WOODWORKING QUIZ for 28 Sept 04

[Ralph Jones]

Good Evening Friends,
I am going to build a room addition that will have a 24' span. the center of the original house being a 24' span measures 60". I am going to cut rafters rather than use trusses and want the ridges to match.

What pitch should I use for the rafters to be the same height?

Respectfully,

[Shane Watson]

Well I don't know, but ya better hurry up cause Ozwinner is pinching ole' records left right & center to melt down for shellac. So it don't matter what pitch you like listening too, ya better hide them 24' & 60" from him, mind you I can't remember those size LP's!

[IanA]

Ralph,

If I understand the question correctly the pitch will be close enough to 22 1/2 degrees.

[Ralph Jones]

Good Evening Friends,
Here in the U. S. roof pitches are figured in inches per foot of run, so for a roof that measures 60" in the center of a 24' span it is known as a 5/12 roof pitch. In other words the rise in the middle is 5" per each foot of run which would be half of the span or 12'. So 5" x 12' = 60". Now if the room addition was of a different width the ridges would not match.

Thank you for your support.

Respectfully,

[IanA]

Ralph,

Pitch in Oz also used to be expressed the way you explained.

Roof pitches of 1/4 were pretty common for a period.

Expressing the pitch in degrees seems to have displaced the old system here.
Maybe it crept in with metrication.

Anyone else care to comment on the change?

[echnidna]

I preferred the old way as you could do it with a tape.
Degrees mean using a roofing square

[IanA]

Bob,

I'm inclined to agree with you. If you were marking out a pattern rafter you would use a tape in preference to a roofing square.

In Ralph's case you would probably measure the existing rafter length and copy it. The angle of the plumb cut you would just set out at the same time.

Trussed roofing in Oz is always expressed in degrees, which is not a problem as they are computer designed.
The percentage of trussed roofs is very high and increasing, and individually cut and pitched roofs are becoming rarer, except maybe for additions to an existing roof.

[duckman]

I preferred the old way as you could do it with a tape.
Degrees mean using a roofing square

Roofing square? Bob you're showing your age

I was taught how to use a square at trade school, back in the early '70's, but never used one on the job. Hancock's Roof Book and a tape was all that was needed.

[Ralph Jones]

Good Evening Friends,
I have used the framing square, since I was thirteen as my grand father being my apprentice teacher made me do so. It is much easier and faster to use the rafter tables on the face of the blade of the square for figuring the length of common rafters by reading the number under the pitch and multiplying by the run. In the case of the quiz on a 5/12 pitch you look at the 5 on the square and in there column just below it is the number 13. Now by simply multiplying that 13 x 12 you will find the length of the rafter from the centerline of the building to the plumb cut on the birdsmouth. then you would add in even feet the length of the overhang from that point.
Now you will notice that it comes to 156" /12 = 13, so you had the correct answer right from the square as 13' is the length of the common rafter for a 5/12 pitch.

I am going to attach a square picture showing the pitches of the different heights.

Respectfully,