Thanks Will for giving me even more insight to this situation. I may be looking at this the wrong way, however, snapping chalk lines, I think, can set up a problem of its own. It looks to me that the primary reference face of the plate is the outside vertical face and the top of the plate would be the secondary ref face (or vice versa). The two ref faces meet to form an arris. This arris lies in the plane created by the outer surface of the rafters. If a chalk line is used to "remove" the crown then the plane formed by the rafter tops will drop below the arris by 3/16" at the centre of the plate. I suppose I could simply plane off the protruding arris before sheating the roof. I realize that by not using the chalk lines I end up with a curved (by 3/16 in the middle of 16') roof surface if it does not flatten itself. Again, I may be looking at this the wrong way. In any case I have not used chalk lines. I think either way since it is 3/16" and a rough shed it would work out.

I certainly see clearly why it is said that crown is undesireable in plates!

Norm Hart