Help! I hope to learn if a formula exists to convert roof pitch to degrees…….for example I see that a 12:12 is basically a 45 deg. cut, whereas a 5:12 is 22.5 deg.
The project I’m looking at involves 3.6875:12, 7.375:12, and 1.6875:12.
I’m wondering if the original framers just picked degrees, like 17.5, 33, 8.
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Replies
Apparently you slept through high school math. The formula you want is from trigoniometry, and it's the tangent formula, where you divide the rise by the run, you get a decimal number, then lookm in your CRC tables for the corresponding angle. (I learned that the second time through.)
How did you get a number like 3.6875?
I'm sorry, I thought you wanted it done the right way.
Yes I did sleep through trig class, but I was awake when they taught me to spell trigonometry. HaHa. Sorry, didn't mean to look a gift horse in the mouth.
I got my #'s by holding a 12" level on the underside of the rafters and measuring the rise. This usually results in an even #, such as 5.0", 6.0", 12.0"............ I measured in many different places to rule out the chance of crown or swag and still came up w/ those screwy #'s.
Yes, there is a formula. Inverse tangent will work. Draw a right (one with a 90 corner) triangle and pick an inside corner you wish to calculate the angle. Take the length of the oposide side and divide by the length of the adjacent side then hit the inverse tangent key on your calculator.
In your first example: 12/12 = 1, inverse tangent = 45 (degrees)
In your second example: 5/12 = .417, inverse tangent = 22.6 (degrees)
bushwacker.. email me your fax number and i'll fax you my graphic pitch display...
converts pitch to degrees..
anyone else...same deal
as for figuring rafters and trussses.. we have to match existing about half the time
if we can do it we usually measure both gable ends and average it
once i have the total rise... and the total run. it just becomesa trig exercise.. with modifications for heel and ridge
if you are not into trig , get a ConstructionMaster IV... pretty foolproof
Mike Smith Rhode Island : Design / Build / Repair / Restore
Edited 3/24/2005 7:47 am ET by Mike Smith
How are you going to fax a speed square?
why don't you find outMike Smith Rhode Island : Design / Build / Repair / Restore
Tharrett has it right. The pitch divided by 12, then the "=" key. Then hit "inverse" and "tangent". (Your calculator have have "ARC" instead of"inverse")
If at first you don't succeed, skydiving is not for you.
I got my #'s by holding a 12" level on the underside of the rafters and measuring the rise. This usually results in an even #, such as 5.0", 6.0", 12.0"............ I measured in many different places to rule out the chance of crown or swag and still came up w/ those screwy #'s.
Let me tell you a little story......
I was called on to fill in for another foreman during his vacation about 10 years ago. The job was a high pressure roof replacement on a restaurant that had a severe fire, but had to open for a big event. Without a single delay, we only had enough days to complete the job. Before he left, he used his vast experience over 20 years to measure the roof trusses which we had pulled strings to get made in one week. He had another veteran help him measure the span of the roof. They then worked together to measure the slope of the existing remaining roof by:
holding a 2' level and a framing square on the sides of the 100 year old rafters.
He called in the span, the overhang tail lengths, and the slope at 4.25/12.
He then left for vacation.
The day the trusses arrived, we had 10 people there for the erection and the subsequent tie-in. The crane set up and we boomed up our first truss. Imagine my surprise when the guys yelled down to me and asked:
"Why is the ridge on the truss six inches lower than the ridge on the existing roof ?"
Why ? Because you cannot accurately measure the slope of a rafter by using a level (especially a 12 incher) and a framing square. Don't do it. You measure the difference in elevation between the rafters' bearing points and the point at the ridge. Take the heel cut into consideration. Then, do your math.
And save yourself from taking a fellow employee out and shooting him !!carpenter in transition
Ayep, and, if you are going to use a level, you have to hold it parallel with the trusses, not level, because the trusses may not be level, but they always form the run of the equation..SamT
Sam
I think what I was saying was:
Don't ever use a level and a square if you want a really accurate measurement. You must measure the OVERALL rise and the OVERALL span or run to get the correct slope numbers. Not something measured with a two foot level.
You are correct, the wall plates could be out of level, further complicating the problem of using a level.
carpenter in transition
Ayep
I bought Larry haun's book Bulid A House, and I think it's pretty good ... seems to cover all the basic stuff well.
But I think there's an error in his explanation of roof slope. There's a diagram on page 114 that shows how he measures the slope, and it looks to me like the starting point on the left side of the diagram is different for the rise & run.
I'm sorry, I thought you wanted it done the right way.
I thought the run was measured to the center of the ridge.
Run = 1/2 Span
carpenter in transition
A guy measuring that way for trusses tells me he's just lazy and doesn't give a crap. Matching trusses up to existing framing is always difficult. Ridges can be off center, rafters bowed, and any number of other problems. I always measure BOTH ends, check the span, and check the overall height in at least a couple of places. And they still aren't always perfect.But no way would I check the pitch with a smart level or anything like that, unless it was just for a quote. That's just poor service, IMHO.
Give a man a fish; you have fed him for today.
Teach a man to fish; and you have fed him for a lifetime.
Teach a man to sell fish and he eats steak.
unfortunately, these were our own employees.
maybe the guy had Disney on his brain.......
carpenter in transition
O.K. - Sorry - I thought you meant a truss salesman had measured the things that way. I can kinda see how someone without a lot of experience with trusses could make that mistake. But once you've been burned a few times you learn to be careful. Bot that it's ever happened to me, of course.
(-:
Paying alimony is like feeding hay to a dead horse.
An easy way to get the pitch is to use a framing square. Place the tongue on the barge and pass the blade over the edge until it is level. You know the tongue number is 14.5" for the rise and read the blade number where it intersects the edge of the rafter for the run. Do this in several places and get an average.
It's another snow day so here I am in the middle of the day twiddling and squandering my time at Breaktime.Mike Callahan, Lake Tahoe, Ca.
Tim, I agree. Measure the total rise, total run and subtract the heel. Then divide the numbers into any units that you want. So, instead of dealing with angles, your dealing with ratios.
I've got a better story...that's too long to tell. Let's just say an old codger pulled a meausurement off of a round storage tank using a framing square. Don't bother to aks how...it cant be done. Anyways after projecting a line for 600', the company proceeded to spend a week pouring foundations. At the end of the week on Friday, the supertintendent noticed the major mistake and ordered the $10,000 worth of foundation work to be yanked out.
It took the excavators an hour to dig out the concrete and we started all over.
blueJust because you can, doesn't mean you should!
Warning! Be cautious when taking any framing advice from me. There are some in here who think I'm a hackmeister...they might be right! Of course, they might be wrong too!
It is e = mc squared.
e is your roof run
m is your roof rise
c is the number of times you cut class in high school trig
Must have darn good eyes - not to mention an incredibly accurate level and ruler - to read those measurements down to the ten-thousandths of an inch! ;)
Tan 8 degrees = 0.14 = 1.687/12
Tan 17.5 degrees = 0.315 = 3.784/12
Tan 33 degrees = 0.649 = 7.793/12
going the other way....
Tan^-1(1.6875/12) = 8.005 degrees
Tan^-1(3.6875/12) = 17.082 degrees
Tan^-1(7.375/12) = 31.574 degrees
Jon
thanks everyone, It's a #$%&!! contemporary with a rotten breezeway (8 degree shed roof) connecting a 17.082 degree garage roof to a 31.574 degree home. To further the fun, the two structures are not parallel, and the catwalk below the shed roof isn't perpendicular to either. Also, the garage is about 10" above the home (elevation.)
17.082 degree garage roof to a 31.574 degree home
I cheated a bit, used AutoCAD, the garage is 3 11/16 in 12--so it was probably "supposed" to be 4/12. The house roof is 7 3/8 in 12--guessing that was "meant" to be 7/12 and "proud" (rather than 5/8 in 12 "out" of pitch).
Of course, it all depends on where you measure, too. Underside of the rafters some times not the same as the average across the shingles--and neither ever matches the rakes on the gables after enough years.Occupational hazard of my occupation not being around (sorry Bubba)
Don't know how you came up with those numbers, but they're all wrong.
Boss, I too agree with you. Where in the &&^***&^% did jazzdogg get these numbers.? they are WRONG!!
I think your chart has all of the degrees values doubled.
"I think your chart has all of the degrees values doubled."
They aren't doubled - They're just wrong.
They say television is making people dumber.
What do they mean by that?
You definitely can't get the angle by simply multiplying the rise by 7.5, which is what jazzdogg appears to have done.
"Listen, strange women lying in ponds distributing swords is no basis for a system of government."Jon
funny tag line...snork
Spheramid Enterprises Architectural Woodworks
"Sell your cleverness, Purchase Bewilderment"...Rumi
OKAY.. .the easy way (and I mean EASY) is spend the $169.00 and buy a smart level, it reads degrees, deviation in 12", and level. The new ones even have a tone when you are plumb or level. I too slept thru class, but bought the level. No more guessing at the bubble, and everybody sees it different. Just checked a roof last Friday for an addition, put the level on the roof, pressed the hold button, took the level down and read the 4 1/2 12 pitch. It is one of those tools you will ask yourself why you went so long without it.
I don't reallty trust smart levels. They're fine if everything is dead straight. But put on rafters that are crowned or bowed and you can be off a fair amount depending on where you set the level. Unfortunately, I know this from experience...
Why is it that no matter what color bubble bath you use the bubbles are always white?
So what system do you use that takes into account the deflection of the rafters?
I don't try to account for deflection of old rafters directly. If there's a serious bow down, I talk to the framer about it and figure out what they want to do. If they're re-roofing the old part they may want to shim up the old rafters or sister them. Or we can drop the overall height of the new trusses some and they can feather the difference out over several feet. Sometimes it's a combination of a couple of things. There's no one thing that will work in every situation. But I absolutely NEVER would just check one place in the roof and order trusses based on that. I've seen it go wrong too many times.
My karma ran over my dogma.
I agree that you should never order trusses based on one reading. I will take as many as necessary until 75% are the same reading. My question is (based on the comment that you would not use a smart level to read a roof pitch due to rafter bowing and sagging) how do you calculate the roof pitch so that it takes into account the aforementioned problems.
Typically I get the span first. Then check the heel heights on BOTH ends. Then check the overall height at the center. And make sure the peak is in the center. If the roof is badly bowed, we can cut down the heel height(s) a hair, and drop the peak a half inch or so. It's really up to the framer.Then figuring out the roof pitch is the easy part. It's just math.
If you upset your wife she nags you. If you upset her even more, you get the silent treatment.
Don't you think it's worth the extra effort?
Take the rise and divide it by the run and take the inverse tangent of that. That will give you degrees. Most scientific calculators have a inverse tangent button. Sometimes it is labled 1 over tangent or tangent to the -1.