|Response from Mitre 10:
You would need to dismantle the part of your spouting or downpipe that leads to your water tank. In some cases this requires cutting through the pipe that goes from the roof guttering to the tank and blocking off the tank completely. Fit another length of pipe just temporarily to direct the roof water elsewhere. This way, when you wash down the roof, the dirty water will not go into your tank water.
There are different cleaners available that you could use. 30 Seconds Powered Moss Remover is one of the options that we recommend. It can either be applied directly from the shaker canister onto your roof or else you can dissolve it in a quanitity of water and then spray it on. Apply when the roof is dry. This will take some time to work, so you would have to keep your water tank disconnected for a few weeks. This may not be viable for you, so you could try the liquid 30 Seconds Outdoor Cleaner as another option. This cleaner works very quickly so the work will be done in a weekend. Visit your local Mitre 10 store and select the particular product that you personally prefer.
Once the moss is removed, make sure that your roof is rinsed repeatedly with fresh water and is thoroughly clean and free of chemicals to ensure that the runoff is completely clear and uncontaminated before you reattach the pipes to your water tank.
Please read the directions and safety precautions carefully on the packaging to make sure that you`re applying the product correctly. You can always test the product on a small section of your roof first to make sure that you`re happy with the result before you apply it to the entire surface. Also, be very careful when climbing onto your roof! If possible, wear and secure a safety harness and use your ladder correctly.
Please consult with our staff at your local Mitre 10 store and discuss your options with them. They may have some other valid suggestions.