Firstly I should state; this is in reference to the roof cleaning in the UK, concrete or clay tiles and not 'Shingles' as used in the States.
There seems to be a lot of mistruths on various websites at present in reference to the subject.
Without a doubt in the wrong hands a pressure washer can damage roof tiles. An incorrect nozzle, extreme pressure or High flow rates or a combination can remove the granular surface of a concrete tile.
However, an experienced roof cleaning company would know what lance / nozzle to use, set sensible water pressures and water flows and, as a result, would not damage the roof tiles. The key point is to use ‘controlled pressure washing’, set pressures and flow rates to remove the weathering from the tile without causing any damage to the surface. Please click here to view photos on our facebook page which shows close up images of roof tiles.
Several major tile suppliers have recommended not to use a pressure washer. The reason for this is quite simple; by stating this they are protecting their products from inexperienced or the DIY enthusiast from possibly damaging their tiles. In a similar way the majority of paving manufacturer’s state not to pressure wash their paving slabs. What do you do when they become dirty and slippery, you can’t apply chemicals, as again manufacturers do not recommend them, and do you replace them? No, you clean them by pressure washing!
The NFRC (National Federation of Roofing Contractors) do not recognize the discipline of roof cleaning, they do not see moss as a problem. However, in the real world heavy deposits of moss on the roof is a problem. Moss falls from the roof tiles into the guttering system which can cause blockages and possibly dampness. The cynic in me wonders if they have adopted this stance in order to protect their members i.e. roofers
Incidentally, the main reason that we get approached by homeowners to clean their roofs, is not a concern of the condition of their roofs. It is simply that the moss is falling from the roof onto their new conservatory or patio, which is understandably a nuisance and a problem. Hence, it needs to be removed to protect their investments!
In contrast to the NFRC, all roof coating / roof tile coating suppliers require the roof tiles to be pressure washed before coating in order to validate their guarantee!