Roof tile sealer comes up most often as a line on a restoration quote, or as an add-on after a roof clean. It sounds sensible — seal the tiles, keep water out, extend the life of the roof.
Sometimes that is roughly right. Often it is paying for something your roof does not need, and in a few situations it causes problems that did not exist before.
What roof tiles actually do
A pitched tile roof is not waterproof and was never designed to be. It is a shedding surface. Tiles overlap so water runs down and off, and some wind-driven water always gets underneath. That is what the sarking — the membrane under the battens — is for.
This is the central point. Making the tile face less absorbent does not make the roof more waterproof, because the tile face was not where the water was getting in.
What tiles you have
Concrete tiles are made from sand, cement and oxide pigment, with a factory-applied acrylic surface coating. That coating sheds water and holds the colour. Over decades it erodes, and the tile becomes more porous and more prone to holding growth.
Terracotta tiles are fired clay, usually glazed. The glaze is vitrified — effectively glass — and it does not erode the way an acrylic coating does. A hundred-year-old glazed terracotta tile often still sheds water perfectly.
That difference decides most of the answer.
When sealing makes sense
Older concrete tiles with a genuinely eroded surface. If the factory coating has gone — the tile looks chalky and dull, the surface feels rough and sandy, water soaks in rather than beading — then a sealer or coating restores a shedding surface and reduces how readily growth re-establishes. This is a real benefit.
As part of a proper coating system during restoration, where the sealer is the primer under a membrane, not a standalone product. In that context it is doing a specified job.
That is close to the entire list.
When it does nothing
Concrete tiles with the factory coating intact. If water still beads on your tiles, the surface is working. Adding a sealer duplicates a function that already exists and starts a maintenance cycle you did not have.
Glazed terracotta. The glaze is non-porous. There is nothing for a sealer to penetrate, and it will sit on the surface and eventually peel or discolour. Sealing glazed terracotta is close to a straightforward waste of money, and we would push back on any quote proposing it.
Fixing a leak. This comes up and it is important. If your roof leaks, the cause is a cracked tile, failed ridge pointing, a flashing problem, damaged sarking or a blocked valley. Sealer applied over the top does not address any of those and masks the diagnosis. A leak is a repair job.
Where it causes problems
Unglazed terracotta. Fired clay breathes. A film-forming sealer on unglazed terracotta traps moisture in the tile body, and in a frost or a strong wetting-drying cycle that causes spalling of the tile face. The same breathability argument applies as on sandstone.
Sealing over growth. Coating a roof that still has live lichen or moss locks the organism in under the film, where it keeps growing and lifts the coating off in patches. Preparation is the whole job, and it is the step most often shortcut.
Sealing a damp roof. Traps the moisture. The coating blushes and the bond fails.
Catchment roofs. If the roof feeds a rainwater tank, any product applied to it needs to be suitable for potable water contact, and the tank must be disconnected through the work and the first rains after. Not every roof coating is rated for this, and it is worth asking directly. See roof cleaning on tank-fed properties.
The commitment nobody mentions
An uncoated concrete tile roof needs an occasional wash and nothing else. A coated roof is on a maintenance cycle — typically recoating every ten to fifteen years, and a coating that is allowed to fail looks considerably worse than bare tile ever did.
That is a reasonable trade on a roof whose surface has genuinely gone. It is a poor trade on a sound roof that simply needed cleaning.
The test
Get up safely, or have someone do it, and throw a cup of water on a few tiles in different areas.
If it beads and runs off, the surface is intact and you do not need a sealer. If it soaks straight in and darkens the tile, the coating has eroded and sealing has something to offer.
Also look at the tile face. Loose sandy grit coming off under a finger is an eroded concrete tile. A smooth hard surface is not.
If you are being quoted for it
Ask which of your tiles are being sealed and why. Ask what the preparation is. Ask what the product is and whether it is suitable for a catchment roof if that applies. Ask what the recoat interval is.
Sealing is frequently bundled into a restoration quote where the real work needed was re-bedding and re-pointing — we cover that in what roof restoration costs in Sydney.
For most Sydney roofs, a proper clean every few years is the whole maintenance requirement. See roof cleaning, and terracotta roof cleaning if that is what you have.
