I want to replace the roofing on an older shed style covered patio. The patio was built 45 years ago (+/-). The rafters are 2x8 with an odd spacing I've never seen before, alternating 21 inches OC,then 17 inches OC then 21 then 17, with blocking about 4'. Was this a thing back then or is it a one off homeowner project? The framing is still solid and I don't want to tear it down, just re-roof. The roofing material is corrugated fiberglass panels only, they are original. If this was a thing, is there a roofing plan for how to start laying panels? What width panels would I use?
We use 19.2" oc joists (5 spaces for 8' plywood) when we use Trus-Joist open web joists. They are so expensive that keeping size down and spacing maximized saves costs. (True-Joist has a software program to figure this out...)
Maybe the alternating spacing allow fasteners to secure the corrugated roofing on the "down" curve of the corrugated shape.
I think this old-time carpenter was ahead of his time.
There is no reason I can see for the alternating spacing, except for the builders choice or homeowners. So I believe you have a one off framing job.
Since the average of increment is 19", it is probably more accurately 19 3/16". This is a not so commonly used framing center to work with 8' material. I can't say if that is what they were striving for that, but it seems to look that way.