Having conquered the heights of Sweetwater Mountain it is obligatory to strike The Titanic Pose before further work can commence.
The problem was caused (we think) by an excessive buildup of dirt under one of the eaves. All the rain falling on the roof over the bedroom (immediately under my right hand in the photo) runs into a gutter which drains at one end onto the roof below, running down further into another gutter and finally into a downpipe. Because this section of roof is overhung by the eaves it does not get much direct rainfall, so all the tiny specks of dirt washing down had built up over time. You couldn't grow tomatoes in it, but there was plenty of soil that needed brushing and washing away into the gutter, and then scooshing with a hose to clear the gutters. Supposedly the previous owner had been on the roof back in April, but I don't think he could have cleaned this section at that time, as this seemed more like ten years' soil than ten months.
The skylight over our loft had been getting dirty too, so I gave that a scrub.
The accumulated dirt cleared away, it was time to dust the ridges with Moss-B-Gone. The idea is that the powder washes down both sides and kills any mossy growth, which in turn washes away in subsequent rain. The roof material is a composite and has a rough texture, so it is not nearly as treacherous or breakable as a tile or slate roof.
Nonetheless you will note that I stayed on the side of the roof that does not offer a juicy 30 foot drop onto concrete to the careless or slippery-footed. It had been warm and dry when we started, but a light rain began as I progressed. I checked the various chimneys and roof vents while I was up there, as occasionally in a high wind one of them rattles. We shall soon hear if my ministrations have made any difference.
My Moss-B-Gone depleted, there was time to strike one more manly pose before returning to ground level. Hopefully the scrubbing and washing will have stopped the dirty dribbles, and when we have a warmer drier day in the Summer I'll go up again to check, and take some photos from the roof. In the meantime I feel I have earned a cup of tea and some biscuits.