This picture above is a great example of what can happen when you turn your back on a roof. Now just to be clear, this is no judgement on the owners - this was up really high and they had no idea what gifts the birds had bestowed upon their roof. In amongst all that there were four cabbage trees, two pohutakawas and enough grass to cover your average putting green. And yes, the trees did get replanted!
As a broad stroke rule of thumb, if you have a porous or gritty roof (concrete, clay, decro-mastic), you'll have moss. If you have a smooth roof (iron, steel tiles, glazed), you'll have lichen. They'll all accrue algae and mould and given long enough, they'll get grass too.
So what to do about it? The problem can generally be tackled one of two ways. The first option and generally speaking, the best course of action, is a roof treatment. Treatments are gentle and they don't cost too much to achieve. You don't see an instant result with a treatment but rest assured, the growth will die and wash away with rain. Depending on how thick the accrual is, you may be waiting three to four months to really get traction.
The second option is to pressure clean. Pressure cleaning achieves an instant clean, however it costs considerably more to achieve based on the labour element and it should really only be considered if you're proposing to paint the roof afterwards. The reason for the caveat is that pressure and roofs are not a match made in heaven and they should really only meet when needs absolutely dictate they must.
GreenAgenda is big on roofs and on roof treatments in particular. If you're having your house washed, seriously consider having the roof treated at the same time. There are great efficiencies to be had while on-site; having a roof treatment at the same time as the housewash will save plenty when compared to doing it in isolation.
We're advanced height safety trained, we understand roofs and wherever possible, we'll treat your roof without having to set foot on it.
Look after your roof and it'll keep you, your house and your family dry for decades.