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I cant understand this math. ive been over and over it. Each time I get a little further, then I loose it again.

ref:

How to calculate rafter thrust? -Help! at http://www.eng-tips.com/viewthread.cfm?qid=105386

Can someone help me figure cable load?

4/12

24" rafter spacing.

No rafter ties at all. A few existing collar ties.

ceiling joists run parallel to ridge.

house is 27' x 30'(ridge).

7/8" roof planking and asphalt shingles.

thanks,

pa

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Figure 1,476# per rafter.

I get about 1215 pounds using a 20 psf live load.

In any case, not much. The connection details will be more important than the cable.

Agreed. When you have support at every rafter pair it is not hard, but when someone wants few ties then the forces add up and connection details are difficult. People do not realize that when you consider allowable edge distance, fastener spacing, etc. when using bolts you run out of space for enough bolts.

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