raising the purlins sounds good to me.
you could still use splines if you wanted, to join the purlins together above the principal rafter. If the splines were long enough, they would show from below. Actually, it should be possible in this case to orient the splines perpendicular to the top face of the principal rafter, then they would show even more...
roger, in your first post, you say that the "purlin could be severly weakened" Did you mean the principal rafter?
