adjusting ridge hieght, and common rafter

[speaker]

hello,
I was wondering how to ensure that my ridge board is going to be the right height, and that my common rafter will be the right length!
example: 12' span
6:12 pitch
run = 6'
rise = 3'
2 x 6 rafter (4 1/8 material left after seat cut - plum)
total rise 3' - 4 1/8" (top of ridge)

sq root of(common rafter length) = (run(sq)) + (rise(sq))
= (72(sq)) + (36(sq))
= 5184 + 1296
(sq root) of CLR = 6480
CRL = 80.498 (or) 80 1/2 " (from top (long point of 6:12 angle) to top side of rafter, then measure down 4 1/8" to get outer edge of seat cut
are these calculations correct

hope someone can help, thank you

[dhill]

Looks like you did all the calculations. Next thing to do is set the ridge, cut a rafter and see if it fits.

[Joe Carola]

Speaker,

Your close but in order to get the exact ridge height you have to deduct for half the thickness of the ridge. If your using a 2x ridge you would take half that tickness which is 3/4" and deduct that from your run.

6' - 3/4" = 5' 11-1/4" = Run

6/12 Pitch the rise would be 2' 11-5/8"

4-1/8" H.A.P. cut

2' 11-5/8" + 4-1/8" = 3' 3-3/4" Total Rise to top of ridge.

[speaker]

Ya right ,I forgot about deducting the 3/4 of run for the ridge thickness in my post, but I must admit I didn't even think about the fact that by doing so I must adjust the rise also (HELLO IS ANYBODY HOME). So if you think about it when calculating the triangle (rafter - rise - run) the thickness of the ridge is not even included in the equation, the roof is calculated as 2 seperate triangles with a 1 1/2 plumb line down the middle. (I am kinda thinking out loud)

Yesterday I did an experiment, for the HAP purpose mostly, because I over looked that when I was planing rafters for a cottage I am going to build, and as soon as I put it to paper, I said to my self "SELF" we got a problem

so...

I cut a 2 x 4 rafter in my garage using my work table and wall as a template for "top plate/ridge", overhanging the work table. my work table is 32 5/8 away from the wall (in this case thats my run) I wanted to create a 6:12 rafer. so
(32.625*6/12) = 16.3125 + a prefigured HAP distance of 2 5/8 (2.625) which i added to the rise (for ridge height purpose only)

Got the HAP from this site http://www.josephfusco.org/Articles/Roof_Cutting/raftercutting.htm

(I couldn't do his calc. with my calculator???) I had it set to degrees, but my numbers were not even remotely close (I"m doin somrthing wrong there for sure) and is there an easier way of calculating this?

so, anyhow sorry.....

run is 32 5/8 or (32.625)
rise 16 5/16 0r (16.3125)
= (32.625)sq + (16.3125)sq =
= 1064.391 + 266.098
= 1330.489
= (sq rt) = 36.476 or (36 15/32)

with no need to cut 3/4 off for ridge thickness as there is no ridge for this experiment

I cut this with my 12" over hang (13.42) added to the plumb line where the outer seat had been cut.
I marked the wall at ( 18.9375) or (18 15/16) representing the top of the ridge, and then placed the rafter. (16.3125 + 2.625)

Conclusion,

my overhang was perfect
the angle was bang on
but the rafter was sitting 1/8' above the line or (would be ridge)

Now what you pointed out to me will no doubt help me, but in this situation where there was no ridge it shouldn't matter right? if this is correct then why was I off 1/8" out

but for future reference, when I am cutting my rafters you definately saved a lot of headaches!
Thank you Joe

[speaker]

Speaker,

Your close but in order to get the exact ridge height you have to deduct for half the thickness of the ridge. If your using a 2x ridge you would take half that tickness which is 3/4" and deduct that from your run.

6' - 3/4" = 5' 11-1/4" = Run

6/12 Pitch the rise would be 2' 11-5/8"

4-1/8" H.A.P. cut

2' 11-5/8" + 4-1/8" = 3' 3-3/4" Total Rise to top of ridge.

Joe, thanks for your help here.
If my top plate was 2 x 4 (2 x6 rafter, 6/12 pitch) and I wanted to cover the entire plate with the seat, that would leave me with 4-3/8 H.A.P.
six or 1/2 a dozen?

[Joe Carola]

Speaker,

Your close but in order to get the exact ridge height you have to deduct for half the thickness of the ridge. If your using a 2x ridge you would take half that tickness which is 3/4" and deduct that from your run.

6' - 3/4" = 5' 11-1/4" = Run

6/12 Pitch the rise would be 2' 11-5/8"

4-1/8" H.A.P. cut

2' 11-5/8" + 4-1/8" = 3' 3-3/4" Total Rise to top of ridge.

Joe, thanks for your help here.
If my top plate was 2 x 4 (2 x6 rafter, 6/12 pitch) and I wanted to cover the entire plate with the seat, that would leave me with 4-3/8 H.A.P.
six or 1/2 a dozen?


Speaker, Your Welcome and as for a 2x4 wall with a 3-1/2" seatcut and a 2x6 rafter with a 6/12 pitch the H.A.P. would be 4-3/8".

The 4-1/8" H.A.P. I wrote in my first post was from you in your first post and I was just adding it on to what I showed you the rise was.

If you were to use a 2x6 wall and a 5-1/2" seatcut the H.A.P. would be 3-3/8".


Joe Carola