If you live anywhere between Shirley Hills and Monks Orchard, you have probably looked up at some point and noticed the back slope of your roof has gone a solid dark green. The question worth asking is not how quickly you can get it off, but whether it needs to come off at all, and what a careless job could cost you. Here is how we look at it when someone calls us out.
Shirley has more mature tree cover than most of South East London. Oaks, pines and sycamores line Wickham Road, Shirley Church Road and the roads running up towards the Addington Hills, and all that shade and leaf fall lands on roofs that already stay damp for months. On the 1930s semis and post war estates that make up much of the area, a north or east facing slope will barely dry out between October and March. Moss takes hold in the mortar joints, along the tile laps and behind the ridge.
The moss on its own is rarely what damages a roof. Problems start when clumps break away in heavy rain, wash down the slope and pack solid in the gutters and hopper heads. The gutter then overflows against the wall, and a few winters later you have a damp patch on a bedroom ceiling and green staining down the render. A fair number of the roof cleaning enquiries we get actually start life as a damp complaint.
A thin covering of moss on a sound, well laid roof is a cosmetic issue, not a structural one. If someone tells you moss is eating your tiles, be sceptical. What matters is where the moss is going and what state the roof is in underneath it.
A petrol pressure washer will strip a roof clean in an afternoon and look spectacular on a before and after photo. The trouble is that concrete tiles of the type fitted across Shirley in the 1930s and again in the 1960s and 70s have a thin surface layer over an aggregate core. Blast that off and you expose a rough, porous surface that holds water and grows moss back faster than before. High pressure also forces water sideways under the tile laps and up into the roof space, and it dislodges old ridge bedding.
The approach that lasts longer is slower. Moss is scraped off by hand from a tower or roof ladder, the gutters and valleys are cleared out properly, any slipped or cracked tiles are reset, and a biocide is applied that kills the remaining spores over the following weeks so the roof weathers clean rather than being scoured clean. On a lot of older Shirley roofs the sensible extra is re-bedding or repointing the ridge and verge while the access is already up, because that is usually the part actually letting water in.
Prices depend far more on access than on roof size. A standard three bedroom semi with clear ground at the sides is straightforward. A terrace with no side access, a rear conservatory in the way, or a steep roof needing scaffold rather than a tower will all push the figure up. Treat these as broad ranges to sanity check a quote rather than a price list.
Also worth knowing locally: many of the larger trees around Shirley are covered by Tree Preservation Orders and much of the area near the Hills falls within conservation controls, so do not let anyone lop back an overhanging branch on the day without checking with Croydon Council first.
Expect a treated roof to stay reasonably clear for around three to five years in this area, less if you are directly under trees. Clearing gutters each autumn once the leaves are down, and keeping branches from actually resting on the tiles, does more for a roof than repeat cleaning ever will. Anyone offering an annual clean on a roof that does not need one is selling you a subscription, not maintenance.
Late spring through to early autumn is ideal, because biocide works best on a dry roof with a few settled days afterwards. Winter cleaning is possible but the treatment is less effective and access is more hazardous in wet or icy conditions.
Sometimes, if the damp is caused by gutters overflowing against the wall. If the damp continues after the gutters are running clear, the cause is more likely to be failed ridge bedding, a slipped tile, or defective pointing, and that needs looking at separately.
Gutter work at normal semi height can often be done safely from a tower. Anything involving walking the roof, working at the ridge, or a steep or high roof should have proper scaffold, and it is reasonable to ask how the work will be accessed before you accept a quote.
Tell us about the job and we will get a quote back to you, most days the same day.