Roof pitch expressed in degrees?

[RRG]

Looking at Velux skylights and their installation instructions give roof pitches stated in degrees instead of 3/12, 6/12, etc.
Can someone tell me what pitch an 85degree roof would be?

[grumpydasmurf]

An 85 degree roof would be a very steep roof. Let's see... 22/12

Damned near straight up like a verticle wall.

[bkrahmer]

If my math is right (I think it is), 85 degrees would be 137/12. I don't think anybody's ever built an 85 degree roof... Double-check those numbers.

[RRG]

Here is the fist page of the pdf instructions.
You can see the 15-85 degrees.

[Rich]

That's essentially saying it is acceptable to install on a near vertical slope. 15-85 would be approx... 3/12 and , as bkrahmer said, 137/12.
Something that steep is most likely only for radical greenhouses or something. I've seen a couple office buildings that are that steep but not many.

[grumpydasmurf]

If my math is right (I think it is), 85 degrees would be 137/12. I don't think anybody's ever built an 85 degree roof... Double-check those numbers.

90 degrees is a wall and a wall is 24/12 therefore a 85 degree must be 23 or 22/12. I did the math rather basicaly... 12/12 is 45 degrees 10/12 is 40 degrees. Therefore a 85 degree is 22/12. I'm probably wrong but I think I am closer than 137.

If a velux skylight is installed on less than 3/12 (15 degree) you need to install one of their pre-fabricated skylight curbs or their warranty is void.

[grumpydasmurf]

This thread has really peaked my insterest now.

[bkrahmer]

90 degrees is a wall, but cannot be defined in the terms of rise over run because run is 0. Anything divided by 0 is mathematically 'undefined'. 60 degrees, the lower pitch on my gambrel, is roughly 20/12 or 21/12. 137/12 is correct for 85 degrees. To find the rise over run for any degree value, you multiply 12 * tan(x) where x is the degrees.

[Tom R]

Amazing but true!!

[Rich]

yep.. I did the cad just to make sure.. and it's just a tad over 137/12

[Joe Bartok]

90 degrees is a wall, but cannot be defined in the terms of rise over run because run is 0. Anything divided by 0 is mathematically 'undefined'. 60 degrees, the lower pitch on my gambrel, is roughly 20/12 or 21/12. 137/12 is correct for 85 degrees. To find the rise over run for any degree value, you multiply 12 * tan(x) where x is the degrees.
Right on! To find the rise of an angle, you MUST use the formula RUN*tanANGLE
And we just went through something similar to this on another thread a while ago. You can not treat your rise/run the same as you would the angle. Multiplying or dividing the angle does NOT mean the tangent or rise/run is doubled or halved (or any other proportion, for that matter).
The attached ANGLE_BISECTOR.pdf shows why. Halving the angle means the same as halving the arc of the circle. But note where the line of the bisector meets the line of the tangent (= rise/run). This situation gets worse as the angle increases. And as bkrahmer notes, if we take our reasoning to the limit and the angle is 90 degrees, it's tangent is running off to infinity ... but: the half-angle is restricted to a value of 45 degrees.
Related to this topic, in case anyone's interested, I've posted a couple more pdf's regarding working with pitches directly on a framing square. The most common scenarios are half-splitting an angle, adding pitches and subtracting pitches. I've tried to keep this "Math-Lite" and expressed the formulas directly in terms of pitch.

[Joe Bartok]

Here's the Half-Angle diagram, couldn't squeeze it in on the last post.

[grumpydasmurf]

You guys are too smart for me. Half of what you said after my last post just went in one eye and out the other.

I still think I am right.

[Rich]

I still think I am right.

Nope..
If you can't read the numbers 137.1606, 12.0000, and 85degrees.

[dmcroof]

Here's an illustration pitch/degrees.

http://www.roofersreview.com/gallery2/main.php/v/dennis/Roof_pitch_degree.jpg.html?g2_imageViewsIndex=1&g2_navId=x6c8249f6

hope it helps

[Tom R]

That's a big help for everyone, - - thanks, dmcroof, - - and welcome to the forum.

[Rich]

Right on dmcroof - welcome to the forum.

[grumpydasmurf]

Ahhh it's all just an elaborate conspiracy against me. ;)

[Rich]

LOL

[Dragon]

Hey I've seen walls at 24/12.

Granted it was on an A frame.

;)

[dmcroof]

Glad I could help.
Gotta keep Grumpy in check.
:wink:

[R Birch]

I’m fairly new to this forum but it has struck me odd that discussions about Roofing Materials / Techniques and Roof Framing are lumped into one. I mean no disrespect to Grumpy by this, but it seems that he is more of a Roofing Carpenter than a Framing Carpenter. Shouldn’t these different Trades be on separate pages? :?

Any other folks feel like I do on this?

[grumpydasmurf]

I’m fairly new to this forum but it has struck me odd that discussions about Roofing Materials / Techniques and Roof Framing are lumped into one. I mean no disrespect to Grumpy by this, but it seems that he is more of a Roofing Carpenter than a Framing Carpenter. Shouldn’t these different Trades be on separate pages? :?

Any other folks feel like I do on this?

Grumpy isn't a carpenter at all... and I agree 100%. I had asked that question long ago but at that time this forum was NOWHERE near as popular as it is now.

Even in new construction, it's usually the framing carpenters that install the trusses/rafters and sheating. There have been a few times we have installed the sheathing in a new building but very very very seldom.

When it comes to remodeling weve rebuilt roof framing (usually due to rot or mold), but it doesn't take a genius to copy what was removed. IMO roofing and roof framing have nothing to do with each other.

[Rich]

I’m fairly new to this forum but it has struck me odd that discussions about Roofing Materials / Techniques and Roof Framing are lumped into one. I mean no disrespect to Grumpy by this, but it seems that he is more of a Roofing Carpenter than a Framing Carpenter. Shouldn’t these different Trades be on separate pages? :?

Any other folks feel like I do on this?

Good point Richard - it's kind of a relic if you will from when I first started the forum and not knowing where it was going to go or if anyone would frequent it. So when I started creating multiple personalities to talk to myself I figured it was easier if they were only one forum instead of 2.. less clicking :)

[Rich]

Ok - well here's a start on seperating Roof Framing and Roofing.. I'll get more seperated as I find time.

[R Birch]

Well dang, that was easy. My next suggestion is;

“How‘bout a round of applause for Rich!”

Clap, clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap fart (oops) clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap clap………………….. :lol:

Cheers! :D 8)

[Rich]

LOL

[Don_P]

I stumbled in here while researching a beam problem...I swear Joe I didn't even have to type in your name, just your math :lol:
I see a couple more folks I recognize and have enjoyed talking to before. This looks like a good group, hope you all don't mind if I sit down a spell.
I have made a javascript calculator that I really meant to work the other way around but does give 137/12 pitch @ 85 degrees slope.
The calc is posted here;
http://www.ls.net/~windyhill/Calcs/hipbackangle3.htm

(Its the top one on that page, type in 137 as the unit rise, hit solve and it gives 84.99 degrees)

[Rich]

Welcome to the forum Don_P. Stay as long as you like :)

[Tom R]

Yeah, Don, - - welcome, - - besides, - - we need more guys that can 'comprehend' Joe B's math-level! :D

[Anonymous]

Hi Don:
If anyone can comprehend my math, you can!
I enjoyed our last discussion, and I'm looking forward to more in future.

[Anonymous]

Here's a non-mathematical question. I was logged in on that last post. How come I've been demoted to the status of a mere "guest" ??????

[Don_P]

If anyone can comprehend my math, you can

You all misunderstood me, I stumbled in here trying to comprehend my first round of Joe's math :lol:

[Tri-State Buildings]

If my math is right (I think it is), 85 degrees would be 137/12. I don't think anybody's ever built an 85 degree roof... Double-check those numbers.

Bkrahmer,
It's very easy to determine the degree roof per pitch. Take the degree of pitch of roof and divide by 3.75. Ie: 45 degree angle is a 12/12 pitch, or in your case a 85 degree angle roof is a 22.67/12 pitch

Kevin Weaver
Tri-State Buildings

[Tri-State Buildings]

[QUOTE=Don_P;18999]I stumbled in here while researching a beam problem...I swear Joe I didn't even have to type in your name, just your math :lol:
I see a couple more folks I recognize and have enjoyed talking to before. This looks like a good group, hope you all don't mind if I sit down a spell.
I have made a javascript calculator that I really meant to work the other way around but does give 137/12 pitch @ 85 degrees slope.


Don,
I think you guys are making this harder than what it is. I replied to Bkrahmer, but in case you miss that post I am posting it to you as well.
It's very easy to determine the degree roof per pitch. Take the degree of pitch of roof and divide by 3.75. Ie: 45 degree angle is a 12/12 pitch, or in your case a 85 degree angle roof is a 22.67/12 pitch

Kevin Weaver
Tri-State Buildings

[Rich]

That works for a 12/12 pitch but not for much else. Try 33.69 / 3.75 = 8.984 when in actuality it is an 8/12 pitch. That's far enough off to cause problems for any framer.

A 137/12 pitch is 85 degrees. Pitch and degrees are not a direct relationship.

[Joe Carola]

Don,
I think you guys are making this harder than what it is. I replied to Bkrahmer, but in case you miss that post I am posting it to you as well.
It's very easy to determine the degree roof per pitch. Take the degree of pitch of roof and divide by 3.75. Ie: 45 degree angle is a 12/12 pitch, or in your case a 85 degree angle roof is a 22.67/12 pitch

Kevin Weaver
Tri-State Buildings

You just proved that what you posted is wrong. It only works for a 12/12 pitch.

85° roof is not a 22.67/12. It's a 137.1606/12.

22.67/12 is 62.10621°


10/12 Pitch Roof in degrees is 39.80557°

Your way.

39.80557 ÷ 3.75 = 10.61482 That is a 10-5/8/12 pitch. Doesn't work.

[R Birch]

Degrees are pieces of a circle; pitches are pieces of a triangle.

(Pie are round, cornbread are square. =)

[Don_P]

I'll take a 12/12 of pumpkin pi please.
No, better make that 90 degrees
Umm, make it 15 minutes.
Ahh, this is getting too hard, just give me the whole thing