Water leaking in behind the facia of your roof could be caused by a couple of things:
1) your roof sheets have a hole in them or your screws or nails (or equivalent) are not watertight anymore, or
2) the more common reason is where roof sheets are cut too short or they don’t overhang far enough into the gutter.
What happens in this case is that water has this capillary action which means the water travels back up the underside of the sheet just a small amount. Now when you add wind, the wind blows this water further up the underside of the sheet and if there’s not enough overhang into the gutter, the water will actually find its way over the back of the fascia and inside your ceiling or somewhere else it’s not meant to be.
This problem can be overcome by having your sheets hanging a minimum of 50mm into the gutter. If you already have your roof sheets installed you need to do one of two things:
1) replace your roof sheets with new ones, or
2) assuming that you have at least a 20mm overhang into your gutter, grab a pair of pliers or multigrips and bend down the valley of your roof sheets into the gutter.
Now this may help reduce the amount of capillary action and also reduce the effect the wind has when blowing the water back up the underside of the roof sheets. It may not work; it may work; but it’s definitely worth a shot.
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