Water-repellent treatment is the best-selling and most misunderstood product in roof maintenance. It does a real job, provided you know what it does — and above all what it does not do.
What it does
An impregnating water-repellent treatment penetrates the first few millimetres of the material and leaves a compound there that sharply lowers its ability to absorb. Water no longer spreads out: it beads and runs off. A tile that no longer absorbs dries faster, freezes less and offers a far less welcoming surface for moss.
A second effect, mentioned less often: a tile that does not load up with water stays lighter and conducts less cold. The thermal gain stays marginal, but it is real.
What it does not do
A water-repellent treatment is not waterproofing. It does not fill a crack, does not replace an underlay, does not make up for a lifted flashing. If water is getting in through a mechanical fault, it will keep getting in after the treatment.
It does not get rid of moss for good either. It slows down how fast it comes back. Any promise of « never any moss again » is your cue to close the quote.
Colourless or coloured
The colourless version leaves the material’s own colour untouched. The coloured one — coloured roof coating — evens out a faded roof and adds an extra layer. It only goes on a substrate that still holds: if the tile crumbles under your finger, the paint will not last more than two or three years.
The cases where it does nothing
- On natural slate: it has to stay breathable, and a film-forming product makes it split.
- On a roof due for replacement within five years: better to keep the money for the renewal.
- On a damp or badly cleaned surface: the product does not penetrate and is gone by the first winter.
- Below 5 °C, or when rain is forecast within 24 h.
Actual duration
Eight to ten years on a tile in good condition, five to seven on a tile that is already porous, ten to fifteen for coloured roof coating applied properly. Those figures assume serious preparation: that is what makes the difference, not the product.
