The short answer
There is no fixed timetable. Moss regrows fastest on shaded, damp slopes and slowest on open, sunny ones, and a roof cleaned without any treatment will usually recolonise sooner than one that was treated. Spores arrive on the wind constantly, so regrowth is a matter of conditions, not luck. Treatment and tackling the cause slow it down; they do not make a roof permanently immune.
Why a cleaned roof does not stay clean for ever
Moss spreads by spores, which are tiny, light and carried on air. Every roof receives them all the time. Whether they take hold depends on what they land on: a surface that stays damp, a little organic grit for the roots to grip, and shade that stops the sun drying the slope. Remove the moss and those conditions remain, unless something has changed.
That is the plain answer to the question, and it explains why one house on a lane stays clear while its neighbour greens over. A clean roof is a clean start, not a permanent state. On an open, south-facing slope that dries quickly, regrowth may be slow and patchy. On a north slope under overhanging branches, the same spores meet an ideal bed and the green returns sooner. The growth also tends to come back in the same places, usually along the lower edges and in the laps.
We cannot give a sourced figure for how long regrowth takes, because it depends on the slope, so be wary of anyone who quotes you a number of years as though it were a certainty.
What treatment does and what it does not
Hand scraping removes the growth but leaves the surface as it was. A treatment applied afterwards, usually a biocide that is sprayed on and left to work, acts on the spores and roots that remain, which is why a treated slope tends to stay clear for longer than a scraped one. Our guide to soft washing and biocide treatments explains how it is applied and why it is left to work rather than rinsed off.
Treatment also does not look instantly effective. Dead moss often stays on the roof for a time and washes away gradually with rain, so a treated roof can look worse for a while before it improves. That is normal, and it is a reason to leave the roof alone rather than attack it with a brush.
None of this is a guarantee that moss will stay away. A treatment slows regrowth, and the conditions on your roof decide how much.
Shade, trees and damp: the real drivers
The single biggest influence on regrowth is moisture. If your roof sits under trees, or hard against a high wall, or faces north, it will hold damp for longer and stay green. Leaf litter in valleys and behind chimneys adds a bed for roots. Where branches overhang, they also drip water onto the same patch long after rain has stopped.
Trimming branches back to let in light and air is often the most effective step, and it costs less than repeated cleaning. Clearing debris from valleys and gutters helps too, as does keeping the gutters running freely because water that cannot leave the eaves soaks into the bottom courses and feeds fresh growth there. Zinc or copper strips fitted below the ridge release trace metal ions in rain, and they have their place on some roofs, though they will not rescue a very shaded slope on their own.
Read our guide to why moss grows on Stroud roofs for the causes in more detail.
Why regrowth is not always a sign of a poor job
When moss returns, people sometimes assume the cleaning was done badly. Often it was not: the roof is simply in an environment that favours moss. A conscientious clean removes what is there and treats what remains, but it cannot change the shade, the aspect or the neighbour's sycamore.
The signs that something was missed are different. If growth reappears quickly and in a thick, even carpet across the whole slope, it may be that the biocide was not applied evenly or was washed off before it could work. If it returns in streaks, runoff from a dirty area above may be feeding it. Either way, tell whoever carried out the work and ask to see photographs of the slope as it was left.
That is why a before-and-after set of pictures matters: we take them for each slope when we survey and when we finish, so a later comparison rests on the record rather than on anyone's recollection. Seasonal change also confuses people. Moss often looks brightest and thickest in a wet winter and shrinks to a brown crust in a dry summer, so judge regrowth at the same time of year.
Planning a routine that suits your roof
Because regrowth varies, the best plan is a light routine rather than a fixed interval. Look at each slope from the ground twice a year, once when the leaves are down and once as spring arrives, with binoculars if you own a pair. Note where green is returning and how thick it is. Early growth is cheap to deal with: MyBuilder's 2026 guide puts manual scraping at about £5 to £10 per m², against £15 to £20 per m² for soft washing.
If you are tempted to treat the roof yourself with a product from a garden centre, our answer on whether roof moss killers are safe for plants and pets covers what to check on the label, and doing it yourself explains the limits of what is safe from the ground.
When regrowth has become a regular chore, our roof cleaning and moss removal service starts with a survey that records the covering's condition, and the work is backed by a 10-year workmanship guarantee in writing.