trouble calc rafter length with ridge beam set....

[GR40RCapri]

Hi Joe (and all..),

I want to start off with saying I thouroughly enjoy this site and the knowledge. I'm having some serious issues figuring out correct rafter length...I know how to do the math, and the answer is coming out the same no matter which way I figure it (Pythagorean Theorem, rafter tables, etc...) but it doesn't equal what I measure with the tape, and the 1st rafter i cut, the top is about 3" above the top of the ridge beam...( I set the beam 1st, as i'm working alone....)

I had re-cut the rafter to fit properly, but my concern is this:

this building is kinda practice before I plan/build our 30x50 2.5 story addition....so my dilemma is, I know i'm supposed to be able to build the rafters on the ground and go up...i want to understand where I'm making my mistake before I'm 25 ft in the air...

some back ground...

building (shed) is 12x16. actually, it's 148" (I didn't cut the joists, and the end plates made up a bit of extra.)

run is 74" from outside of wall to center of building
height of top of ridge is 74" above plate ( I figured a 12/12 pitch) and easy to calc PYT. Theorem
ridge is 1.5" thick...
rafters are 2x6's

I figure this... (Pythagorean Theorem) 74squared +74 squared = C squared (or rafter length)

5476 +5476 = 10952

sq rt of 10952 = 104.65 or basically 104 5/8's

But when I measure from the outside corner of the top plate to the top of the rafter, it's just about 101....this is the same on both sides, of the rafter, and I checked 4 points of the building...everything is plumb and square...

I'm at such a loss, and sick of pulling my hair out...

I found this calculator, and it seems to say i needed to set my ridge @ 80+ imches to make a 12 pitch with a run of 74" http://windyhilllogworks.com/Calcs/ridgehgtclc.htm

to enter the #'s I did to find this they would be in this respective order:

74 12 1.5 5.5 74 5.5

Am I measuring something wrong? the rafter looks good, just too long...i cut it down to about 101, and the top of the plumb cut is @ the top of the rafter. but when I go up on the roof, and attach both rafters to the ridge, with equal amounts of each rafter above the ridge ( maybe 1/4") and put the square on both, it's not totally square ( which would signify a 12/12 pitch...)

thanks for the help/discussion..

Jay

[Joseph Fusco]

Hi Jay, and welcome to the forum.

First I'd like to say thanks for the kind works about the forum, it is a good place without much b/s Second get yourself a Construction Master! and make your life a bit easier LOL. I was just posting a note in the tools section when I saw your post, I'm running out and I'll be back in just a bit to go over your roof and to see if we can get it straightened out for you.

Thanks,
Joe.

[Joseph Fusco]

Ok Jay,

There are a few things that need to be dealt with whenever you are framing a roof whether it is a fixed ridge height or a conventionally framed roof. One big one is to remember to SUBTRACT the H.A.P. from the rise measurement to get the total rise to use in your calculation. From the looks of your calculations you didn't make the adjustment for the H.A.P. Now since I don't know what you used on your rafters it's going to be hard to tell what the correct length should be. But, if you are using a 2x6 rafter with a 12 pitch and using the 2/3rd's rule, it should be about 5-3/16". The next thing is to SUBTRACT 1/2 the thickness of the ridge from the RUN, which it looks like you didn't do either. Now you could have just as easily removed that thickness from the rafter after you cut it by making the adjustment and I don't know if you did that either.

Now if you recalculate with those two corrections in mind you'd get:

Total Rise = 74" - 5-3/16" or 68-13/16"
Total Run = 74" - 3/4" or 73-1/4"

Making your length 68.8125² + 73.25² or 10100.723
Giving a rafter length of: sqrt(10100.723) = 100.50 or 100-1/2"

Since you already have your ridge in place the last thing you'd need to do is recalculate the pitch angle or the Inch Rise.

You get that by: (total rise*(12)) / (total run) or (825.75)/73.25 giving a 11.25" in 12" pitch.

Roof Framing Basics
Calculating the H.A.P.
Fixed Ridge Height

Hope this helps

[GR40RCapri]

Wow....

thanks Joe... quite the in-depth answer...thank you!

I DID subtract 1/2 the width of the ridge beam...

but, I did not know, realize, or find anywhere in my research, that i needed, or should have subtracted the HAP....

I did use a 2x6, and I calc'd the 2/3rds rule, as you've suggested

what i've came up with by trial and error was just about 100.5" long... so it's together, roof is sheathed and shingled. I'm now finishing siding and doors.

But I still want to learn/study the right way.... I enjoy this stuff....it's not like work to me...I should have went to trade school and became a carpenter.

Thanks!

Jay

[Joseph Fusco]

Anytime Jay,

Glad it was helpful. If you go to my site www.josephfusco.org you will find a good deal of info on roof framing that should help you even further. If you have any other questions feel free to ask

[ace]

Jay,

Joe has answered your question good.

My only input would be that to achieve a true 12/12 pitch or 45 degree pitch then you need to raise the ridge by the HAP like the site you linked to suggested.

Bit late now though

Dave

[Joseph Fusco]

HI Dave,

It funny you should say that because I came here just now to say the same thing

[ace]

So to set the height of the ridge exactly it would be -

Total rise (74") plus HAP (5 3/16") minus 3/4" which is the rise of the rafter above the ridge.........unless the top of the ridge was bevelled..........

Is that right Joe?

Dave

[Joseph Fusco]

Dave,

That's absolutely correct. I like the fact that you mentioned the adjust at the ridge as well, most folks forget about that one. Nice job.

[pyrolater]

Joe,

Can you re explain the where you subtract the hap from?

I'm still a bit confused on that part.