Look across Bellevue Hill from Bellevue Park and the roofscape is mostly terracotta, with slate on a share of the older and grander houses. Both materials can last a very long time. Both become brittle with age, and both are damaged far more often by the people working on them than by the weather.
Terracotta
Unglazed terracotta tiles are porous. In a shaded, leafy suburb they collect moss, lichen and black algae, mostly on the south and east faces and wherever overhanging trees keep the surface damp.
The right method is soft washing: a biocide applied at low pressure that kills the growth, which then releases and weathers off over the following weeks. The roof does not look finished the day the crew leaves, and lichen in particular takes a month or more to clear. More: the first 48 hours after a soft wash.
Pressure washing terracotta is the wrong answer. It strips the surface of the tile, opens the body to water, and on old brittle tiles breaks them. Glazed terracotta has a coating that pressure can lift. More: why we never pressure wash a roof.
Slate
Natural slate needs very little cleaning. It is dense, sheds water, and grows less than terracotta. What it needs is inspection.
Slate roofs fail at the fixings rather than the stone. Nails corrode, slates slip, and a single missing slate lets water straight into the roof. A slate roof should not be walked on by anyone who does not specifically work on slate, because slates crack under foot and the cracks are often invisible from below.
If a slate roof has growth, it is treated from roof ladders or a platform with a gentle biocide. If it has slipped or broken slates, that is a slater’s job before anything else.
Ridges, valleys and flashings
On both roof types the weak points are the same: ridge capping bedding and pointing, valleys that collect leaves, and flashings around chimneys and parapets. A big Federation roof with multiple gables and valleys has a lot of all three.
A roof clean is the best chance to see them. Cracked pointing, slipped ridge caps, rusted valley irons and lifted flashings get photographed and reported, even when they are not part of the job. More: what is in our photo report.
Trees and roofs
Overhanging branches are the main reason Bellevue Hill roofs grow moss. They shade the tiles, drop litter into valleys, and drip water long after rain stops. Cleaning a roof under a tree without addressing the tree buys a shorter interval than expected.
Many trees in Woollahra are protected, so pruning may need council approval. It is worth asking the question before booking a roof clean.
Does the roof need restoration instead?
Sometimes. If tiles are cracking in numbers, the ridges need rebedding, or a slate roof is losing slates regularly, a clean only shows the problem more clearly. That is a roofing job. More: roof cleaning versus roof restoration and does roof cleaning damage tiles.
Timing
Soft wash in a dry spell, any season. Avoid windy days, which carry the mix onto gardens and neighbours. Inspect slate annually, ideally after the wet autumn. For the rest of the year, see the Bellevue Hill maintenance calendar and our roof cleaning service.
