Green growth on paving, a terrace or a north-facing roof slope is algae, moss, or both. They grow where three things come together: moisture, shade, and a surface to hold on to. Take any one away and the problem goes on its own; leave all three and it comes back after every wash.
Washing is easy. What lasts is what you do about the cause - so start by asking why the growth appears in this particular spot.
In short
This is not dirt carried in from outside.
Algae come back wherever the shade and damp remain.
Shade and standing water are the cause.
Drainage, a cut-back shrub and a cleared gutter do more than a litre of anything.
The slipperiness goes with the growth.
If a surface is slippery only after rain and in shade, that is usually biofilm, not the material.
A sealer is not a moss treatment.
A sealer limits water uptake, but the growth has to come off with a cleaner first.
Cause first, preparation second
Walk the area and check three things. Does water leave? A puddle still there a day after rain is a ready-made habitat. Does sun reach it? A branch over a terrace keeps it damp all day. Is something running down onto it? A leaking gutter or sill makes a vertical green band no preparation will stop.
The difference is measurable: with the cause removed, washing lasts several years; with it left in place, every season, at the same cost each time.
Where it grows and what that usually means
| Place | Usual cause | What beats washing |
|---|---|---|
| Paving against a north wall | Shade all day | Cutting back planting, if that is what shades it |
| A band along a gutter or sill | Water running down from above | Fixing the leak |
| A hollow in a terrace | Water standing after rain | Improving the fall or the drainage |
| Joints between slabs | Moisture held by the joint | Topping up and compacting the joint |
| North-facing roof slope | Shade and slow drying | Usually the clean itself; the conditions will not change |
What to wash it with
For facades and roof tiles, the proven CTS Chemistry cleaners clear growth effectively: C-11E for facades and C-11D for clay and concrete roof tiles. On paving, terraces and other horizontal surfaces, where algae come with other organic soiling, D-12 or EC-87 do an excellent job.
Check the dilution and dwell time in the product's technical data. Before you wash a whole wall or roof, test it on an inconspicuous patch.
How to wash a roof and a wall step by step is covered in our guides to cleaning moss off roof tiles and cleaning algae off a facade.
Preparations for green growth
Effective cleaners for green growth, organic soiling and facades.
PremiumŚrodek do czyszczenia elewacji i fasad Guard Industry
329,00 zł
62,45 zł/L
PremiumSolvent-based block paving sealer, wet look SK-2 CTS Chemistry
60,27 zł
54,12 zł/L≈ 7,22 zł/m²
StandardFacade and render cleaner C-11E CTS Chemistry
20,66 zł
19,43 zł/L
StandardRoof tile cleaner C-11D CTS Chemistry
33,21 zł
24,35 zł/L
PremiumSolvent-based sealer for concrete fencing EI-13 EKSIL
232,47 zł
46,14 zł/L≈ 5,43 zł/m²
StandardSilane facade sealer that deepens colour EH-16 EKSIL
145,00 zł
25,75 zł/L
Does sealing help
Indirectly. Moss and algae need moisture, and a sealer limits water uptake - a surface that absorbs less and dries faster is less hospitable to them. A sealer does not remove the cause, though: with standing water and full shade the growth will come back regardless. Treat it as a complement to changing the conditions, not a replacement for it.
If you are doing both, the order is fixed: remove the growth, then dry and prepare the substrate, and seal last. A sealer applied over algae will shut them in underneath.
Common questions
Does moss damage block paving?
The growth itself mostly spoils the look and makes the surface slippery. The damage can come from moss roots, which widen the joints, and from the moisture the growth holds - especially over winter. How fast depends on the sub-base and the drainage.
Will soda or vinegar remove moss from paving?
Sometimes, on small areas - but they are hard to control, and on coloured paving and limestone acids can do damage. A ready-made cleaner is more predictable: it comes with a set dilution, dwell time and a list of the substrates it is made for.
How often does it have to be repeated?
As often as the conditions return. With drainage improved and sun let in, it can be every few years; in permanent shade over standing water, every season.
Can roof tiles be cleaned without going on the roof?
Partly, yes - some roof tile preparations are sprayed on from a ladder or a platform. Working at height is a risk of its own: on steep slopes, hire a professional.
Is green growth the same as efflorescence?
No. Efflorescence is white and comes from inside the concrete; growth is green and lives on the surface. They are removed with different preparations - we gave efflorescence a guide of its own.
