Breaking the Mould
Mould is easier to clean than the soot of old, but with the return of clean air, nature has reclaimed ground in a way not always foreseen. Problems sometimes arise when a building does not receive its full share of sun. In other places there are not enough sunny days in a year to stop deleterious growth. The issue does not affect all materials in the same way. Most roofing materials will someday need cleaning, but not all, and not at the same intervals. Choosing a roofing material with regard to its susceptibility to mould and moss encroachment is good design practice in shady locations. Below is a brief summary of material behaviours towards biological growth and reactivity to a non-aggressive biocide treatment.
Natural slates are not porous and display a smooth surface. As a result the material dries rapidly and is not naturally prone to roof growth with the notable exception of yellow lichen in coastal environments. The material cleanses readily upon treatment. It is a product of choice for shady locations.
Fired clay of good quality is not porous. The surface dries rapidly and sheds the bio film satisfactorily when treated. The roof surface is however more textured than slates offering intersects where growth can take a foothold.
Old clay tile roofs are fragile and are best treated from the eaves.
Concrete tiles are not indicated in the vicinity of tall trees. The high porosity combined with a rough surface provides a natural host surface. Removing mated moss from concrete tiles is labour intensive. The material cleanses well, but more slowly than other materials. The alkalinity of the surface seems to benefit black blanket moulds. In recent years smooth and less absorbent concrete tiles have arrived on the market. They are a better choice for higher humidity regions.
Zinc is not a natural host to live contaminants. It is however susceptible to organic acid releases from rotting leaves, moss and lichen and should be avoided on lower draining slopes and box gutters.
Lead is also not a natural substrate for life. It is vulnerable to mild organic acids but paradoxically resistant to harsher mineral ones. Mossy tiled roofs draining into lead gutters will eventually jeopardise its integrity.
Copper has biocide properties. Water running over copper will acquire some of its cathodic properties by contact and hampering further downstream growth. The protection may not be sufficient to maintain the tiles in pristine condition along the whole rafter length. On canal tiles the protection is limited to the trough, where it is still the most needed.
Aluminium is not particularly affected by acid releases. Water running over it will become anodic and display the reverse effect to that observed on copper. The illustration of this is seen in the splash area of TV aerials.
Flat roof felts of bituminous kinds are not unduly prone to moss, but ponding, cluttered designs and raised walkways will host them. PVC felts become very slippery, but as is usual with algae, the treatment is rapid.
Organic materials are best used in full light where they can dry rapidly. Durable presence of mould and algae will eventually jeopardise their integrity as roof covering. As the lining of wood shingles washes away, the grey cellulose needles are exposed and offer a slightly abrasive surface hosting algae. The treatment will reinstate the silver grey appearance for some time. Thatch, if badly affected will need combing beforehand.
For the existing stock, periodical maintenance is the solution. The general wisdom is to opt for the milder cleaning processes before considering harsher ones. Blanket moss has to be removed by hand, bagged and disposed of in all cases. The tiles can then be treated by the appropriate biocide.
After the treatment, the dead bio-film will begin to loosen under diurnal and seasonal cycles, until the roof has regained the true original colour of its material. The rate of cleansing will vary greatly with the type of material, and the exposure to wind and driving rain. Not all chemicals are appropriate, and a few only authorised by the Health and Safety Executive (HSE). All formaldehydes, caustic soda or bleach based chemical are proscribed.
The application method and knowledge is as important as the biocide itself. Jet cleaning is seldom appropriate to roof cleaning. The pressure removes surface aggregates, creating a moisture retaining surface favourable to the return of the contaminants. Tile impregnation and coatings are ways of mitigating the damage done by jetting, but at a cost sometimes higher than simply replacing the tiles by new ones.
In conclusion, the return of clean air means new parameters for the choice of roofing materials by the Architectural Technology professional, and the specification of a non aggressive maintenance procedure for existing roofs. This is easily searched for on the Internet. A comprehensive approach to roof cleaning and the provision of specification support is a sign of an experienced specifier.
The author is Managing Director of TVSPLtd, which specialises in a roof cleaning process known as the Algoclear Pro Roof System, supplier to HD Roofing Services Ireland.