How to cure black mold on limestone

Written by paul dohrman
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Many roofs have black stains that streak downward, widening as they approach the gutter. This is from limestone powder used in fibreglass shingles to give it weight and increase lifespan, an alternative to petroleum-based shingles used before the energy crisis of the 1970s. Bacteria and mould feed off the limestone. Houses that use limestone walls have a similar problem. The streaks tend to be found on the north-facing side of the house. The lack of sunlight means it’s cooler and moister. Shingle manufacturers recommend low-pressure washing with bleach.

Skill level:
Easy

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Things you need

  • Garden hose or low-pressure washer
  • Bleach
  • Zinc strips (optional)
  • Zinc-doped shingles (optional)

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Instructions

  1. 1

    Use a cup of bleach for each gallon of water to wash the roof or limestone wall. Don’t use a pressure washer, which can damage your shingles. Use low-pressure water instead, from a garden hose with a spray nozzle. Some people have found that pressure-washing does not even remove black mould, so it’s not worth the risk to your shingles.

  2. 2

    Keep it wet with bleach for 20 to 30 minutes.

  3. 3

    Wash it away with soapy water.

  4. 4

    Consider nailing a strip of zinc over the length of the top of your roof. Such strips are commercially available for this purpose. This will keep further growth from coming back. Though mould likes limestone, it doesn’t like zinc.

Tips and warnings

  • When you get a new roof, you can choose shingles that contain metals, such as zinc, that won’t permit mould to grow.
  • It costs on the order of £260 to £390 to have someone remove the streaking with high-pressure power washing and fungicide. This money could instead go toward the purchase of your next roof--a mould-resistant roof at that--to permanently solve your staining problem. Consider that the bleach application would have to be done every half year or so.
  • Be very careful on the roof. Bleach is very slippery. You may want to do each side on a different day, cleaning only the side opposite to the one on which you are standing.

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