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Blog
Breaking the Mould
| Posted on 10 January, 2013 at 5:14 |
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. Manmade
slates offer
some of the advantages of natural slates, but not always to the same degree. Fired clayof 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 tilesare 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. Metals:
Zincis
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. Leadis 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. Copperhas 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. Aluminiumis 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 feltsof 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 materialsare best used in full light where
they can dry rapidly. Durable presence of moulds 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 nonaggressive
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
Mossgo Roof System, supplier to HD Roofing Services Ireland. |
Categories: Effects of moss growth., Roof maintenance
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