As a young carpenter (or should I say, “helper”) I was always amazed at the skill of the roof framers. They made it seem effortless to cut and fit roof rafters with only the aid of a framing square. The whole process mystified me. In those early years, I tried several times to do it myself, but I never had success. In time, an older carpenter named Rich Murphy took me up on a roof and helped me lay out and cut a reverse gable using a framing square and his roofer’s pocket reference or “bible.” That experience sent me on the quest to master the art of roof cutting. I can’t say I ever mastered it, but I’ve come a long way since Rich graciously took the time to help me understand what was going on.

Of course, that was decades ago. Today, I use a calculator to find rafter lengths and angles. Without a doubt, a construction calculator is the quickest way to find your way around a roof.

Construction calculators are pre-programmed with Pythagorean formulas for finding the values of right triangles—and roofs are all about right triangles. These calculators eliminate some of the memorization, and all of the charts. They put all the information in a nice, clean, easy-to-understand interface.

There are also two laudable software versions available as smartphone apps: one from Calculated Industries, and one from BuildCalc. They essentially do the same thing as an actual construction calculator, but I prefer the real thing—if I’m going to drop a calculator in the mud, I’d rather it not be my pricey smartphone!

#### Using a Construction Calculator

The values on the calculator that we use for common roof framing are: Pitch, Rise, Run, and Diagonal. If you have any two of those values, the calculator will quickly figure out the rest of the right triangle—which means it will tell you everything else you need to know about a rafter.

Most of the time, the two values I have are the run of a building and a specified pitch, which is why I used these values for the example in this online tutorial:

Looking at our model roof, I need to find the two elements that will give me all the information needed to frame the roof.

The building width, in this example, is 6 ft. 3/4 in., including the sheathing.

Each rafter only spans half the width of the building, and they start at the face of the ridge beam. For simplicity, and to prevent error, the first thing I do is deduct the full width of the ridge beam from the building width: in this example 6 ft. 3/4 in. – 1 ½ in. = 5 ft. 11 1/4 in.

I write this down on my template rafter as the adjusted overall run. Then, I divide that by 2 to get the actual run of each rafter. The result on my Construction Master calculator is 2 ft. 11 5/8 in. Next, I press the Run key, instructing the calculator to use that dimension as the ‘run,’ which is the first element of the right triangle I am working with.

I need one more element, and in this case I know the pitch of the roof, which is 6/12. So I enter the number 6 into the calculator, followed by the Inch key, and then press the Pitch key. Note: It’s important to remember to press the Inch key when entering the roof pitch—without it, the calculator treats the number entered as ‘degrees of pitch’ instead of the rise/run ratio of the roof.

Now the calculator has all the details it needs, and it can provide me with every bit of information about that triangle. For instance, I’d also like to know the diagonal measurement, which will help me layout the seat cut. All I have to do is press the Diag key, and the calculator displays the measurement: 3 ft. 3 13/16 in. I write this measurement down on the template rafter, too.

Next, I press the Rise key, and write that number down: 1 ft. 5 13/16 in.

Be sure to go through all the calculations a few times, clearing the calculator in between. If all the results match, you can rule out any keystroke errors.

#### The Layout

The next step is to layout and cut the rafter. First, I attach a set of stair gauges to my framing square, so I can make precise, repetitive marks. In this case, I attach the gauges for a 6/12 pitch—6 in. on the tongue of the square and 12 in. on the body of the square. I carefully align those measurements along the edge of the rafter material, and then set the gauges.

Laying the square on the top of the rafter material, I start by scribing the plumb cut at the peak of the rafter. Keep in mind that, for most framing jobs, the tongue (the skinny side) is the vertical cut, and the body (the wider side) is the horizontal or seat cut.

I make this plumb cut at the peak with my saw before marking my seat cut (or “bird’s mouth,” in some vernacular). This way, I have something to hook my tape measure on, which is very handy for long rafters.

Measuring from the tip of the rafter, I mark off the diagonal measurement along the top edge of the rafter.

Then, using my framing square (some carpenters choose to use a speed square, but speed squares aren’t as precise, especially on fractional pitches), I draw the parallel plumb line across the rafter, marking along the tongue of the square. This line represents the plumb line on the rafter at the edge of the building.

The seat cut (or “bird’s mouth”) is referenced from this line. If you are framing from scratch, and not matching rafter heights (which will be explored in a future article), you will need to decide on what size the seat cut should be. Most codes require a minimum of 1 ½ in. of seat bearing on the top plate. I like to keep the seat cut the same width as the wall, including the sheathing.

In my model here, and on most of my jobs using 2×4 walls, the seat measures 4 in. with the sheathing. With wider plates, you cannot cut into the rafter more than a third of its overall width—this would weaken the structure too much. I generally go with 4 in., and it works well with most roofs.

To do a 4-in. seat cut, I rotate the square 180 degrees from the plumb cuts I’ve marked so far—this way the stair gauges will be referenced against the bottom edge of the rafter. I then slide the square along the bottom edge until the 8 in. mark on the body intersects the parallel plumb line I drew earlier; you’ll see that the line I trace will be exactly 4 in. long.

To me, that’s the quickest way to draw the seat cut.

Then, while I have the square there, it’s easy to slide the square over to mark for the rafter’s overhang (if I have an overhang less than 12 in.). In my case, that’s 6 in., and I draw another plumb line so I can start to cut the rafter.

#### Setting the Rafters

Before I set my rafters, I like to set the ridge in position first. That’s why I recorded the Rise measurement of our rafter.

I could calculate this on my construction calculator, but honestly, I find it easier to draw it out—it’s much safer, since drawing makes it easier to keep track of the numbers. By drawing it out on a story pole, I find the post elevation, and I can then cut the story pole to post the ridge.

To get to the top of the ridge, I need to measure the rafters HAP, or “Height Above Plate.” Looking at the illustration (below), you can see the triangle that our construction calculator calculated. The calculator has no idea about the depth of the seat cut, or the size of the rafter material—it’s easiest to measure from the seat cut to the top edge of the rafter I’ve cut, and that is the HAP.

I add that measurement to the Rise and label it. The post height will now be at the top elevation of the ridge. In this example, my rafter has a 4-in. HAP. |

Next, since I want to post the ridge, I measure the depth of the ridge beam (in my example, 5 ½ in.), and measure down from my HAP line mark. This line represents the height of the post. I now know if I cut that, it will fit.

Mathematically: (RISE + HAP) – Ridge Beam depth = Post Height |

In real life (not mathematics), not everything is perfect. I usually deduct a 1/4 to 1/2 in. more, to allow me to shim the ridge into position perfectly. It’s a lot easier to shim a 1/2 in., than to have to cut a 1/2 in. off *after* the ridge is on the post.

The process is pretty straightforward—no complex charts or tables. And as Tom Brewer says, we all love it when a plan comes together and actually works!

(SketchUp drawings by Wm. Todd Murdock)

Once again I feel compelled to comment. The word “Pitch” is often mis-used, as in this and many other articles. One might say that “it’s a 6/12 pitch” This is incorrect. The proper term for expressing the roof angle is “slope” in that instance. Pitch is properly expressed as a fraction, such as 1/4 pitch or 1/2 pitch. The above would be a 1/4 pitch since the unit rise is divisible into 24 by 4.

Thank you

Bill,

And I feel compelled to correct you. According to Wikipedia, the most untrustworthy reference source, but a great location for modern usage, pitch is the proper word to use:

“The roof’s pitch is its vertical rise divided by its horizontal span (or “run”), what is called “slope” in geometry and stair construction. It is typically expressed with the rise first and run second, with the run denominated by the number 12, giving a ratio of how many inches of incline there is to each foot of run. For example, 3:12, 4:12, 5:12, and so on.”

Gary

You’re right Gary, stairs have a “slope” and roofs have a “pitch”. At least that is exactly how I was taught 40 something years ago. I’ve heard a few guys use the word “slope’ while talking to the framer cutting rafters referring to a roof and all they got was a ‘Huh’? back. And the guy cutting the steps will always correct someone with “you mean slope” it’s not a roof. So there it is there.

I’ve cut a few rooves. I’ll interchange “Pitch” and “Slope” synonymously for both stairs and roofs since the concept behind both terms is basically the same. We’ve all used or heard the term “step-off” to describe rafter layouts and stairs too.

But the terms “Run” and “Span” should not be confused with each other. I have read many books describing “Pitch” as Bill B. has explained it. I have always felt that “Rise to Span” ratios were a confusing use of the term “Pitch”. (How would this usage apply to multiple pitches, or irregular roofs?) I must not be alone on this because so many modern tools and calculators base Pitch on the Rise to Run dimensions of a single sloped surface.

Span is the width of the Roof

Run is generally one half the Span

“Effective Run” is the Run (Span/2) minus the ridge deduction (usually the Ridge board/2), and is the dim used to calculate the Rafter length.

The most basic thing a roof cutter needs to know and understand is a rafter length is simply the Hypotenuse of a right triangle, and the “ER” is the base of the triangle.

Oh, and I thought 6/12 was a fraction, > > > unresolved of course.

Richard

And here I thought, pitch was the gunk that collects on my blades and bits…

That caught my attention also. If memory serves, pitch is rise to span, slope is rise to run.

Regards,

Brian

My goodness, we have a lot of linguistics and English majors on board, but I’m not sure if the debate between pitch and slope really adds a lot to this article. :)

Gary

You should say the run of the rafter is 1/2 the span of the building, not “the span of the rafter is 1/2 the buildings width.” I know it’s the same but it drives me nuts when a guy holds a bubble on a doorjamb and says- yep, it’s level…. How about proper use of “run” and “span” while we’re at it.

Vernacular is the key —

As I learned Pitch in the Early days , it was the Rise of a building over its entire run , The denominator was not usually 12. More like 1/2 or 1/4 – Quite frankly for the field carpenter, a useless term. Over time the ratio over 12″ (1 foot) was more commonly used as it was easy to reference on the framing square. The term Pitch when used over 12 became to be understood as the standard , and is commonly called the Pitch of the Roof. Just look at the Calculator entry the button is labeled “pitch’ not slope or incline. The Construction Master folks wanted us to know what that key was for, so they used a Vernacular term.

Like my Mom used to say , Ain’t Aint in the dictionary. Sorry Mom it is now……

I’m glad these calculators are available, however, I can step a rafter with a square quicker than I can go to the truck and get the calculator. Double step when you can 5/12 doubles as 10/24

Nice article Mike. I won’t get into the english debate. Suffice to say around here we call a 6/12 ‘pitch’ for example. Article is well written and the sketchup diagrams make it easy for anyone to understand. I still step off my rafter cuts with a framing square and stair buttons like you show. I haven’t got used to using a calculater, even though I bought one a few years ago. I always forget where it’s at in my van.

Mike H.

Perfect timing. Thank you gentlemen. I have a very fancy screened porch to build next week. This will be practiced in shop tomorrow and done on site next Tuesday.

Thanks again,

Yannis N. Tsakiris

http://www.citydecksinc.com

http://www.citypropertiesinc.net

Mike,

Thank you for writing a great straight forward article.

I love how you take a complex task and break it down to simple parts. Thanks for sharing your insights.

Will you be at the JLC in Portland, Oregon? If so, looking forward to learning more from you and Gary.

Dan

If I’m cutting a simple gable roof, I’ll have a CMC in my nail bags. But, not if I’m cutting a complex roof. Ben at BuildCalc has done an excellent job on the user interface of BuildCalc and it is the best construction calculator for smartphones.

Android tablets are a great alternative to smartphones. I can see android tablets replacing handheld calculators as the price of the android tablets keep going down in price.

For complex roof framing I use my *********************

Sim

******************* = Chappell Master Framers’ Square?

Which would make an excellent topic for a follow up article, hint, hint.

I am waiting for the right project to come along to order mine.

I am not a pro carpenter (I am a wood worker), but am considering building a modest-sized house which I call a ‘cabin concept.’ Tremendous article. If I do build the house, the roof will be designed with rafters, for loft space. While the Pythagorean concept is basic, the application to actual roof construction brings a boat-load of questions for someone in my shoes. The article seems to clarify the whole thing.

Sincere appreciation to Gary Katz and all contributors, who make TiC the great web-site it is.

Gary, thanks for setting our English majors straight, keep it coming.

I’ve always used the step off method which has never failed me. What is great though was using the calculator to find the post height. If you can find the book “Residential Carpentry ” by John Capatosto, you will be able to find all the step methods needed for cutting rafters, including shortening the rafter for the width of the ridge, both common and hip. I have referred to this book for years and it hasn’t failed me yet.

Thanks for the great article!

Although I’ve read this in books and done a couple of simple rooves I never quite got the hang of it but now that I am armed with this information things will be simpler on my next project.

The information was presented in a readily digestible manner that helped me to really understand where to go with it.

Thanks again,

Chaim

I know Mike has game, I’ve met him and seen him at the shows. I just can’t believe we are kicking around beginner roof framing, I thought we were past that. Grammer boys and guys that think they can “walk” a framing square faster and more accurate than a calculator, please…

What do you mean by reverse gable though?

The great thing about our trade is that you’re never “past” anything. It’s always good to review the basics in order to avoid “accumulated error” which, amongst other things*, is what you can get when you step off rafters instead of using a calculator.

BTW David Lemke, I believe you’re always free to write and contribute an advanced framing article for TIC to consider publishing. Sure beats complaining.

Regards,

Sonny

* accumulated error also occurs when you’ve accumulated a lot of experience and think you know it all…but really don’t.

Sonny,

Don’t mean to complain just felt like subject was getting nitpicked. Screwed up many a rafter and still do. However you get there is how you get there caveman style, calculator, or book. I use a little of all three. Sometimes forget column is for basics to never ending pursuit of perfection. Might try my hand at an article soon, thanks.

While there are a dozen ways to skin a cat, and do rafter cutting,

the best way I have found is the way they did it before phone apps, and that is with the Reichers framing book..

It will give you everything you need, and no worries if you hit the wrong button, (whether it be the (X), or (divide) the only way you could mess this up is incompetence, and then you might look for a job at Wal Mart.

It will also give you lengths on the valleys and jack rafters

This book is bible, and if you spill coffee on it, it will still work, no need to go to Radio shack for another calculator.

http://www.amazon.com/Full-Length-Roof-Framer-Riechers/dp/0899669077

Here’s my two cents on the “pitch” debate:

I learned roof framing the hardest, and most maddening, way possible –– from H.H. Siegele’s books. Actually, these were compilations of articles he wrote, about 100 years ago, in the union magazine. He was a xenophobic man with a condescending attitude, but he did know his roofs (or rooves, as a lot of the guys here like to say).

He was way off, though when it came to the definition of pitch, IMO. For him pitch was rise/span, so an 8/12 rafter had a 1/3 “pitch.” Never mind that with a shed roof or a salt-box, you were talking about 1/3 of something that didn’t even exist!

I’m content as long as I know what a guy is talking about –– dictionaries are descriptive, not prescriptive, so we are in charge of the language.

Regardless of the method used cutting rafters is more fun and rewarding than rolling trusses.

I do love the slope/pitch terminology debate. Kind of like the dado vs plow, soffit/plancher and miter/mitre round and rounds I have been in before.

TJ

Good article, it might be good to do one on simple rafters using a “regular” calculator.

An old misunderstanding of the code regarding the seatcut is being repeated. If the overhang is <2' there must be at least 3-1/2" of rafter left above the seatcut. The limit at the other end of this is, the level portion of the seatcut should not extend inboard of the wall. The misunderstanding stems from viewing this bearing cut as a "notch"… and I've opened another can of grammer.

The one thing I do different is I use a straight peice of 1X and clamp it to my square instead of using square dogs. I don’t know how many of those things I’ve lost or the screws fell out of them . And I think the straight edge will span any irrregularities in the 2X better. JM2C

Although CM can make roof cutters job easier, it will NOT make you a roof cutter; Learn the square!

Using a CM to figure rafter lengths without knowing the basics can lead to a ton of headaches when cutting complex roofs, especially bastard hips and true valleys. Keep in mind that all the numbers a CM spits out are mathematical points and not the actual measurements. You have to where to add and subtract for material thickness. Ex: For hip jacks you need to subtract half the thickness of the hip on the pitch plus 1-1/16 for then plancher cut. If you don’t do this your “step off” length will be off and the last few jacks will be out of square by a mile.

@ Jay Stein:

I agree with a lot of what you say, although I don’t like to be a member of the “Just Say No” crowd. To clarify one of your points, I assume that by “bastard hip” you mean what I’ve always called an “irregular hip, that is, two different pitches –– say, 4/12 and 12/12 –– meeting at a hip rafter. In that case, you can no longer just assume a hip that runs at 45º to the plate, you have to solve for it in plan view first. This is a bit confusing with a CM, because you have to call one of the runs the “Rise” for the calculator to solve it. Then the “rafter” that the CM gives you will be the actual run of the irregular hip.

Whew! Hard to do this stuff in words! Did I get that right?

But can you clarify what you mean by “1 1/16 for then plancher cut?”

@ Harlan,

I am not in the “just say no” camp either, however to fully understand roof cutting one needs to learn how to crawl before they can walk.

I will try to better explain what I mean, but please keep in mind that I am a carpenter and writing is not my strongest skill.

For now, lets forget about bastard (irregular) pitch, for simplicity I will use a common gable roof.

For my example I will be using a 20′ structure with a 10 pitch.

Using the CM calc 10′ run 10″ pitch, you end up with a common rafter length of 13′-0 3/16″; this gives the total length from center of the building to the wall line.

Now we have compensate for a 2x ridge board which is 1 1/2″ half of that is 3/4″, if we just take 3/4″ off the mathematical length we end up 12′- 11 7/16″. However if we use the CM to figure how much to shorten the total mathematical length, we end up with a different number.

Using the CM again, punch in 3/4 run (half of the total span of a 2x ridge) than punch 10″pitch, doing this we end up with a hypotenuse of 1″; this is the actual number you would use to shorten the rafter for ridge material.

Using 1″, subtract that from the total gable rafter (13′ -0 3/16) and you end up with the true length of 12′-11 3/16″, now we end up with a 1/4″ difference.

Now I understand this seems minimal, and for a gable roof it is, however when you start getting into hips and valleys, especially bastard pitches, that 1/4″ starts to compound (the same methods should be used to figure shortening for hip and valley jacks) and can add up to inches, causing wasted time and material.

The plancher cut is the 45 cut at the top end of a jack rafter.

If you cut a 2x on a 45 the face of that cut measures 2 1/4″, half that being 1 1/16″. Remembering that all mathematical points are center lines, this is what you would add to get the real length.

The only laudable smartphone app for unequal pitched gable roofs, that will give the correct length of the rafters, is Rafter Tools for Android. Deducting 1/2 the thickness of the ridge for the rafter framing lengths for unequal pitched roofs is wrong. See my help file for the correct way to deduct for the thickness of the ridge for unequal pitched gable roofs.

http://www.raftertools.com/help/unequal_gable.htm

Sim

@Sim,

Yes! That is what I was trying to get at,

I used a common gable with regular pitch for simplicity. I felt that getting too much into bastard pitches took away from the original article. I was also trying to make the point that a calculator while a great tool, it is just that, it won’t replace basic training.

The first step to a good house is a good foundation, the same philosophy goes for skills to build it!

The stepper the roof the greater the thickness of the ridge becomes–ex 12/12 you should us 7/8ths for half the ridge , simply think about it-to reach the center(which is the goal) is a longer distance -all calculations are from plumb cut to plumb cut, if you use the actual length on your first common then your jack will lay out perfect. Just my experience

Wow! After reading the article and all of the comments, I know why I’m a finish carpenter.

@ Jay Stein:

By “plancher,” I thought you meant what I call the “soffit.” But I understand now. Never heard it called a “plancher cut,” though. I like your trick of measuring the cut face of a jack to find the length at 45º. Good trick, as long as your hip stock is the same as your rafter stock.

I caught a guy making a related mistake when we were building a timber frame with a hip roof. This guy was marking the top edge of a jack with the 45º edge of a Speed Square. I showed him how to square around to the other side, step back the thickness of the stock to mark a plumb line, then connect the dots on the edges. That’s the safest way, although if you know how, you can mark the edges with a rafter square.

Yes, the risk of measuring along the wrong plane is greater when using a calculator –– the rafter itself is such an excellent visual aid. I’m still in the habit of marking the plumb cut full length, then measuring back at 90º to that line to shorten for the ridge.

By doing that, a guy could catch himself if he made the mistake of measuring along the top edge. But if you see the “end grain” of a piece of “1x” staring at you, you know you’ve done it right.

Gary please pull out your Audel’s Carpenters and Builder’s Guide from 1939 and turn to the “roof slope vs. roof pitch” chart.

This was and still is my bible, dating back to 1972. I’m quite surprised that you and many others are unfamiliar with this fact. I enjoy these historical tidbits and pay homage to them daily. Maybe “This is Carpentry” should explore this 4 book set and write some articles on it.

Bill,

I respect your experience–the years you’ve invested in the trade and all that you’ve learned over four decades as a carpenter.

I too have a set of Audel’s books and–being short on experience and lacking instruction–I used them a lot when I started out working as a carpenter. But the problem is, they were published in 1939. That means some of the information may still be applicable and some may not: terminology, building materials, and tools have changed a lot over the years. In 1939 there were no circular saws, no nail guns, no cordless drills, no Titebond II, no power miter saws, and no construction calculators that are far more accurate and efficient–especially when you consider cumulative error. Construction

Calculators do not have a key that says “Slope.” They have a key that says “Pitch.” If we had used terminology from 1939 in this article, at the very least it would have confused all directions connected with using a modern calculator. The purpose of this magazine is to provide comprehensive articles that help carpenters improve the quality of their work, speed productivity, and hopefully add a little enjoyment to a sometimes difficult career.

I think some articles on Audel’s would be welcome, but I suspect they’d be more about how things used to be done and how they’re done today.

Gary

Some really detailed instructions here. A great guide.

I have to say that my biggest pet peeve in forums and blogs are the guys who want to bring up insignificant details and argue how the author is wrong and they are right. Like they are trying to show how much better they are than the author and want everyone to know it. When it comes to terminology most framers I worked with used pitch and slope as the same. Maybe they are using incorrect terminology but everyone on the crew knows what each other means. The article is titled Common Rafter Framing not the definition of pitch.

Road-Dog

And on a lighter note from a finish carpenter who cuts a roof now and then; “pitch” is that material you get on your hand when you pick up a board to cut it into a rafter!

Great article!

I taught carpentry for a few years. I don’t claim to be a skilled roof cutter like many who frequent the forums. What I did find out was that it really didn’t matter what the definition of pitch was, or is. Knowing that definition wasn’t necessarily a help in explaining how to cut a rafter. I taught stepping off with a square, reading from a rafter book and using a calculator. Each has it’s advantages. What’s really inportant is what happens at both ends of the rafter after you know the length. For that a good understanding of a framing square is invaluable. I always taught there’s only two cuts on any rafter – level and plumb. The level cut is always marked on the unit run side of the square (12″ for a common rafter) The plumb cut is marked on the unit rise side of the square (varies with the pitch/slope angle or whatever you want to call it.) You can use the i/s, o/s , top or bottom of the square when marking your cuts as long as you’re consistent. In other words you can’t use the i/s for the rise and the o/s for the run.

Great article. Wish I’d had the information before my last porch build. :)

Thanks for taking your valuable time to put together all the information; it’s very much appreciated.

This is the first article that makes sense to me. Everything I have read about using the Pythagorean theorem was so aggreivating because when using the outside of the wall and the rise from top plate to ridge is not where the rafter actually sits because there is not a seat cut yet and of you did use the exact measuremts than you would have no tails as they would be removed. Now please I cant wait for your ” matching raftter heights article”. Thanks

Hi everyone, can someone please elaborate on the use of wooden shims between the ridge beam and the rafters? Specifically, I understand the need to make minor adjustments without having to cut some length off but I have received mixed reviews on this from a number of people. We are having new construction done and several of the rafters are not laying flush to the ridge beam. I’ve attached one photo as an example. I thank anyone in advance who can offer some commentary on my question.

A shim is a quick fix

If the rafters aren’t working out its because the wall is out of plumb or the ridge is off center. one shim ok. multiple…

Quick, hire a better carpenter.

I was putting a gable roof on a dog house that has an outside measurement of 32 1/4”(16 1/8 run). I laid out a 2×4 with a 45 degree angle for the ridge and pulled down 22 1/16 and 1 1/2 up from the bottom of the ridge cut and the bird’s mouth. I subtracted for the ridge ahead of time. When I put everything together with the ridge the span is still 1 1/2 inches to long. I believe I did everything correctly. If I take out the ridge it is 32 1/4″. What did I do wrong? Is this some mathematical anomaly?

I figured it out….

I have a hip roof with a 12-12 pitch on the main side of the roof while the end is an 18-12 pitch. How do you figure the length and angles of the cuts on an uneven roof for the hip rafter ???

Billy hope this helps you

Construction master pro calculator

Example

Building is 30′ wide Main roof 12/12 end cap is 8/12

1. Enter run 14′-11″1/4 ( this measurement will also be where your 8/12 common rafter starts)

2. Enter pitch 12 inch

3. This will give you rise height, rafter length for 12/12 side

4. Then push 8 inch conv hip/Val this will give you the irregular hip/Val length plumb cut/cheek cut

5. Then clear calculator

6. Enter the rise height from above

7. Then enter 8 inch pitch

8. This will give you the run which is where your first 12/12 common rafter will start and pushing diag next will give you the common rafter for the 8/12 side

Hope this helps

Ben,

Thanks for the response! I’ve been hoping that Mike would respond to that question, but….it’s been hanging there for a LONG LONG time!! :) Glad you cleared it up! I would have answered it if I could but I couldn’t…no way.

Gary

Why is the full 1.5″ width of the ridge deducted to get the length of the rafter and not only 3/4″? I don’t understand how the rafter can reach the center of the wall by taking the complete 1.5″ off. The ridge board being dead center has 3/4″ on each side of the center line.

This kind of made me think about this one too but the method of deducting the overall width of the ridge beam from the span will actually have the same output when subtracting just 1/2 also of the beam’s total width.. because for everything else you will just be computing for the “deducted” length of the span already.

Robert-

If you follow the procedure carefully – Notice the Ridge is deducted from the ENTIRE width of the structure first , than the result is divided in half to get the actual rafter run. This eliminates the need to deduct 1/2 the ridge . it also tells you where to put the edge of the ridge post as measured from the Outside of the building.

Thanks Mike. I see what you mean.

I have struggled with getting rafters right forever! I have also read everything I can find and watched countless videos but never fully understood the processs untill now. This is the best video on rafter cutting in existence. I got the calculator and followed the video instructions with perfect results!! Thank you for settting me straight.

I’ve attached a pic of how the rafters on the shed looked.

Is your shed open ceiling? What size is the shed? And what pitch is your roof? I’m planning on building a 12×12 shed with an open ceiling. This looks like what I want to do except I want 2 ft. gable overhand and eaves.

Tom —

Thanks for your comment!

I struggled for way too many years myself .. It really is pretty simple , I wish my math teacher had told me what I was going to use all that math for ……

Thanks for the Photo too !

Mike

Thanks so much for the rafter video… it makes laying them out a breeze.. thanks again..

Calculating roof geometry was made much more simple for me after reading “A different Approach to Rafter Layout” by John Carroll. The article is in Taunton’s book “Framing Roofs. Rather than outside the wall to ridge, the author made calculations from inside the wall for effective run of rafter at ridge. This eliminates extra addition (or subtraction) with regard to seat cut. Also puts seat cut in the correct position intersecting inside edge of stud wall (Gary might say, “see the triangle”). Height to bottom of ridge intersecting with bottom face of common rafter also easy. Bottom of ridge board for common gable roof can be set to bottom face of rafters. This combined with Gary’s regard and guidance that lead me to construction calculators really improved my game as a contractor and novice roof cutter.

Thanks For your Comment Mike

Unfortunately Johns Approach has some significant drawbacks. You can’t always set the seat cut at the edge of the Plate. On wide plates it will create a code violation, In addition, when dealing with differing HAP’s it gets really confusing. The method we show here is the most common in use, and it does require a little understanding of where the triangle is. I tried his method but found it to be more confusing than the traditional methods. In addition, you become the only guy on the jobsite measuring to the inside of the plate – to me its like switching the gas and brake pedals on my car ( how do you hook your tape ?)

We chose a simple Traditional approach using the modern Calculator. Better to speak the language of your trade than have to explain yourself ..

So… Your triangle is fixed; your roof pitch angle is 22.67deg; your plum cut on the rafter top against ridge is 22.67deg; then the height of the ridge depends on how deep your seat (birds mouth) cuts are? And what size (width) rafters are used? I’ve been using the wrong triangle!

I meant 26.57 deg.

Mike, I actually need to read your matching rafter heights article, have you written it yet?

How do you find pitch when total rise and run are known?

http://www.jlconline.com/framing/framing-to-a-fixed-elevation.aspx

Read this article Sawyer

Mike

What adjustments in layout are made when the bearing walls are of two different widths but you want the pitch the same on both sides?

The Seat cut remains the same for both sides –

The center of building is determined from outside to outside – so the ridge from the perspective of the Interior will be slightly off center

On a room addition I am using 2×10 rafters for a 3/12 pitch.

This is for a cathedral ceiling so my seat cut depth is 1.5″ and seat width is 6″(2×6+1/2″sheathing).

I can not see how to get the “plancher” cut level for a 6″ fascia

without eliminating the birds mouth.

The rest of the house uses a 6″ fascia

Any thoughts?

Your seat cut does not need to be full width of plate.

Additional info –

The rest of the house was at 5-1/4 on the fascia width.

I now have approx. 6-1/4 and am going to go with it.

I had thought about a deeper seat cut – but that makes a wider seat which extends inside the wall and doesn’t work with the cathedral ceiling. If the ceiling was not cathedral (i.e. flat) then that would have worked.

Thanks for the article. It has made my life simpler. I especially like getting the ridge thickness issue out of the way right up front.

Perfect, absolutely perfect, I have spent two days searching my screen for a straight forward “how to cut rafters” and every thing I found was missing something for me, untill I found you, Thanks a great deal!

Question: If calculating the cuts for a flat sloped roof, is the run measured then from the outside of the down slope wall to the inside of the up slope wall?

James –

If a roof is flat , it has no slope.

Better off drawing a Picture so we can understand what you are trying to describe .

Mike

Flat as in no peak. Assume a straight connection between an 8′ high wall and an 11′ high wall 12′ apart. I can easily calculate pitch and the angles for the cuts, but the formula requires span and translating the span instructions above to a non-peaked roof seems to indicate that you use the outside of the down hill wall and the inside of the up hill wall.

What a beautiful illustration and in layman’s terms. Extremely easy to understand. I too was always mystified in my dad’s skill with a using a framing square in construction. I wish I had taken the time in my youth to have learned from him. Thanks much.

Well done.

I have a question… if you are say using a 2×6 as your ridge whcih are generally 1 1/2 wide… wouldn’t you take 1/2 of that to reduce the overall run of your rafter? In the video you you compesated the 1 1/2in from each side of the rafters giving you a total of 3 total inches of compensation. I am confused why your wouldnt take 3/4 from each rafter so that in total you have achieved the 1 1/2 width you need from each side

Disregard I just re-read above you deducted it from the width if the building not from the run of the rafter

Don’t subtract from the rafter.Always subtract from overall span prior to calculations.

I agree with Andrew of 11/09/2012. 40 plus yrs of carpentry & never used a cal to lay out rafters.I use the framing square & double check it with myFull Length Roof Framer book by A.F.Riechers.

I know that this is an older post, but I was hoping that you could assist. I am putting together a timber frame barn and the roof trusses are half lapped at the peak. The beams are 4×8 and the roof pitch is 40 degrees or 10/12. My question is that I am going to have an overhang of 10 inches and I can’t figure out the distance from the top plumb cut to the bottom plumb cut in order to mark my seat cut, any thoughts? The plate is a 8×8 and the building dimensions are 16′ x 24.’ Any help would be appreciated.

The overall width of the building in S.Cal would be the span.Half the span is the run.Learned to cut roofs in the 70’s from a master roof cutter.

Great article and videos, very informative and easy to understand. I did find in my research that the maximum seat cut depth for a roof rafter is no longer 1/3 the beam depth (D/3) but the 2012 International Residential Code (Figure R802.7.1.1) limits it to 1/4 of the beam depth (D/4).

Building an A frame rabbit hutch/run. I’ve already built one, have difficulty with the angle cuts and knowing what they should be. The rise is 22″, the run is 17 1/4″. What should my cut angles be? I do not have one of those nifty stair squares.