What is the depth of the ridge? Is it the same depth as the plate? If so, you can disregard that since they will drop the same amount. If the ridge is deeper than the plate, (say 12" vs 8") you have the shrinkage of the difference compounding the problem. In this example, that's 13" total of oak -- could be as much as 5/8" or 3/4".

If they are the same and you are just looking at 9" of oak shrinkage that would be more like 1/2".


I try not to do posted ridges in green wood. You can leave a gap between rafter plumb cuts and don't screw opposing rafters to each other - just to the ridge. That way when the ridge settles, the rafters don't thrust.

But with 1/2" of ridge drop, you can't really leave gaps that big. You would need a gap of 2x(tan roof pitch x 1/2". That would be 1" if it is 12 in 12 and 3/4" if it is 9 in 12.