Last edited 11 Nov 2020

Pattern staining and soiling in stonework



Stone is a tremendous material used in the construction of many period properties. With time, stonework weathers which arguably contributes to the overall character and appeal. Although naturally resistant, stone and other materials typically used in construction must be adequately maintained.

Worryingly, the likes of sandblasting, chemical treatments, sealants and repointing with inappropriate materials are becoming increasingly common. Many are inappropriately sold by inexperienced tradesmen and the long-term effects can be damaging. Caution should be exercised and a professional opinion ought to be sought by anyone looking at cleaning stone or associated materials.

An overview of pattern staining and soiling

Pattern staining occurs when a foreign composite settles or emerges on the internal or external surface of a material.

There are various causes of pattern staining, such as pollution deposition, salt expulsion, staining from other materials, and so on. Algae soiling is a random growth on buildings that is caused by organic biological deposition on surfaces, such as moss and lichens.

Staining and soiling can sometimes be seen as adding character to buildings; however, others argue that they are unsightly. Usually, these cause detriment to building surfaces, and ought to be rectified.

The most likely causes


Typical materials used in the construction of period buildings, such as; limestone, flint, timber, lead, cast iron, and clay are able to withstand rainfall by either being impermeable or having a natural absorption / expulsion process. These processes depend on adequate exposure to the sun and good ventilation. However, too much rainwater,such as from overflowing gutters or down pipes,can exhaust the material’s adaptive processes.

Accordingly, masonry areas that are poorly ventilated, lack sunlight, or become oversaturated from rainwater can darken and stain. Stains develop deeper in materials the longer they remain oversaturated becoming more difficult to eradicate. Surrounding mortar will begin to lose strength and be more easily washed away, thus compromising structural integrity.

Clay tiles are a common foundation for the growth of moss. Moss can be cleaned away from the surface but it would advisable to avoid using sealants that many tradesmen will promote. The types of sealants typically used may trap in moisture and seal in unwanted foreign particles that have not been adequately cleaned away. Building materials should be able to breathe,allowing moisture absorption and subsequent evaporation.

Pollution and acid rain

Soot and dirt in the atmosphere from chimneys, motor vehicles etc, can settle on masonry. Certain pollutants (such as bird droppings, mineral, or vegetable deposits) can contain acidic components or soluble salts, which can deposit directly on stone causing staining and/or erosion.

Pollution may mix with rainwater in the atmosphere creating acid rain. Acid rain coming into contact with the a stone surface tends to produce gypsum by a chemical reaction. This allows dirt and soot to bond to stone surfaces, causing staining. In open areas, normal rainwater may wash away the staining; however, in sheltered areas with limited access to fresh water, an impermeable skin can form from the acid rain and result in cavitation of stone material.

Acid rain can cause corrosion staining to run off lead. Lead materials such as flashing, may well be joining a stone wall and the run off can stain the stone.

Limestone sills are typically vulnerable to acid rain. Acid rain chemically reacts with the calcium carbonate present in limestone, causing run-off white staining from lime salts. Crystallisation of the lime salts may contribute to further decay. Lime run-off stains can be cleaned immediately with water and brushing. However, if left to carbonate,rectification becomes more difficult and a more abrasive tactic using chemical agents may be required.

Air abrasive techniques are a common physical method along with wet techniques such as washing, steaming and brushing. In addition to physical methods, it might be wise to consider whether chemical techniques would be worthwhile.

If the soiling is non-water soluble, then air abrasive techniques can be adopted which include techniques using a variety of machines.


Iron staining to stone

Cast iron typically requires treating every 3-5 years to prevent rusting and decay. Failure to do so will compromise its functionality via corrosion. Not only will rust stain adjoining stone, but the functionality of the original iron material will be compromised. In coastal areas with higher atmospheric salt content, corrosion will be accelerated. Precautions for additional maintenance would be appropriate.

Routine weather proofing and maintenance should avoid failure. Filters could be added to the top of drainpipes to minimise build-up of debris in the gutters and prevent subsequent water stagnation and rusting of pipes. Where pipes have broken or there is extensive decay, replacement might be required.

Metal embedded in structure derived from masonry or mortar can also produce iron staining. This can usually be cleaned mechanically unless the staining is well bedded and the material is particularly strong.


Salt crystallisation

Salts are found naturally in the atmosphere and within the chemical composition of materials. External salts can accumulate or deposit on surface materials, while internal salts can leech out via efflorescence.

Efflorescence takes place when salts escape a porous material when drying gradually. Typically the appearance will be a powdery residue on the surface. Rates of evaporation can influence the appearance and distribution of salt in materials, often noticeable by white curves. It is important to note that excessive salt crystallisation within pores exerts internal pressures on materials. This could result in subsequent damage such as cryptofluorescence.

Salts could be absorbed into the porous material from the atmosphere, or from another material used in the fabric of the building such as the mortar. If the stained wall is close to a road, then salts could be splashed and deposit on the wall’s surface.

Salt is water soluble and can generally be cleaned using abrasive water techniques. Many try to rectify salt crystallization by painting masonry which is not advisable as moisture and salt locked in could cause an increase in internal pressure, resulting in spalling.

Organic biological growths

Organic growth, such as the growth of moss, lichen, and algae tend to affect the transpiration rate of materials (they absorb rainwater, preventing drainage). This presence of organic growth offers a moist bed which may promote the growth of damaging creeper plants (such as ivy). Ivy has been known to cause cracks in masonry and mortar. This is especially damaging in cold weather as the cracks filled with moisture may freeze and the expansion during the freezing process can cause further cracking and/or spalling.


The presence of moss may be especially damaging to tiled roofs where moss facilitates moisture ingress into timber batons causing premature decay. Moss may also fall, or spread into gutters which can lead to them overflowing. Mosses are known to secrete acids, which can etch into some stones causing decay.

Maintenance will limit and prevent further organic growth. Using sealants on clay tiled, or any tiles roofs should be discouraged. If biological growth is present, then it should first be determined whether it actually presents a risk to the building’s integrity prior to treatment as the rectification process may prove more damaging than the original growth.


Before remedial works are to be considered, the opinion of a professional surveyor or masonry specialist ought to be sought. It should be ascertained whether the presence of staining is damaging enough to warrant remedy as invasive remedial work can itself be damaging.

Prevention measures (such as sealants) are not always ideal and can be more damaging in the long-term. Staining and weathering to masonry can add to the aesthetic appeal of a building but careful consideration should be exercised in assessing whether that appeal is at the expense of structural integrity.

--Gary Yeardley

Related articles on Designing Buildings Wiki

Designing Buildings Anywhere

Get the Firefox add-on to access 20,000 definitions direct from any website

Find out more Accept cookies and
don't show me this again