Green deposits on surfaces explained: material comparison

Material comparison: susceptibility per surface type

Not every outdoor surface is equally susceptible to green deposits. Susceptibility relates to three material properties: porosity, roughness and moisture absorption. The higher these three properties, the faster deposits establish themselves and the deeper the attachment.

Concrete: porous but variable

Standard concrete is porous and absorbs moisture, making it a structurally vulnerable surface for green deposits. Deposits find grip in the surface structure and can penetrate deeply into the pores of older or damaged concrete surfaces.

The susceptibility of concrete varies greatly with the surface finish. Smooth polished concrete is considerably less vulnerable than rough cast concrete or concrete with an abraded top layer. Coated or impregnated concrete provides additional protection as long as the coating is intact.

When cleaning concrete, high-pressure cleaning is often effective on heavy deposits. For sensitive or older concrete surfaces, however, high pressure can damage the top layer. Water-based methods as a supplement give more control on sensitive concrete surfaces.

Sandstone and soft stone types

Sandstone is one of the most vulnerable stone types for green deposits. The material is soft, porous and absorbs moisture quickly. Deposits penetrate more deeply than on concrete and the material itself is sensitive to damage from overly aggressive cleaning methods.

Chemical cleaning agents are often unsuitable on sandstone. Acidic agents effective on lime deposits or concrete can attack sandstone. On sandstone, a soft brush with water or a mild water-based cleaning fluid is the safest approach. High pressure is generally inadvisable on sandstone.

The article on how green deposits form explains why soft stone types structurally have higher growth risks due to their moisture absorption profile.

Wood: organic and moisture-sensitive

Wood is naturally organic and thereby already provides a favourable environment for organic growth. Untreated or old wood is particularly vulnerable: it absorbs moisture, has a rough structure and offers many attachment points. Moss preferentially grows in cracks, fissures and unevennesses of the wood structure.

Deposits on wood retain extra moisture, accelerating the weathering process of the wood. Over the long term, untreated deposits contribute to rotting of the surface layer. Cleaning wood requires a gentle approach: a soft brush and water-based cleaning fluid. High-pressure cleaning damages the wood structure, particularly with softwood or aged wood.

Treated wood impregnated, oiled or lacquered is less vulnerable as long as the treatment is intact. Cracks in the finish layer immediately increase vulnerability.

Tiles and joints

The tile surface itself is barely vulnerable to green deposits on glazed or polished tiles. The smooth, dense top layer offers little grip and dries quickly. The joints between tiles are however particularly vulnerable: they are rough, porous and retain moisture.

Deposits preferentially establish themselves in the joints and spread from there over the tile surface. A targeted approach to the joints is therefore more effective than a generic surface treatment. Joints can be treated with a brush and water-based cleaning fluid or with low water pressure directed at the joint.

Unglazed or roughly finished tiles such as certain terrace tiles or concrete pavers are themselves also more susceptible to deposits than glazed tiles. The distinction between tile type and joint is therefore relevant when assessing.

Facades of brick, render and wood

Facades form a separate category due to the combination of material variation and vertical orientation. Although water runs off vertical facades, rain and wind-driven moisture can keep the facade persistently moist, particularly on north- and west-facing sides.

Rough brickwork and untreated brick are more susceptible than smooth render. Damaged render layers immediately increase vulnerability as moisture can penetrate more deeply. For facades, the adhesion strength of the finish layer is also relevant: overly aggressive cleaning damages the render layer or paint and makes the surface even more vulnerable.

Timber cladding requires the same gentle approach as other wooden outdoor surfaces. High-pressure cleaning is inadvisable on most facade materials unless the facade is specifically suited for high-pressure treatment.

Matching cleaning method to material

Water-based methods such as ozone water are applied on a wide range of surfaces as a supplement to mechanical cleaning. Effectiveness differs per material: on porous materials a longer contact time is relevant, on smooth materials a shorter treatment may suffice.

For a structured working sequence when cleaning outdoor surfaces, the two-cloth method offers a practical framework as an additional step.

Maintenance intervals per material type

Different materials require different maintenance intervals. On sandstone and porous concrete in shaded environments, deposits can visibly re-emerge within four to eight weeks after thorough cleaning. On smooth ceramic tiles in a sunny environment, the same surface may remain deposit-free for months.

Knowing the typical return speed per material type and per location helps in planning a maintenance cycle. Adapting maintenance timing to the actual growth pattern is more efficient than following a fixed calendar frequency regardless of the surface and seasons. A zone-by-zone assessment at the start of spring provides a good starting point for annual maintenance planning and helps prioritise effort.

Material type and choice of cleaning agent

The material type also determines which cleaning agents are permissible. Acidic agents that dissolve lime deposits on tiles can attack sandstone or limestone. Alkaline agents effective on concrete can damage sensitive finish layers on facades.

Water-based cleaning systems such as ozone water have a neutral chemical profile applicable to a broad range of surfaces. They can be used as a supplement to mechanical cleaning without risk of chemical damage to the material, provided concentration and contact time are correctly matched to the surface.

Articles in the green deposits basics cluster

This article is part of the basics cluster on green deposits. The other articles cover additional aspects:

• Removing green deposits - overview of methods and surfaces
• What is green deposit - definition and composition
• How green deposits form - growth factors and conditions
• Green deposits as a problem explained - impact on material

Context: cleaning with ozone water

For a broader understanding of surface cleaning with ozone water, cleaning with ozone water explains the principle and application possibilities.

The ozone water machine produces ozone water on site for professional cleaning applications without traditional chemicals.

Costs and affordability

Cleaning costs per surface type vary greatly. Smooth, less porous surfaces are faster and cheaper to clean than porous or vulnerable materials requiring more care. Water-based systems have a different cost structure than one-off chemical agents.

More information via the cleaning guide or via contact.

Experiences from practice

💬 "I treated my sandstone facade with a high-pressure cleaner and noticed afterwards that the stone had been damaged. Since then I use a soft brush with a water-based fluid - much better results and no damage." - User from Zeeland

Further reading

Continue with green deposits as a problem explained or view the full cleaning guide.

Metal, glass and dense plastics

Metal surfaces such as steel, aluminium and zinc are barely susceptible to green deposits as long as they are smooth and undamaged. The smooth rolled or sprayed surface structure offers few attachment points and dries quickly. Corrosion, damage or unfinished edges locally increase vulnerability.

Glass and smooth plastics are similarly low-susceptibility. They are non-porous, dry quickly and offer little grip to organic growth. In combination with roof or facade structures, however, they can serve as overflow zones for deposits that then establish themselves on adjacent more porous materials.

Combinations of materials on a surface

In practice, many outdoor surfaces consist of combinations of materials. A terrace has tiles and joints. A facade has brick, mortar joint and possibly wooden frames. A fence combines wood with metal fittings. Those combinations mean that susceptibility is not uniform across the surface.

The most vulnerable zone determines the cleaning frequency and method. For a terrace, the joints are the weakest point. For a facade, the transition from render to woodwork is a vulnerable zone. Knowing the combination allows more targeted cleaning and joint-specific or zone-specific treatments to be applied.

The article on what green deposits are describes the different types of organisms that can grow on surfaces and how they can be distinguished visually.

Testing moisture absorption: a practical approach

To understand how porous a surface is, a simple water test works well. A few drops of water on the surface give immediate insight: if the water is quickly absorbed, the material is porous. If the water sits as a droplet, the material is low-porosity or has a repellent coating.

This simple test is also useful when comparing two zones on the same surface. A zone with damaged coating absorbs more quickly than an intact zone. That gives direct insight into which zone needs extra attention during cleaning and maintenance.

The ozone water machine produces ozone water on site and can be used on a wide range of surface types as a supplement to mechanical cleaning methods.