mark-- thanks for the reply. Yes, there is a housing to consider: 1 inch deep, therefore 1"x4"x5" total volume of wood removed. By your 25% rule, would that mean 20sq.inches in addition to the 7sq. inches for the spline, for a total of 27sq. inches, just over 50% (yikes!) of the rafter's dimension?? Maybe this idea won't work. . . I haven't done similar math to the traditional scheme of using dovetails on the purlins (with or without housings), but it seems to me intuitively that such joints remove as much if not more wood. Your thoughts?