The short answer
A moss free guarantee is only as good as its wording. Moss regrows on any damp, shaded roof, so a sound guarantee names a period, says what the firm will do if growth returns, and sets out what you must do, such as clearing gutters. Ask for it in writing before you pay, and treat a promise of permanent results with caution.
Why moss cannot be guaranteed away for ever
Moss spores are in the air everywhere. They land on every roof, and whether they take hold depends on water, light and surface, none of which a roofer controls. A cleaned roof is a clean start, not a sealed one. On an open, sunny slope the growth may take years to return, and under a damp, tree-shaded edge it can be back within a couple of seasons.
This is why a flat promise of a moss-free roof makes little sense. A firm that offered one would be promising to out-argue the weather. What a guarantee can sensibly promise is narrower: that the work was done properly, that the treatment was applied as described, and perhaps that the firm will return if growth appears within an agreed time. Our page on zinc strips for moss explains one way of slowing regrowth, and what it can and cannot do.
What a guarantee might actually cover
Read the document, not the headline. Some guarantees cover the workmanship, meaning that if the cleaning damaged the roof or was done poorly, the firm puts it right. Others cover regrowth for a stated period, either with a free re-treatment or with a partial refund. A few combine the two. And some are simply a statement that a treatment was applied, with no remedy attached.
The questions to ask are practical. How long does the guarantee run? What counts as regrowth: a few spots, or a full cover? Who decides? Is the firm obliged to come back, and within what time? Does the promise depend on you doing something, such as keeping gutters clear, trimming trees or having the roof checked each year? What happens if you sell the house, and does the guarantee transfer to the buyer?
If the answers are not written down, treat the guarantee as a sales line. A short email from the firm confirming the terms, sent before you pay, costs them nothing and protects you if memories differ later.
Aftercare you may be asked to do
Almost every sensible regrowth guarantee comes with conditions, and most are reasonable. A roof that is continually wet because the gutter overflows will grow moss however well it was treated. A branch that drips onto one slope will do the same. A firm that sets conditions is not looking for a way out; it is describing what the roof needs.
Typical aftercare is simple. Keep gutters and downpipes clear. Cut back overhanging growth. Leave the biocide to do its work for the weeks it needs, and do not scrape or jet the roof meanwhile. Let the firm know promptly if growth reappears, ideally with a photograph, so the cause can be looked at. For access and timing questions, our page on roof cleaning access shows what a visit involves.
How the method changes the result
The result a guarantee can rely on starts with the method used. Steam and hot-water systems such as DOFF clean by heat and gentle flow, and MyBuilder's roof cleaning guide, updated on 21 August 2026, prices DOFF cleaning at £20 to £30 per m², against £15 to £20 for soft wash. We describe the process on our page about DOFF steam cleaning. Whichever system is used, the question is the same: does it leave a clean surface, kill the roots and avoid harm to the covering?
A treatment that only blasts the visible moss away leaves the roots and the spores behind and may leave the roof rougher than before. A promise attached to that kind of work deserves less trust than one attached to a patient, low-pressure clean followed by a biocide.
Your rights besides the guarantee
A guarantee adds to your legal position and does not replace it. The Consumer Rights Act 2015 treats every service contract as including a term that the work is done with reasonable care and skill. If it is not, you can ask for the job to be repeated or for a price reduction. A roofer who damages tiles during a clean has a duty to put that right whatever the paperwork says.
The reverse is also true. Statutory rights protect you against poor work, but they do not promise that moss will not return, because moss returning is a matter of nature and not of workmanship. That gap is why guarantees exist, and why reading the terms matters. Keep your quote, your invoice and any photographs together so you can show what was done and when.
Our own approach, and selling the house
For our part, the labour on your roof carries a decade of written cover for workmanship. That is a promise about the quality of what we do, and we spell it out so nobody mistakes it for a forecast of how fast the weather and the trees will bring the moss back. If you ever want to know what a guarantee covers before you agree to anything, ask for the wording and read it at your own pace; a firm that is confident in its terms will not mind.
If you are cleaning a roof to prepare for a sale, a written record of the work, dated photographs and a note of the method give a buyer's surveyor evidence that someone has kept an eye on the roof; see roof cleaning before selling. To book a survey, ring 01793 626096 or use the enquiry form, and find the wider service under roof cleaning and moss removal.