Can I cut a roof without knowing Trig?
MSLiechty
| Posted in Construction Techniques on
Been reading with interest Blownfuels roof framing project. I’ve never cut a roof entirely on my own, but have stacked a few, and didn’t get past basic algebra.
Is knowing trig necessary to understand the basics of roof cutting?
ML
Replies
Is knowing trig necessary to understand the basics of roof cutting?
No. There are numerous methods for figuring roofs - stepping off with a framing square, snapping lines of a full-size drawing, using a rafter table. Trig makes it more fun, I think, but is not a necessity. Especially if trig gives you a headache! Takes all the fun out of it.
edited to add: oh yeah, and there's always the Construction Master!
CaliforniaRemodelingContractor.com
Edited 9/29/2008 1:39 am by Huck
One way to do it is to make accurate scale drawings (or have someone make them for you) and then take measurements off the drawings.
You don't know Trig, and you want to cut a roof??!!!
Take out a loan, go back to school and get that degree before you touch a ladder!!
Most roofers I know have a PHD in math or at least a Masters.
Wow!.....glad you posted this and I got to you in time.
Bwahahaha - you beat me to it.View Image
Whew thanks Jer. Not sure if I can continue, Good then you were there to keep me off taht ladder.... :)ML
MSL,
Cutting roofs can be one of the more enjoyable tasks in home building and one of the simplest, it just depends on your approach.
http://www.constructionforumsonline.com
MSLiechty
I built my own roof trusses without knowing how to add, well almost..
My Math skills are near nonexistant.. what I did is set the wood in my driveway and laid it out so I spanned the width I needed. I then laid the timbers out and drew the intersecting cuts directly on the timbers.
Only later when everything was up did I calculate the pitch.. one was at 17/12 and the other was at 27/12
Oh I could have laid things out so I came out to a 12/12 pitch or 24/12 pitch but then the roof peaks would have been at differant heights..
Lots of ways to skin a cat. My way I was sure everything would fit and work..
I know nothing about trig and have cut roofs as complicated as most I have seen here.
Old books on rafter cutting for multi faceted tower roofs help refresh my brain for those, for the rest of them I used the framing sq. for years, then the construction master or "The Roof Framers Bible".
I am not going to even compare myself to Joe for speed of getting really complex roofs figured out but they all fit.
I use that old greek guys formula when I need to.
Speed sq. is reserved for the fact that is is easy to use as a guide for drawing lines.
ML, I've cut roofs since the 70's and I don't know trig. I can theoretically explain all the angles and dimensions on most roofs (parabolic roofs would be an exception) using simple geometry.
If simple geometry is beyond the abilities of a carpenter it's still possible to cut most roofs by having a solid knowledge of how to make the framing square do the work. The geometric formulas and functions are built into the framing square and it eliminates a lot of the need for substantial geometric knowldege. Most carpenters can get by with knowing two or three basic ideas: square, parallel and the pythagorean principle.
Jim, I'm not a framer, but I can do it and have done quite a bit of it over the years. I never learned how to use the scales they give you on the side of a framing square. You know these?
I would love to learn sometime just out of interest. I probably would never use them at this point. I imagine they're in a book somewhere. Were you union? I wonder if they teach these in the union.
Should have saved the little book that came with your square.
Joe H
I went through a union carpenter program and we did learn how to use all those scales but I'd have to refresh my memory on some. I've never used any of them except the rafter tables. I stopped using the rafter tables years ago when I ditched the stanley professional squares and bought a dozen "homeowner" squares. The homeowner squares were superior because they had the 1/8th scale on all the sides of the square which I used extensively for dozens of measuring applications on a daily basis. The professional square had twelfth and tenth scales which drove me nutty because I had to mentally convert the numbers. Back in those days, I was a bit of a perfectionist focused on speed and I couldn't deal with the reality that I was wasting a ton of time doing the mental math to convert my numbers. When I switched to the homeowner square, I learned how to get all the "formulas" for the rafter scale using my square by holding it on the edge of a board. It's pretty simply stuff actually. They did make us layout stuff with the octogon scale. We had to know and use the board feet scales. The brace scale is fairly useless.
MSLiechty
Oh by the way making dormers in the roof wasn't difficult either.. I simply used a level, held the timber up level and drew a pencil line at the intersection..
Unfortunaltly, I went to college, even took calculus. You know, I find it easier to use a framing square or speed square, that to go to the formulas. I know both ways and the old way is a lot easier.>G<
I failed every math class i ever had but became a roof cutter on a framing crew by just using the square.
When i was in the union we could buy a little blue rafter cutting book and i always kept mine with me.
Now theres all sorts of books and a good general carpentry book will show how to step off the square.
If you do step it off dont forget to subtract one half of the ridge thickness but put 2 up before you do it or add a block.
I always cut 2 and we tried them on each end and in the middle.
If you want post the measurements and i can help you
I cant use the computer so well but can run it by you.
Learning to step off with the square is one of those fundamental skills that I believe should be learned early in the carpenters career. I believe the "speed square" short circuits that knowledge and ultimately the carpenter is slower as a result of the not proplery learning the fundamentals. I probably only stepped off one or two rafters a year after I started using a calculator but the fundamental way in which I used the square never changed and I used it all day every day for dozens of applications and it was used in many ways that the speed square would never work.
I learned how to do the stepping off, but it's been many years since I helped frame an addition or house and I just don't use it. I do make a lot of stairs though.
The little blue book is still kicking around the bottom of my truck somewhere.
I really did enjoy framing & learning about it, but didn't work with enough real framers that knew their stuff to learn any proper way. I picked up what I could.
As a trim carp & installer I will lay my golden hammer at the feet of a good framer who knows & practices plumb, square , level & especially straight line.
eye iz anaineer, dew trig errydae,
butt wem eyd bild ah shed, it awl bye aye! Neber knead ta uze tirg fore dat!
Plane geometry is the mathematics that explain why the lines on that drawing are the length they are; trig is simply a way to figure out the same thing without going to the trouble of doing the drawing first and then measuring it. I came into building after having spent 15 years as a graphic designer and 8 years in the Merchant Marine, so graphic solutions come naturally to me.
For example: In navigation, there is something called the current triangle which enables you to solve the problem of drift. Ships don't run on tracks; they kinda slide through the water in several directions at once, so even if you point your bow due north and call for 14 knots, after 2 hours you might very well be 30 nautical miles NNW of where you started. Which can be embarrassing if there happens to be a buncha rocks over there....
The current triangle can be solved with trig--I once programmed my old HP calculator to do it (237 lines of code!), just to bug a captain who used to tease me unmercifully about "book larnin'." But the graphic solution is what's used every day, and it's done with vector addition. Sounds complicated but just think of a vector as an arrow that is defined by its length and direction.
Simply stated, you plot the speed through the water (from the patent log, a sort of water-born speedometer) and the ship's head (from the compass) as one vector, and the speed and direction of the current (from current tables or charts) as the other vector. The ship's vector is plotted from where you are/were/think you should be; the current vector is plotted from the arrowhead end of the ship's vector.
To solve the triangle, you simply 'add' them together by plotting a third vector--the resulting SOG and heading--from the ship's vector starting point to the current vector's end point. Simple as falling off a log and a lot less painful.
Like I said, you can figure it out with trig from the raw numbers...but unless you've got a programmable calculator, drawing the actual triangle on the chart is faster.
Cutting a roof is kinda like that. You can do it with trig...but if your blueprints are accurate (and with ACAD, you can have precision out to four decimal places just for the asking) you can just pick the numbers off the drawings, set the angles on the CMS, and cut.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
I love this stuff, and I wish I had the interest back in my salad days that I do now instead of daydreaming and watching the beautiful curve of Janet S's neck and shoulders.I uh...don't do that anymore soooo...it wouldn't be a problem (clearing throat)...
but if your blueprints are accurate (and with ACAD, you can have precision out to four decimal places just for the asking) you can just pick the numbers off the drawings, set the angles on the CMS, and cut.
Your drawings give you rafter lengths?
Joe Carola
Your drawings give you rafter lengths?
Mine do, Joe, altho I can't speak anyone else's. Ya gotta remember I design what I build, so I do my own drawings. I like to do a detail of the gable elevation showing every piece in the roof, to scale. Saves me lookin' like an idjit in front of the hired help when it comes time to start cutting....
When I was still doing drawings by hand, I used to get out the slide rule and figure the angles and lengths and mark my copy of the drawings with all the necessary dimensions (never the customer's copy!). But since Imerc donated his outdated ACAD software to me a few years back, I just draw it the way I want it to look, and then tell the program to measure it for me.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Dinosaur
Using your math skills how much time do you save over an actaul layout?
I mean you have to still measure and cut things which means handleing the wood etc..
I found laying the timbers out on the ground, setting what I wanted up pyhsically and then marking it was pretty fast. Once I had a master I simply dupicated it.
Now granted these were giant 6x9 19 foot long white oak timbers and they aren't as light as a 2x4 would be But since I would still have to handle each timber to cut it, birds mouth it etc.. the physical work didn't increase.
Granted the layout took me a few minutes to do and check to ensure everything was perfect but not that long actually because I could eyeball everything to ensure it was correct.
The only added labor I see was the labor of setting the master on each timber. That however was offset by the time saved by not having to measure anything twice. (measure twice-cut once) Simple tracing prevented errors such as marking the wrong side or mis-measuring a length etc..
Using your math skills how much time do you save over an actaul layout?
I don't know if I actually save any time overall with my math 'skills' as you call 'em...and BTW that's praising them more than they merit. I still remember proudly showing my father--an engineer--a convoluted series of geometric contortions I'd concocted to solve for chord length on something or other...and having him ask me bluntly why I didn't just use the Law of Sines. (Answer: Because up until then--I was about 16--I was blissfully ignorant of its existance. But I couldn't tell Dad that--he'd a disowned me and then horsewhipped my HS math teacher. So I went to the used bookstore and bought an ICS Trigonometry workbook for $2 and started to learn it on my own.)
What I do save by having it all worked out in advance is time on site. When there's a whole crew getting paid by the hour, I don't want to be paying them to watch me scratching my head trying to remember if I forgot to add or subtract half the ridge-board from the hypotenuse of the framing square....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Your taking the challenge out of the situation. What fun is that?
Dinosaur
Gee when I had people helping me I had them do the lifting while I did the aligning and layout. Took me maybe 5 minutes. I cut a master and used that to trace all the rest. I know I could set a master on top and trace it faster than I could take measurements and mark angles.
I watch some guys build cabinets by carefully measuring and remeasuring stuff while the really fast guys use story sticks. That's what I used to pattern my building after.. OK so a tape measure is lighter than a timber but at least if I toss a timber on top I won't be reading the wrong side of the mark or confuse 9/16ths with 7/16ths. Ever try to read a tape upside down when you're rushing and mismeasure something?
I can honestly say that I didn't waste a single timber because I measured something wrong or forgot to add back in the offset etc..
View Image
Frenchy - I've seen pics of your house, and it's awesome. But a DIY homeowner building one house one time isn't the same as a professional carpentry foreman who has to lead a crew doing numerous roofs, numerous styles, day after day. A cut-up roof (meaning mulitiple planes, hips, cripple hips, etc.) is better to figure in the quiet of the home or office - making a pattern to re-use on the jobsite only works on common rafters.
View Image “Good work costs much more than poor imitation or factory product” – Charles GreeneCaliforniaRemodelingContractor.com
Using your math skills how much time do you save over an actaul layout?
Takes about 5 seconds on CM to get a rafter length.
I mean you have to still measure and cut things which means handleing the wood etc..
Yes, no matter what way you calculate a rafter it still has to be picked up and set on the bench. What's your point?
I found laying the timbers out on the ground, setting what I wanted up pyhsically and then marking it was pretty fast. Once I had a master I simply dupicated it. Now granted these were giant 6x9 19 foot long white oak timbers
Are you for real Frenchie? What does that prove? It proves that you don't know how to layout a rafter with a framing square or calculator. Why would you do that, especially with 6x9x19' timbers?
Joe Carola
Framer
first it's frenchy with a Y, IE is the femine form of a name. Y is the masculine form. <grin>
Second. I know how to measure things and I also know that every time you measure something you should measure it twice to make sure you are correct.. Once cut the board stretcher is always out of reach.. <more grins>
That is something you never need to worry about when tracing something..
Try it yourself. See how long it takes you to measure off 225 & 7/16ths inches and make a 54.4 degree cut plus the required fish mouth. Now set a timber on top and compare the time you spent doing each task.. (just for information a 55 degree cut becomes noticable when abutted to another 55 degree cut at the peak, it looks just plain sloppy!)
Finally when working with big timbers such as this a person tends to set tools and tape down and use both hands. Inevitably when you're done moving the timber around you next have to look around for where you set the tools down and go over and get them. Now perhaps you don't need to do that with those little sticks you like to work with. I mean I'm sure you handle a 225 inch long board with one hand and your tape and square with the other. <big grin>
Second. I know how to measure things and I also know that every time you measure something you should measure it twice to make sure you are correct.. Once cut the board stretcher is always out of reach.. <more grins>
Measure what? I'm talking about laying out a rafter from a framing square table or a calculatyor. Your talking about getting a rafter length by lift up timbers in the air to get a rafter length.
That is something you never need to worry about when tracing something..
Tracing what? Y ou have a new house to build, there's no tracing anything to get a rafter length. You get the rafter length by using a framing square or a calculator and then make a pattern. Take that pattern and use that to scribe the rest of the rafters.
I found laying the timbers out on the ground, setting what I wanted up pyhsically and then marking it was pretty fast.
How do you figure your rafter lengths? You said you lift them up in the air and scribe or trace them, why would you do that and not figure out the rafter length from a framing square or calculator?
Your going by your one time experience cutting rafters on your house compared to me cutting rafters for 25 years starting out from using the tables on a framing square to a calculator?
Explain what you're talking about again and how you figure out a rafter length.
Try it yourself. See how long it takes you to measure off 225 & 7/16ths inches and make a 54.4 degree cut plus the required fish mouth. Now set a timber on top and compare the time you spent doing each task.. (just for information a 55 degree cut becomes noticable when abutted to another 55 degree cut at the peak, it looks just plain sloppy!)
Try what myself? I get my run and punch in the pitch and hit diagonal (5 seconds)mark my plumbcut and pull my length and mark the birdsmouth and overhang and cut.
What are you talking about? How do you figure a rafter length?Joe Carola
Framer
Apparently you need to spend a little more time reading. I never lifted my rafters into the air until they were assembled and ready to go up. If you've looked closely at my pictures you'll note just how tight all my joints and fish mouths were..
I'll repeat for you,
What I did was lay the timbers in my driveway to determine length and angles. once that was exactly determined I cut a master and traced it onto all the other timbers.. I then assembled the rafters with their scissors braces (figured out the exact same way)..
and flew them up into place..
they fit exactly.. you couldn't get a business card anyplace into the joints.. heck to be fair you couldn't get a popcorn fart into any of them.. Not bad for my first attempt.. from someone who can barely add 2+2 (it's four right?)
My point is that once I figured that out. I know I could slide a timber on top of another timber and mark everything correctly faster than you could measure and mark everything with a tape measure..
Plus the 54.4 degrees is critical.. 60 would leave an ugly gap and even 55 degrees is very noticeable..
IT's 54.4 degrees. Now try to lay that out accurately with your framing square..
Joe when i learned there were no calculators so i had to learn my steel square, I rooted through bookstores and found old books on the square. I even had dividers to check stair measurements. I talked to some framers that use a calculator and was very impressed with there abilities. But i have never seen anyone like my dad and a few old carpenters that could lay out hip roofs, octagon, hexagon roofs from the ground and send up all the rafters that fit perfect the first time. Im sure there are some but now with the use of trusses theres even less roof cutters. Its been years since i worked for anyone but i got many jobs no one else got because i could cut roofs and i dont think i was as good as i could have been. I should buy a calculator and study it but its all trusses now at least here in OR. I was always amazed at how much there is to learn on the square and for awhile i had a lot of it down. 2 years ago i helped my boy frame and they all laughed at me for always useing my sq i guess the little speed sq is the mark of a hot carpenter nowadays, I dont even own one;]
Speed sq. is useful for wiping the sawdust off the floor when snapping lines. Gee's bobbys keep up with the times would ya. ;-)
They can't get your Goat if you don't tell them where it is hidden.
"Speed sq. is useful for wiping the sawdust off the floor when snapping lines."That's already been replaced with a blower tool and air compressor.
bobbys
? Is that hard?
I laid mine out in the driveway drew a few lines checked a few measurements and went to work making timbers..
everything fit perfectly when I raised them into place..
I guess not knowing anything is better than knowing a little?
"I laid mine out in the driveway drew a few lines checked a few measurements and went to work making timbers.. everything fit perfectly when I raised them into place.. "That's a very simple concept that has been proven over the test of time. I've framed and cut a lot of roofs using the actual floor as the "template". That kind of foresight is often overlooked in the rush to frame walls...which get in the way of that style pattern making.
Joe when i learned there were no calculators so i had to learn my steel square, I rooted through bookstores and found old books on the square.
That's exactly how I was taught using the tables on the framing square. Best way to teach a young kid at first to figure out what the triangles are and mean. I was taught to step off just to show me what it meant. After that my boss showed me how to use the numbers on the tables and how to multiply to get the rafter lengths. Steeping off is a waste of time.
If someone were to step off, they should at least go twice the amount of pitch to step off. In other words, doing this only up to an 8/12 because your square is 16". If you have a 5/12 pitch roof, instead of holding the square at 5 and 12 and then step off 12 times for a 12' run for example, hold the square at 10/24 and step off half the steps at 6 times instead. less steps and less room for era.
Joe Carola
Who's Trig?
I love my little Sharp Scientific calculator from Office Depot for seven bucks. It has trig functions. Carpenters do not need to know much trig but a little bit really helps. Here are some handy tips. Rafters and hips are easy if you know the pythagorean theorum. Angles are easy to calculate dividing the rise by the run and then taking the inverse tangent. For example if your stairs rise 7 inches and run 11 inches then the angle is 32.47 degrees. The diagonal is 13.03 inches using the Pythagorean theorum, the square root of (7 squared plus 11 squared). Simple.
Diagonals of squares are proportional to the square root of 2. For example if the square has 12 inch sides, the diagonal is 12 times the square root of two or 12 x 1.414 = 16.97". If you are proficient with a few simple calculations then there is no head scratching. It only takes seconds to come up with an accurate number.
The step method with the framing square is too inaccurate for my work. Every time you step you could be off a sixteenth. After ten steps you might be off an inch. No good.
My pappy used to say that being a carpenter was a thinking mans job. If you can't think or don't care then you might be better off as a ditch digger.
A good roof framer should know basic trig. I hate working on someone elses' roof on a remodel when the pitch is a 5.20" and 12 or a 11.78" and 12. It just makes things more work than it should be. I know it was framed by some amateur hack.
working on someone elses' roof on a remodel when the pitch is a 5.20" and 12 or a 11.78" and 12. ... I know it was framed by some amateur hack.
Mike, I don't think that's fair. I know many framers who don't know a sine from a sign but who can frame the most egregiously cut-up roofs with nothing more than a square, a tape, an a blunt pencil...and can do it faster and more efficiently than I could on my best day.
Sure--a pencil line has thickness and stepping things off can result in accumulated error. But where is it written that if a roof pitch isn't in round numbers, it's gonna fall down or leak or look awful? And what standard of precision do you want to hold framers to?
The difference between .75 and .78 is only three hundredths of an inch, or a bit under 1/32". Come on, man. If you can really measure that with your square and level while hanging off a ladder on the rake, I'll buy you a large double-double at Timmy's. A 16-foot rafter will grow more than that between 7am and lunchtime on a sunny fall day.
Frenchy likes to do his layout full-size; because that works for him. I like to do mine on paper at 1:48 scale because that works for me. Do either of us frame a roof to within a 32nd of an inch?
I don't think so. Do you?
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Dinosaur
Well said.. My pitch worked out to be 17/12 and 27/12 because that's the length of timbers I had to work with.
I get frequent compliments and I will put the quality of my workmanship up against proffesionals any day.. I have to because my timbers are exposed for anyone to see unlike typical building which gets to hide behind sheetrock..
Even an average carp can use the square and simply multiply the scale numbers by the know run of the rafter in number of feet and then simply step off the inch increments if it is a foot and inches run.
No error with repeated step offs. That is the method I was first taught and I am no math whiz.
They can't get your Goat if you don't tell them where it is hidden.
"Do either of us frame a roof to within a 32nd of an inch? "That was my tolerance level that I aimed for but realistically, in most situation, it's not necessary and if I actually achieved it, it was probably an accident and something was screwed up somewhere else to keep me balanced. I'm beginning to feel like a complete failure because I never learned how to figure my cuts in degrees. Instead I accepted the age old method of just using the rise/run ratios and marking the lines as needed. I'm not denigrating the use of trig but I reject the notion that a skilled layout man can't get an accurate framing member laid out using the framing square and step off method. It all boils down to the same skillset that differentiates the average carpenter and the above average guy. A sharp pencil and good eyes and a detail oriented layout man can achieve a very high level of accuracy. With that said, I will also say that using the calculator is significantly faster especially when accuracy is factored in in an apples to apples comparison. Unlike Mike though, I refused to pay for expensive $7 calculators and stuck to the $4.99 models.
I'm beginning to feel like a complete failure because I never learned how to figure my cuts in degrees.
And here I've been feeling the same way for years cause I always get a migraine when I have to use a framing square. My secret shame, exposed for all to see....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....