dimensioning cutout for stovepipe

[MikeC]

Hello -
After exhausting my key word supply in fruitless searches I'm still trying to find the answer to:

installing stovepipe through the roof, at a 12/12 pitch, double wall insulated, 6" OD with 2" clearance for a total of a 10" total OD (imaginary) cylinder; what is the formula to calculate the roof cutout?

It seems there will be a minor axis (10") and the major axis running up the roof, how to calculate the major axis?

Thanks in advance for any help,
Mikec

[MountainDon]

Yes, the perfect hole would be an ellipse with a major/minor axis.

When I cut the hole through our roof I used the flashing as a guide to the maximum size of the hole. I set the center of the hole by drilling a 3/8" hole at the center. Since the roof already had the metal roofing installed this was more like a retrofit chimney, so I had to deal with cutting both the wood decking and the metal.

The actual cut out was a series of short straight line cuts through the multiple layers plus some finishing off with a sawzall. The hole was not pretty but it is all hidden from view and all the clearances were 2 inches or better.

[Don_P]

Blocklayer does have some calcs for it.
Quick and dirty, the line length ratio for a 12/12 is 1.414 so 10" as the level base of the triangle x 1.414=14.14" as the major axis (hypotenuse) plus some fudge for sheathing thickness.

[MountainDon]

Blocklayer does have some calcs for it.
Quick and dirty, the line length ratio for a 12/12 is 1.414 so 10" as the level base of the triangle x 1.414=14.14" as the major axis (hypotenuse) plus some fudge for sheathing thickness.

That would make for a much more self-satisfying "job well done"   

[MikeC]

That's just what I was looking for - couldn't find the reference on blocklayer though.

I may be wrong, but it appears that as long as the cut opening edges are kept parallel to the pipe, roofing thickness wouldn't be an issue?

Thanks, Don & Don!

[Don_P]

Here's the blocklayer link I saw;
https://www.blocklayer.com/pipe-angleeng.aspx

Yup you got it, if the cuts are plumb then the sheathing thickness is a non issue.

another way with a calculator 10/(cos 45)
or to get line length ratios 1/(cos roof pitch angle).
Carpenters used to keep some line length ratios in their head of common roof pitches. The ratio of the base of the triangle to the hypotenuse, jobsite simple trig. Then if you know the horizontal span, multiply that by the LLR to get the rafter plumb to plumb length, the "line length".

[miketom]

Thanks for the thread. My friend was looking for related information. This helps. Cheers!

[JRR]

A quick and rough process is to wrap the pipe, or whatever, with craft cardboard.  Dip in soapy water at correct angle.  Unfurl the cardboard and cut the wet/dry line.  Reassemble cardboard.

[Don_P]

I like it.
When Edison was working on the light bulb he was needing the volume of the bulb and asked the guys in the engineering dept to get it to him. Hours later he went down to ask for it and they were busily cyphering away with all kinds of calculations and equations going on.

He reached over, filled the bulb with water and poured it into a measuring cup, done!

Happy Thanksgiving Everyone

[JRR]

A variation of Edison's procedure is to first fill to the brim a vessel a bit larger than necessary.  Pour off an amount, known to be larger than expected volume, into calibrated measuring vessel.  Drop in specimen and refill to the brim from measuring vessel ;; the volume left over is "answer".  "Play sand" or diatomaceous earth can be used if specimen is not water resistant.