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When you look closely at roofs you can find
grass, tomato plants, and stonecrop growing in the little crevices, but
the most common life forms found are lichen and moss. Lichen and moss
are generally not an issue where new roofs are concerned but are very
much an issue where old roofs and extensions to existing roofs are
concerned. We know what they look like, but what should we do about
Lichen and moss are very different, but may occur next to each other on
the same roof. Lichen is a composite organism where a fungus and either
an algae or bacteria form a very close friendship and live off each
other. Lichen can survive the very harshest of climatic conditions and
is rarely killed by long drought conditions. The one thing that will
kill lichen is atmospheric air pollution.
There are approximately 15,000 species of lichen but
the one that is most common on roofs is Ascomycetes Foliose. This form
of lichen starts as a small yellow or orange dot and gradually grows out
from the centre getting bigger and bigger leaving the centre to go white
grey (which is dead).
Moss on the other hand is a very simple plant form that
does not have a root or means to suck up or move water around inside its
form. It has to absorb the moisture directly and that is why it likes
damp shady places. Of the 12,000 species of moss the most common moss
found on roofs is Bryophyte Andeaeaceae which grows in green humps, and
has little lantern shaped spore capsules in place of flowers.
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
In many instances the start of moss or lichen
growing on roofs is bird droppings. Look under any TV aerials that are
fixed to chimneys and the volume of lichen and moss is at its greatest.
The nutrients in the bird droppings give the moss or lichen a good
starting point. If the surface of the tile or slate is rough then it
gives the nutrient something to stop it being washed off the roof. Both
moss and lichen like a slightly alkaline to an acidic surface, as the
combination of acid rain and alkaline tile (concrete) produces a very
weak salt solution which it likes. That is one of the reasons that you
see a lot of moss and lichen on houses near the sea. Both of the
organisms and plants like wet conditions, therefore they tend to grow
better on north facing roof slopes.
Moss and lichen will grow on slates and clay tiles but
their preference is concrete tiles or fibre cement slates/sheets.
Generally you will find lichen growing on the exposed surfaces while the
moss will grow in the corners, interlocks and head laps, where they
shelter and get more water, especially the side laps of concrete
interlocking tiles. Gradually moss will filter the water flowing down
the interlock, collect the dirt and eventually build a dam which will
force water flowing down the interlock to discharge into the second
In severe cases the second interlock becomes blocked
and the water flows off the side of the interlock and sometimes drips
off onto the underlay especially where there is a tile clip.
Moss and lichen will not eat its way through a roof
tile or slate but will capitalise on any defect that there may be in the
material, such as a small crack.
Moss, unlike lichen, which will grow
out from the centre and finally die, will not die out naturally, as it
is both hardy and slow growing and needs positive action to kill it.
Therefore prevention or cure tactics need to be put into place, such as
washing weak solutions of copper oxide down the roof every time it
rains. Copper oxide is very toxic.
This can be done using a wide copper flashing at
the head of a roof slope. The wider the flashing the greater the
coverage. A thin copper wire will only protect the top 300- 400mm of the
roof while a 300mm deep flashing should protect up to 8m of rafter
length. After about five or six years when the original polymer coating
on concrete tiles has dissolved away, or after cleaning off old moss and
lichen, it would be worth treating the surface of the tiles with a
biocide (a cocktail of fungicide, bactericide and algaecide) or a very
strong horticultural disinfectant like Jeyes Fluid. This should be done
on a dry day so that it can soak into the surface and dry. With natural
slates and clay tiles this could be done from new as they are not
Tiles that have been cleaned can look a little bald or
discoloured where the surface granules have come off or where the moss
has been shielding the tiles from the sun. After a few years the
elements will weather the surface back to a more even colour and
texture, but the tiles will never revert to their original colour or
The presence of moss and lichen on a roof can make the roof look old but
can also affect the performance of the roof by preventing water run off
from draining out of laps and interlocks. Moss and lichen will reduce
the life expectancy of the roof by a few years over a 100-year life
expectancy and is therefore negligible, but should be discouraged from
forming in the joints, and should be cleaned off when it has become too
- Prevention is better than cure, so
choose a roof covering that is less attractive to moss and lichen
spores by being smooth, and is on a steep roof pitch.
- Scrape and brush as much moss and
lichen growth off the roof, especially from head and side laps,
before treating the roof in any way.
- Take care when power washing a roof
that you blast down the roof, not up the roof, to reduce the amount
of water and debris that is forced through the joints onto the
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774