How long does moss and mould treatment take to work?
The honest answer
Green mould and moss: days to a couple of weeks.
Lichen: months — and up to six for a bad case.
That is the whole answer, and it is worth knowing before you spray rather than after.
Why lichen takes so much longer
Moss sits on a surface. Lichen grows into it. It is a slow organism with a tough outer layer, and it dies at the pace it lived. A roof treated in March can still be shedding lichen in September — that is the treatment working, not failing.
The difference catches people out. Someone treats a path and a roof on the same weekend, sees the path transform inside a fortnight, then looks up at the roof a month later and assumes the product did not take. It did. The roof is just on a different clock.
What you will actually see
- Week one to two: moss and green mould darken, then turn orange-brown. It stops looking alive.
- Week three onward: rain and wind start carrying the dead growth away. You do not need to help it.
- Month two to six: lichen gradually lifts and breaks up. Patches go patchy before they go.
Do not respray early
There is no need to reapply before those timeframes are up, and respraying does not speed anything along. A surface that looks unchanged at a month is lichen being lichen.
The one exception is lichen on the first pass: spray it, leave it 15 to 20 minutes, then spray it again along with the surrounding area. That second pass is part of the first treatment, not a re-treatment.
Frequently asked questions
How long does it take to kill moss?
Days to a couple of weeks for moss and green mould. The growth darkens first, then turns orange-brown, then weathers off with rain.
Why is my lichen still there after a month?
Because lichen takes months, and up to six for a bad case. A month is too early to judge it. No reapplication is needed in the meantime.
Do I need to spray it again?
No, not within those timeframes. The exception is lichen, which gets a second pass 15 to 20 minutes after the first as part of the same treatment.
Will rain wash it off before it works?
Spray onto a dry surface on a still day and give it time to take. Rain afterwards is doing you a favour — it is what carries the dead growth away.
How do I know it is working?
Colour change. Live moss is green and springy; treated moss goes dark, then orange-brown, then brittle.
Working out quantities next? See how much you need for your area, or which growth you are actually looking at.
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