There is a right way and a very wrong way to clear a mossy roof. This guide covers the three removal methods, the damage a pressure washer does and what the work should cost.
Decide first, clean second
Moss is not automatically a problem. On many roofs it is cosmetic and removing it carelessly does more harm than leaving it. Our guide on whether to remove or leave moss goes through that decision, and this guide picks up once you have decided it needs to come off.
The usual reasons to act are moss shedding into gutters and blocking outlets, growth lifting the tail of slates or plain tiles, and thick mats holding water against a north facing slope through winter.
Method one: hand scraping
Manual removal with a stiff brush and scraper is the safest option for clay, slate and Cotswold stone. It is slow, it needs scaffold or a roof ladder, and it does not sterilise the surface, so regrowth begins within a couple of years.
The advantage is control. Nothing is forced under a lap and nobody is standing on brittle slates with a lance. On heritage coverings it is the only method we will use.
Method two: soft wash and biocide
A low pressure application of a roof safe biocide kills the growth at the root and lets weather wash the residue off over the following months. It looks like nothing has happened on the day, which is why some homeowners dislike it, and then the roof clears steadily.
Biocide is the better long term answer because it slows regrowth for several years. It should be applied on a dry, still day with planting protected below, and it is not suitable immediately before rain.
Why pressure washing costs you money
A high pressure lance strips the surface layer from concrete tiles, taking the granular finish that protects them from ultraviolet light and frost. It drives water under laps and into the loft, breaks the tail of old slates and blows mortar out of ridge bedding.
We see roofs that were cleaned aggressively five years earlier and are now visibly porous and shedding. If a firm offers to jet wash your roof cheaply, the saving today is borrowed against the covering's remaining life.
Costs and what should be included
Hand clearing a typical three bedroom semi runs about £450 to £900 depending on pitch and access. Biocide treatment alone is usually £250 to £550. Combined clearance plus treatment on a larger or steeper roof can reach £1,200.
A proper quote includes gutter clearance afterwards, because the debris has to go somewhere, and a check of ridge bedding and tile condition while the roof is accessible. Fitting a copper or zinc strip below the ridge is a cheap addition that slows regrowth.
How often does moss come back?
On a shaded or north facing Oxfordshire roof, visible regrowth typically returns within two to four years after scraping alone, and rather longer after a biocide treatment.
Does moss removal damage the tiles?
Careful hand work does not. Pressure washing does, and on old clay or stone slates even heavy handed scraping can break tails, which is why the method has to suit the covering.
Can I treat moss myself from the ground?
Ground applied sprays reach very little of a pitched roof and encourage people onto ladders they should not be on. If the roof is only lightly mossy, clearing the gutters and leaving it alone is often the better call.
Will removing moss stop my roof leaking?
Only if the moss was blocking drainage or lifting tiles. If water is entering through failed flashing or cracked tiles, cleaning the roof will not change anything.
Do you clear the gutters as part of the job?
Yes. Removing moss without clearing what falls into the gutters simply moves the blockage, so gutter clearance is part of the work.
We will tell you whether it needs clearing at all, and if it does, which method suits your tiles. Quotes are free and there is no upselling.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.