Problems with lichen
Where and when the air that we breathe is highly polluted, moss and lichen struggle to survive, but where the air is clean they will survive, accordingly as the clean air act takes effect the problem of lichen on roof materials is increasing.
There is no issue with the erosion itself on a building, but the acidic releases will affect all natural and manmade roofing products including bituminous sheets, metals, lead in particular, and are known to damage the galvanisation of steel components.
Left untreated, lichen can cause damage to the colour pigmentation and resin coating to bituminous sheets causing degradation to the sheet structure, particularly in cases when moss is also allowed to build up in the corrugation bases which results in the high moisture content at this point and as a result softens and degrades the sheet structure and can reduce the service life of the roof covering, any moss on the roof should should be removed with a stiff bristled brush and the area treated as required.
Air pressure forces are generated on the roof as wind pass over it; ‘Positive’ pressure on the windward side of the roof and ‘Negative’ pressure (suction) on the leeward side.
These forces act with high (ridge) and low level (eaves) ventilation to create convection or stack ventilation within the roof space, which will under normal circumstances expel water vapour from within the roof space restricting condensation formation. When this ventilation is not provided a warm roof structure must be provided with internal air extraction provision.
Restoration of damaged surfaces
Problems with moss
Moss is a very simple plant form which does not have roots or means to collect or move water around inside its form. It therefore has to absorb the moisture directly from the roof surface, this is why it thrives in damp shady places, such as under tree cover or in gutters.
As moss requires collected nutrients draining down the roof sheet in rainwater to exist, it is associated with wet conditions on north facing roofs, or shaded roofs under overhanging trees providing moist leaf mould on which moss thrives.
Generally whilst you find lichen growing on the exposed surfaces of roofs moss prefers damp sheltered areas. Gradually Moss feeding on nutrients present in the water draining down the roof will grow and in effect build a ‘dam’ in the sheet corrugations in order to retain more moisture.
Problems with mould
Mould is ever-present in the environment it is spread by tiny spores which float in the air. It can establish and grow on almost any receptive surface that spores land and find moisture allied to a constant temperature, between 40 and 100 degrees F.
In theory this includes about every place in a domestic situation that a damp environment is maintained from shower rooms to damp fabrics left in a badly ventilated position.
You can easily spot the most visible type of mould called mildew, which begins as a grey coating, this forms into black spots which in turn grow to form larger colonies. This is normally the fungus you notice in grout lines in a badly maintained shower, or on damp walls, and outdoors on the surfaces such as decking and timber particularly if situated in damp and shady areas.
Caring for Onduline Roof Sheets
Preparation: Care must be exercised in preparing the Onduline sheets to be treated, ventilate the building and clear and prepare all surfaces in contact with the mildewed area. Next remove or protect goods or fitments below the area to be treated with a suitable tarpaulin. Finally isolate and protect any electrical goods adjoining the area to be cleaned, if in any doubt seek professional advice in making safe all electrical items. In undertaking this work wear suitable protective long sleeved overalls or old clothes, which should be either cleaned or disposed of immediately after use, it is also advisable to wear work or old shoes. Always wear protective goggles and gloves and use a suitable disposable respirator.
Cleaning process: Treat the underside of the Onduline sheets and timber surfaces in contact with the sheet with a diluted bleach solution (maximum recommended concentration of 1 part bleach to 8 parts water). Use a brush to apply this solution to the affected areas rinse off the solution with clean water after 30 minutes.
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