We've been contacted by a homeowner concerned about grey streaks appearing on the white painted exterior walls of his house.
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.
We say
- Check your roof design for water drainage and assess the condition of the butyl rubber roof before painting the walls beneath it. Talk to your architect, builder or painter if you have any concerns and get their advice. Consider cleaning and sealing the butyl roof before painting and clean it regularly once the walls are painted.
- We think the Building Research Association of New Zealand (BRANZ) should investigate how widespread this problem is and also see if there’s useful advice that can be offered to homeowners and trade professionals.
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2 degree pitch is now required by the acceptable solution E2/AS1, it's a new update from 1.5degrees. All the same it's not a lot of slope. The roof should not discharge over the edge of the wall + a properly designed parapet should also direct the water landing on it, inwards + into a downpipe via the gutter. This will reduce the amount of staining from any type of roof above. Staining will still occur at joints in flashings as well due to dirt collecting + dripping off at these points. Regular cleaning (not waterblasting) is a good option to stop it getting too bad although the butynol situation sounds worse than normal. This may be remedied by cleaning the Butynol + paining it with an approved paint system, to prevent any black leaching out. Check with Ardex, the manufacturer.
We have staining like this and have to constantly paint the wall to cover it. The house is approx 14 yrs old, what can be done to seal this product to prevent this happening?
Resene has a membrane sealant that you paint over the butynol, then paint with roof paint. Stops the rubber deteriorating and causing streaks.
The New Zealand Building code does NOT require a flat roof to be a minimum 2 degrees pitch. The code is performance based and does not provide solutions. The article confuses acceptable solution E2/AS1 with the NZBC code E2. The acceptable solution is not the only solution, so the 2 degree pitch requirement is also wrong.