We've been contacted by a homeowner concerned about grey streaks appearing on the white painted exterior walls of his house.
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The streaks couldn't be cleaned off and the paint seemed to be stained. Streaking was limited to those parts of the walls directly below a butyl-rubber-covered flat roof and wall parapets. We quickly found examples of houses in the same area with similar-looking streaks.
There appear to be three possible causes of this streaking.
A flat roof, with a slope of only a few degrees, means debris doesn't get washed away easily by rain. So dirt and grime can build up on the roof: soot from chimneys, pollutants from nearby traffic and wind-blown organic material such as pollen, lichen, moss and mould.
Butyl rubber is a proven weatherproofing solution for flat roofs, but like all polymers it will gradually deteriorate or “weather” when exposed to the environment. This may result in a rough surface that allows contaminants to build up, and some of the “carbon black” filler material used to strengthen the rubber will add to the surface grime. Carbon black is the fine powder in photocopier toner cartridges. If you’ve ever got that on your hands, you'll know how difficult it can be to remove.
The homeowner’s streaked walls had been painted immediately before the streaking appearing. We don't know the conditions at the time of painting, but there could be a link between applying the new paint and the streaking.
The New Zealand Building Code now requires flat roofs to have a minimum two-degree pitch, and they must be designed to direct rainwater to drains and gutters. A modern roof design should limit the amount of water running down the walls, but it may not completely prevent it.
Many houses with flat roofs were built before these new requirements were brought in. Some older houses will have water run-off down the walls and will also have older butyl rubber roofs, which may be weathered and dirty.