We inspected the roof of a home owner that decided to pressure wash his roof, but after a few days of rain discovered water stains on the underside of the plywood sheathing in his attic. The problem started with moss growing on the north side of the roof where it was heavily shaded by tall trees. This is a common issue for homes with the type of weather we get in Virginia. While we commend home owners for being proactive about maintaining the roof, there are certain things you should know before you take matters into your own hands.
In this case, the roof was about 10 years old – getting close to the end of a productive lifetime. The asphalt shingles were a little worn – the tiny granules embedded in the shingles were becoming thin. Those granules play a major role in repelling the water away from the roof – like water off a duck’s back! When our home owner took his power washer up and blasted away the moss, he blew off another layer of granules. So many in fact, that the shingles lost a serious amount of waterproofing efficiency. We had no choice but to replace the shingles on that section of the roof.
The picture with this post shows a low pressure power washing system, which is highly effecting for cleaning a roof. It will remove moss and algae without damaging the shingles. FYI, there are treatments available that discourage the growth of moss and algae. If you choose to power wash your roof, be sure to take all safety precautions!