Rainwater can cause massive damage if it leaks into your metal roof. It can rot timber rafters, battens and other structural beams inside the crawl space. It can spoil insulation, causing mould and mildew to grow. Water can also dampen and cause dark patches on ceiling plaster — which, once waterlogged, can snap away from the ceiling joists. Another possible issue, blocked gutters, can lead to rainwater gushing over the sides and down your home's external walls.