I've got grp valleys to fit. The boards are fitted as detailed in earlier post and all felt is fitted. What I can't work out is whether to nail the battens so they push the valley to follow the contour of the boards, ie: form more of an actual valley, or so the valley sits without any stress as it comes, a flatter profile in which case it isn't supported fully by the boards. I hope this makes sense. Can someone put me right please?