Moss is not automatically a problem, and the wrong removal method causes more damage than the moss ever would. Here is the honest position.
When moss actually matters
Moss matters when it blocks things. Clumps breaking off and washing into the gutter cause overflow, which is the single most common real world consequence. Moss packed into the joints of interlocking tiles can hold water against the tile and, in a hard frost, force the joint open.
It matters more on shallow pitches, on north facing slopes that never dry, and under overhanging trees. It matters far less on a steep south facing slope where it never gets thick enough to hold water.
On old clay or natural slate, a thin covering of moss and lichen is largely cosmetic and, in conservation areas, part of what the roof is supposed to look like. Scrubbing a Cotswold stone slate roof back to clean stone can be actively unwelcome to a conservation officer.
Moss on a roof that is nearly at the end of its life is also not worth spending money on. If the covering is due for replacement in a couple of years, clean the gutters instead and put the money towards the roof.
How to remove it without damaging the roof
Hand scraping with a soft tool, working down the slope, then clearing the gutters and downpipes afterwards, is the safe method. On concrete tile a biocide treatment applied after scraping slows regrowth, and the moss then weathers off over the following months rather than being blasted off in a day.
Never pressure wash. High pressure strips the granular surface off concrete tiles, which is the layer that keeps them weatherproof, and it drives water up under the laps and into the roof. A roof that looks brilliant after a jet wash is frequently a roof that has been shortened by ten years.
Hand removal with gutter clearance typically runs 350 to 850 pounds on a standard house depending on access and how heavy the growth is. A biocide treatment adds 150 to 400 pounds.
On a shaded roof, expect visible regrowth within three to five years. Fitting copper or zinc strips near the ridge slows it, because rainwater picks up traces of the metal and inhibits growth down the slope.
Does moss damage roof tiles?
Indirectly, mainly by holding moisture and by blocking gutters. On sound tiles, moderate moss does far less harm than an aggressive cleaning method would.
Is pressure washing a roof a bad idea?
Yes on concrete tiles, where it strips the protective surface, and yes on old slate, where it drives water under the laps. It is one of the most common causes of premature roof failure we see.
How much does roof moss removal cost?
Typically 350 to 850 pounds by hand including gutter clearance, plus 150 to 400 pounds if a biocide treatment is applied afterwards.
Do copper strips really stop moss?
They slow regrowth on the slope below them rather than eliminating it. They work best as a follow up after a proper clean, not as a cure on their own.
Should I remove moss before selling the house?
A tidy roof presents better, but do it by hand and well in advance. A freshly jet washed roof can raise more questions with a surveyor than a naturally weathered one.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.