A roof clean is four jobs in sequence, and they are not equally visible. The one that decides how long the result lasts is the one you cannot watch happening — which is why the cheapest quote is very often the one that leaves it out.
1. The survey
Before anything is sprayed or scraped, the roof is looked at — from the ground with binoculars or a pole camera, and from the loft where there is access. What that is establishing:
- Tile type and condition. Concrete tiles, clay, natural slate and fibre cement all behave differently, and a tile that has already lost its surface will not survive the same treatment as one that has not.
- Cracked, slipped or delaminating tiles. These are photographed before the work starts. A roof that is cleaned without that record leaves nobody able to say whether a broken tile was broken beforehand.
- Mortar condition at ridge and verge. Soft or missing bedding mortar is a reason to keep water and pressure away from it.
- Access. The single biggest variable in what a roof costs. A detached house with hard standing on all four sides is a different job from a mid-terrace where everything has to come through the house or over a neighbour’s garden — which is most of inner Dublin.
- Where the water will go. Everything that comes off the roof ends up on the ground, so drains, planting, ponds and the neighbour’s washing all get considered before the first litre is used.
If the survey finds a roof that needs a repair rather than a clean, that is what the quote should say. Cleaning a roof with failed mortar or slipped tiles produces a tidy-looking roof with the same defects and a bill in front of them.
2. Getting the moss off
Bulk moss is removed mechanically — scraped and brushed off the tile surface, working down the roof so nothing is dragged back over cleaned ground. On most Irish roofs this is done without a pressure washer, and the reason is specific rather than cautious.
A concrete tile has a factory surface. High pressure removes the moss and takes that surface with it, exposing the porous body underneath — which then holds water and grows moss faster than it did before. The roof looks dramatically better for a season and then deteriorates on a shorter cycle. That trade-off is covered properly in soft washing versus pressure washing.
Slate is different again: it is not porous in the same way, but it is brittle and it is laid in overlapping courses that a jet of water can drive straight under. Natural slate is cleaned with even more restraint than concrete, not less.
3. Gutters, last
Everything taken off a roof arrives in the gutter, so the gutters are cleared after the roof rather than before. Doing it the other way round means paying for the same job twice, and it is a reliable indicator of how carefully a quote was put together.
The downpipes and outlets get checked at the same time, because a gutter full of shed moss will have pushed material into both. Details are in how to clear a blocked gutter, and it is the same work described in gutter cleaning when it is booked on its own.
4. The treatment — the part that decides how long it lasts
Removing moss does not remove what caused it. Spores are already on every surface and in the air, and a bare tile in an Irish climate is somewhere they will happily start again. A biocidal treatment is what slows that down.
It is applied at low pressure after the roof is clear, and three things about it matter:
- It is not a rinse-off. The product is left on the roof to work. There is nothing to see immediately, which is exactly why it is the stage most easily skipped.
- It keeps working for months. Residual growth continues to die back and weather off after the crew has gone, so a roof often looks better six weeks later than it did on the day.
- Without it, moss returns on a much shorter cycle. This is the difference between a clean that holds for years and one that needs repeating.
What is in these products, and what actually protects the ground underneath them, is covered in what eco-safe actually means in exterior cleaning.
What it looks like afterwards
Worth setting expectations honestly, because this is where satisfaction is usually won or lost:
- The roof will look patchy at first. Moss holds moisture and shades the tile beneath it, so tiles that were covered come up a different shade from tiles that were exposed. That evens out over a few months of weathering.
- Some staining is permanent. Where growth has sat for years the tile itself can be discoloured. A clean removes the organism, not the history.
- Dead growth keeps coming off. Expect some in the gutters for a few weeks afterwards. It is the treatment working, not a job left unfinished.
Weather and timing
Exterior cleaning is not a summer-only trade, but the treatment stage has real weather requirements: it needs dry hours after application to work, and it should not go on in heavy rain, hard frost or strong wind.
That gives Dublin a practical advantage. Being the drier side of the country, the east coast offers more usable windows through autumn and winter than the west does, so a roof in Dublin is easier to schedule out of season than the same roof in Galway, where Atlantic exposure means both faster growth and fewer dry days to treat it in. Cork sits between the two, with a mild, damp climate and a long growing season that keeps growth going for more of the year than the temperature would suggest.
What the whole job comes to, and why two quotes for the same roof can differ by a factor of three, is set out in what roof moss removal costs in Ireland.