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Posted by on April 29, 2007, 4:38 pm
Hi folks,
I'm looking at a small booklet on the rafter/framing square that's a
reprint of a 1923 Audel's publication. On page 42 of this booklet is
a
partial image of the back of an old Eagle square where instead of the
usual Essex board measure table there is an octagon rafter table for
gazebos, turrets, and the like. The image, unfortunately, includes
values for only a handful of the columns. The rows in the table are:
Length of Octagon Hip Rafters per Foot Run
Length of Octagon Jacks Spaced One Foot
Side Cut of Octagon Hip Rafters
Side Cut of Octagon Jack Rafters
Thinking it would be useful to have the complete table, I set out to
calculate the table values. Calculating the two length rows was
pretty
straightforward and my values matched the printed values pretty close.
But, the side cut values in the table are baffling me. Let me try to
explain.
Assuming that octagon roofs are framed with eight common rafters
centred
on each side of the octagon and meeting in the centre at 45 degrees
to
each other, the hip rafters would be centred on the octagon's corners
and form an angle with the commons of half that, or 22.5 degrees. A
zero pitch roof, therefore, would require a side cut on each side of
the
hip of 22.5 degrees.
I would expect then that a rise of only 1 inch per foot of run would
not
change this angle very much at all. The table, however, states that
for
a 1 inch rise the hip side cut will be found by using edge values of
16.
The other edge value being always 12, I calculate a side cut angle of
about 53 degrees.
It's unlikely this is a misprint, so I'm thinking either the framing
technique was vastly different back when the Eagle octagon rafter
square
was being used, or I am clueless about how this table was to be used.
If someone could help me either correct my math, or explain how these
tables are to be used correctly, I'd really appreciate it.
Thanks for your time,
Terry
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