Moss on roof tiles: harmless or a problem?
A problem — a slow, quiet one. Moss holds water like a sponge and wicks it into the laps and valleys where tiles overlap, exactly where a roof is supposed to stay dry. As it grows it lifts fragments that wash into your gutters and block them. The fix is chemical, not mechanical: kill the growth at the root and let weather carry it off. Never pressure-blast a tiled roof.
What moss actually does between your tiles
A tiled roof works by overlap. Each tile sheds water onto the one below it, and the laps — the covered joins — stay dry because water moves down and off. Moss breaks that arrangement. It is, functionally, a sponge, and when it grows in the laps and valleys it holds water there and wicks it sideways into joins that were designed to drain, not soak. The surfaces rain was meant to leave stay damp for days after the sky clears.
The damage compounds quietly. Moss anchors into the porous surface of concrete and terracotta tiles, and as the cushions thicken they hold more water and pry at the surface they grip. Fragments break away — of moss, and of the tile surface with it. A permanently damp tile weathers faster than a dry one, and a weathered tile grows moss more readily. That loop is why a lightly green roof left alone becomes a thickly green roof, not a stable one.
The shade-side pattern
Stand on the street and look at any tiled roof in your suburb. If there is moss, it is almost always on one side — the southern slope, the face under the tree canopy, the pitch a taller neighbour shades in winter. Moss does not care about dirt; it cares about drying time. The slope the sun reaches dries within hours of rain and stays clean. The slope it never reaches stays damp into the afternoon, and moss takes the lease.
That pattern is useful information. It tells you which part of your roof dries slowly — which is also where laps stay wet longest and where the growth will return first after treatment. It also sets honest expectations. Killing the moss restores the surface; it does not change the shade. A treated shade-side slope stays clean for years, not forever, which is why moss control on a tiled roof is maintenance on a cycle rather than a thing you do once and forget.
Your gutters inherit the problem
Everything a roof grows, the gutters eventually receive. Every storm carries moss fragments down the slopes and into the valleys — the internal channels that do most of a tiled roof's drainage — and from there into the gutters, where they mix with leaf litter and silt into a dense, water-holding mat. Moss debris is particularly good at this because it does not dry out and blow away like leaves do. It sits, holds moisture, and binds the rest of the blockage together.
A blocked valley or gutter turns heavy rain into water with nowhere to go, and water with nowhere to go finds the eaves and the wall cavity. So a mossy roof is rarely one problem — it is the upstream cause of the gutter problem you deal with every autumn. Treat the roof and the gutters together and you break the supply line, which is why roof and gutter work belong on the same visit rather than separate call-outs.
Kill it at the root — never blast a tiled roof
The tempting fix is a pressure washer, and it is the wrong one. Blasting a concrete tile strips its finished surface and leaves it more porous — better moss habitat than before you started. The lance drives water up under the laps the roof exists to keep dry, and it shears off the visible growth while leaving the root structure in place, so the green comes back within a season. Add the foot traffic of someone working a lance across brittle tiles and you can break more than you clean.
The correct method is chemical and patient. A soft-wash treatment applied at low pressure kills the moss at the root, and then weather does the removal — dead growth loses its grip and rain carries it off over the following weeks. The roof is not scoured, the laps stay sealed, and because the root is dead rather than trimmed, the result lasts years instead of months. It is a less dramatic afternoon than a pressure lance. It is also the only version that leaves the roof better than it found it.
Getting a real number on it
Roof cleaning does not price honestly from a flat-rate menu. Size, pitch, how established the growth is, access, and the tile material all move the work involved, which is why Schmick quotes roof treatment per job rather than pretending one number fits every house. What that quote buys is a soft-wash treatment done to the method above, photographed before and after, by a crew insured to be up there.
Per-job pricing does not have to mean waiting for a callback. Answer a few questions about the house in Schmick's instant quote and it returns a firm, bookable price in about 60 seconds — a number that holds unless the place turns out to be materially different from what you described. If one slope of your roof has gone green, that is the shade side telling you it will not fix itself. Get the number and decide with it in hand.