this in, but maybe some don't trust themselves with this method...which is fine. Measuring will get it done, sure...but be careful. I have proven on site that it is really easy to err on field measuring. 

If one has a smart phone and build calc, it is super easy. 4' diagonal, 7 in pitch, 10 inch irregular pitch, hip/val, hip/val and it's under "minor run" 

BTW....your numbers were correct :laughing:


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## loneframer (Feb 13, 2009)

Kent Whitten said:


> If you are using a full 48" wide sheet :whistling: :laughing:
> 
> 
> 
> BTW....your numbers were correct :laughing:


If not a full sheet, I have my multipliers for each side. 7/12 side is .604, 10/12 side is 1.097. Width of rip x multiplier = L/S difference.:thumbup:

....and my coffee was piping hot, my flannel jammy pants were comfy and the silence was golden.:laughing:


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## Ninjaframer (May 28, 2011)

ryanshull said:


> Or after your first hard measurement, do simple subtraction and you have your number to add to the short side measurements. I can think of a several ways to get it cut without a complicated formula and a trig calculator.
> 
> Has anyone figured out the formula yet anyway?
> 
> ...


If you want a formula that's easy to remember:
4' divided by the sine of your h/v angle than inverse (arc sine) gives sheathing top angle.

There are more ways to mathematically arrive at your sheathing angle than I can count- I just like playing with the trig calculator so that's what I've been using lately. You can usually do right angle math with less key stokes using trig than a CM using pathagorean formula.

And again a bastard valley sheath top angle can be found by taking cos of main divided by tan of dormer than inverse (arc tan)


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## ryanshull (Nov 1, 2012)

I can respect a framer who knows thier math. hands down. Trig is actually fairly complicated stuff.

But if my laboror doesn't understand sine, or cosine, or tangent, or what it means to inverse an angle, I am not trusting him with a calculator and a formula. If one understands the concepts of trig, it could be very advantageous, but most of us who do get it won't stay on our tools for very long.

When I say you don't get paid to think, I am not speaking of the foreman or the GC obviously, but the guy with the skil saw who may or may not be on the ground because you don't feel comfortable with him walking rafters..........may be better off with a hard measurement..........surely you understand.

Of course the field of laborers around here may not be what it is in your areas either.


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## Ninjaframer (May 28, 2011)

ryanshull said:


> I can respect a framer who knows thier math. hands down. Trig is actually fairly complicated stuff.
> 
> But if my laboror doesn't understand sine, or cosine, or tangent, or what it means to inverse an angle, I am not trusting him with a calculator and a formula. If one understands the concepts of trig, it could be very advantageous, but most of us who do get it won't stay on our tools for very long.
> 
> ...


I see what your sayin- I don't have a single guy working for me that I can hand a roof plan and a trig calculator to and say "get to work". But as Lone points out- I do all that figuring at home before work starts- I have all the lengths, angles, etc done before anyone has a chance to scratch there head. Then it's just "turn & burn!"


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## ryanshull (Nov 1, 2012)

Ninjaframer said:


> I see what your sayin- I don't have a single guy working for me that I can hand a roof plan and a trig calculator to and say "get to work". But as Lone points out- I do all that figuring at home before work starts- I have all the lengths, angles, etc done before anyone has a chance to scratch there head. Then it's just "turn & burn!"


That makes enough sense to me.

I guess if you framed the house yourself, and you know everything is square, plum, and up to spec, that makes a lot of sense.

I am not dogging any craft, but for instance, I dont know how many times I have gone in to lay tile in a brand new home and couldn't find a square wall to work off of. (Of course I knew how to work around it, but that's not the point....)

So in any trade I worked I never assumed someone before me did their job perfectly up to specs. And in the remodeling business you count on nothing!

To be honest, I've done some framing, but never on a production crew. I've done plenty of roofs, but rarely new construction.


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## FramingPro (Jan 31, 2010)

loneframer said:


> I don't overcomplicate it. If you know your rafter length and you know how far the valley or hip is offset, which should be known, divide the rafter length in inches by the offset in inches and you arrive at a multiplier that you can use for all cuts, even rips.
> 
> For example, a 12/12 roof with a true valley has an offset of 1" per 1" of run. On a 12 pitch, your rafter length will be 1.414" per 1" of run.
> 
> ...


How do you figure this? I see the same figures in my rafter bible, if it were me, i would just use the sheathing cut 8 1/2"p then extend my line across, use first pieces as a template, think about all the chalk you would save, if you could just draw that line..


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## Kent Whitten (Mar 24, 2006)

FramingPro said:


> How do you figure this? I see the same figures in my rafter bible, if it were me, i would just use the sheathing cut 8 1/2"p then extend my line across, use first pieces as a template, think about all the chalk you would save, if you could just draw that line..


A stick with two nails works awesome Nick. Short point measurement gets yelled down, you mark it, grab the stick and let the two nails hit both edges and strike your pencil down the side. Blam! Away you go. No figuring or anything....well...except the first figure that is.


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## ryanshull (Nov 1, 2012)

Kent Whitten said:


> A stick with two nails works awesome Nick. Short point measurement gets yelled down, you mark it, grab the stick and let the two nails hit both edges and strike your pencil down the side. Blam! Away you go. No figuring or anything....well...except the first figure that is.


So easy a caveman could do it.


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