Nobody publishes a roof cleaning frequency, and the pages offering one, whether every two years or every five, have made it up. There is no British Standard, no trade body guidance and no cost guide that states an interval, so the honest answer is that the trigger is condition rather than the calendar.
What the trigger is
Clean when the moss is thick enough to do something. Two things count: moss that blocks the gutters or downpipes as it washes off, and moss that holds water against the tiles or slates rather than letting it run away.
Both of those are functional problems with a cost attached. A blocked gutter is one of the commonest causes of what homeowners report as a roof leak, and standing water against a covering is what accelerates the wear the covering is otherwise very good at resisting.
Everything else is appearance. A light growth of moss on a sound, well-pitched roof is not damaging anything and is not an emergency, and a roof that only looks green from the pavement does not need buying anything for.
Not because someone knocked on your door
This is where roof cleaning turns from a maintenance question into a consumer protection one. Trading Standards have issued repeated warnings about cold callers offering roof cleaning, following complaints about damaged roofs and poor workmanship. Which? puts the unsolicited doorstep approach first on its list of rogue trader red flags.
The pitch works because you cannot check the claim. Someone reports moss, a slipped slate or cracked mortar on a part of the roof you have never seen, offers to deal with it while the ladder is on the van, and you are agreeing to work you had no reason to think you needed. The information asymmetry is the whole business model.
If you have already agreed to something on the doorstep, a contract made in your home is an off-premises contract and carries a 14-day cancellation right running from the day after it was entered into. Where the trader never gave you written cancellation information, that right can extend to 12 months and 14 days.
Pressure washing can damage older coverings
Checkatrade lists two explicit drawbacks of pressure washing a roof: it can damage or dislodge tiles, and it is not ideal for older or more delicate roofs. Manufacturers commonly advise against it as well.
That matters more in South London than in most places. Victorian and Edwardian clay and slate are exactly the coverings a high-pressure lance is worst suited to, and a slate that survived 120 years of weather can be cracked or knocked off its nail in an afternoon. Water forced up under a slate also goes somewhere, and on an old roof that somewhere is the underlay.
Hand removal with a biocide, or soft washing, costs more per square metre and is the right method on that stock.
What it costs
Checkatrade's April 2026 figures are per square metre, exclude VAT and exclude access equipment:
- Pressure washing: £5 to £12
- Steam cleaning: £9 to £13
- Hand removal with biocide: £8 to £15
- Soft washing: £12 to £16
- Scaffolding, where required: £16 to £20
On a typical semi that works out at about £637.50 for pressure washing and £862.50 for hand removal with biocide, and scaffolding can double either figure. The gap between the cheapest and the gentlest method is roughly £225 on that job, which is a small premium against the cost of replacing cracked Victorian slate.
Before you book anyone
Ask what method they intend to use and why, ask what they will do about the gutters while they are up there, and ask them to look at the flashings and the ridge at the same time. A cleaning visit is one of the few times someone is on your roof without an expensive job attached, which makes it the cheapest inspection you will ever get.
Moss on a sound, well-pitched roof is not an emergency, whatever the person at the door says.