Why do roof tiles stain and turn green with algae?

Staining and algae growth are encouraged by moisture. A wet surface binds dust and organic matter more easily, and algae/moss can establish. In winter, repeated freeze–thaw can damage porous tiles.

What is the best way to prevent staining and algae on roof tiles?

Clean the roof (and remove moss/algae if needed), allow it to dry completely, then apply a silicone impregnation. Two coats applied wet-on-wet provide reliable water repellency while keeping the surface breathable.

Do clay and concrete roof tiles need different impregnation products?

Yes. Use a product intended for unglazed fired clay tiles and a separate product for natural concrete roof tiles. Choosing the correct product helps ensure consistent penetration and performance.

Can I apply impregnation by spraying?

Yes, with a backpack sprayer. Set it so it does not create a fine mist: apply in larger droplets to minimise loss and achieve even coverage.

Is dipping better for a new roof?

For new roofs, dipping the tiles before installation is simpler and often more effective. A soak of about 90–120 seconds is usually sufficient; let the tiles dry before fitting.

Efflorescence (salt deposits) on walls: Remove the salts with a nitrate remover, let the surface dry fully, then apply a silicone water-repellent impregnation in two coats using the wet-on-wet method. Wipe off any excess.

FAQ – ROOF TILE CARE

How can you prevent roof tiles from staining and turning green with algae?

Clean first (pressure washer) If needed: moss & algae removal Two coats of silicone impregnation

Roof tiles: cleaning and impregnation in practice

We recommend two practical approaches – for concrete roof tiles and for unglazed fired clay tiles.

A) Existing roof: clean, dry, impregnate

Cleaning: wash the roof with a pressure washer.
If algae or moss is present: use MOSS and ALGAE REMOVER to support the cleaning.
Drying: after cleaning, allow the roof to dry completely.
Impregnation: apply silicone impregnation in two coats, wet-on-wet (apply the second coat before the first dries).

Important: Use the correct impregnation for the substrate. Clay tiles can accept the concrete-roof-tile impregnation, but not the other way round.

Recommended products:

ROOF TILE IMPREGNATOR – ROOFTOP TILE (for unglazed fired clay roof tiles)
CONCRETE ROOF TILE IMPREGNATOR – ROOFTOP CONCRETE ROOF (for natural concrete roof tiles)

Spraying tips

Use a backpack sprayer, but avoid fine atomisation.
Apply in larger droplets (spray–wet), for a more even, low-loss application.
Work top-to-bottom and avoid windy or rainy conditions.

B) New roof: dipping before installation

For a new roof, the method is simpler and often more effective: treat the tiles by dipping before installation. A soak of about 90–120 seconds is usually sufficient. After draining and drying, the treated tiles can be installed.

Why does impregnation help?

Tiles differ in porosity, which affects how much water they absorb. When tiles stay wet, they bind dust and dirt more easily, algae and moss can settle, and in winter the material can deteriorate due to freeze–thaw. Silicone impregnation reduces water ingress so these moisture-driven problems are less likely to occur.

Video

See the practical effect in our video:

Video narration (English):
In the video you can see how silicone impregnation changes the behaviour of roof tiles. On an untreated tile, water is absorbed and the surface darkens, which encourages staining and biological growth. After impregnation, water beads up and runs off instead of soaking in. A drier tile stays cleaner, algae has a harder time establishing, and freeze–thaw damage is reduced.

Open the video file

In brief: the safe workflow

1) Cleaning: pressure washer; if needed, moss & algae remover.

2) Drying: impregnate only on a fully dry surface.

3) Product selection: clay and concrete tiles require different impregnation.

4) Two coats: apply wet-on-wet with a sprayer that produces larger droplets.

New roof: dipping before installation (around 90–120 seconds).

Tip: set the sprayer so it wets the surface without a fine mist.

Common mistakes

impregnating on a damp surface → patchy look, weaker performance
only one coat → shorter protection
too fine atomisation → material loss, uneven coverage
using the wrong product for the tile type → uncertain result
working in rain or strong wind → poor penetration and streaking

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