This section is from the book "Carpentry", by Ira Samuel Griffith. Also available from Amazon: Carpentry.

**Lengths Of Octagonal And Other Polygonal Jacks**. The methods of procedure in determining the lengths of jacks for other than square cornered buildings differs from that described for the square cornered building only in the fact that the runs, hence lengths, of the jacks must be determined differently. An examination of Figs. 68 and 72 will show that the runs, hence the lengths, of jacks for the square, the octagon, etc., when the run of common rafter is the same, will vary inversely as the tangent ratio. For example, the tangent of the square is 12" when its run of common rafter is 12", while that of the octagon is 4.97" or practically 5". The runs, and, therefore, the lengths of octagon jacks will differ one with reference to another, 12/5 of those of the jacks for the square building. If an octagon jack rests 24" from the corner, its run, and therefore its length, will be 12/5 times that of a jack on a square cornered building similarly placed.

Second Method: Difference in lengths of jacks may be determined by counting the spacings along the plate and dividing the length of common rafter by that number.

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