im getting to the stage were i need to order roofing tiles etc,,, my roof is cathedral style and is a lean to,,, which is 15 degree i have a red land tile which is good to 12.5 ,, what is best to provide ventilation at the top of the roof ?is it ok to let it vent from the lead flashing by leaving the valley of the lead say 5mm off the tile is there another way as i need to satisfy B C