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Various technologies and methods can be used to clean a roof; each has its own benefits and pitfalls.

Soft washing

Soft washing is a relatively gentle, low-pressure method that uses a chemical solution to kill and detach organic matter from a building’s exterior surfaces. It is widely considered to be a highly effective method of cleaning a roof without the need for technicians to stand on the roof as long as they can get within a few feet of the surface via a scaffold tower or mobile lifting platform. The low-pressure application of soft wash chemicals greatly reduces the likelihood of the roof incurring damage during the cleaning process or water ingress into the property. The soft washing process kills all organic matter and, provided it is undertaken by competent technicians, will leave treated areas spotless, sanitised, and last for a long time.

Soft Washing requires extensive training along with highly specialised equipment and chemicals to produce long-lasting, high-quality results. Access to the areas to be soft washed can be problematic in confined spaces as the solutions need to be sprayed from a few feet away rather than in close proximity to the surface.

Care must be taken to prevent soft wash solutions from entering drains or falling onto landscaped areas or gardens without immediate counter treatment to prevent

Scraping

Scraping is a method of moss removal widely used to remove the most visible organic growth from a roof. In many cases, it can be undertaken from the ground with the aid of an extending pole system. A metal blade shaped to suit the roof’s tiles attached to a pole is used to scrape moss and other debris from the surface. It is an effective method of removing heavy moss growth and – provided the gutters are cleared immediately after scraping – can reduce the likelihood of gutters becoming blocked.

The equipment required is minimal and inexpensive and can be undertaken with little experience or training. Scraping a roof can give immediate visual results but these are often short-lived because the roots of the organic growth are not removed by this process. This can promote the regrowth of moss to a greater degree than before – similar to pruning a bush in the autumn to promote strong regrowth in the spring.

The process of scraping a roof can disturb tiles and pointing; particularly as it is often undertaken by unskilled individuals from quite a distance away, making it impossible to clearly see any issues or damage.

Pressure washing

Pressure washing can be a highly effective method of cleaning all kinds of surfaces and many homeowners will already have their own pressure washer for car washing etc., making them familiar with the principle. Water is projected under high pressure towards the surface in question in order to remove the dirt, moss, lichen, and other debris. Pressure washing can yield visually compelling results with little experience or expertise from the operator.

The use of high-pressure water to clean a roof carries the inherent risk of water ingress to the structure beneath. Water being forced ‘uphill’ on a roof will be forced beneath tiles and into the timber battens, felt, rafters and purlins. A greater risk associated with pressure washing is that the pressure is often so harsh that it can damage and crack roof tiles and remove vital mortar pointing.
Pressure washing can produce visually pleasing results but it is highly unlikely to remove all of the organic matter; causing faster and more prolific regrowth in a relatively short timeframe.

Steam Cleaning

Steam Cleaning can be used in some circumstances to clean roofs but it is more widely used by cleaning specialists to clean stonework and certain types of cladding. Specialist steam cleaning equipment, given the nature of steam at high pressure, can be extremely dangerous in untrained hands or if the equipment is poorly maintained.

High-pressure steam presents risks to the structure which are similar to those of pressure washing; high pressure and steam can remove mortar pointing, crack tiles and cause other damage, although it is more likely to kill all of the organic material and slow down regrowth.

Access

Access to a roof for the purpose of cleaning should only be undertaken by trained individuals with the correct equipment. Roofs can be slippery and in most cases, it is illegal for operatives to stand upon a roof. The widely accepted best practice method of accessing a roof for the purpose of cleaning is via a mobile platform (also known as a ‘Cherry Pickers’) or with mobile scaffold towers.

Homeowners should make themselves aware of their liability in such circumstances and ensure any companies or individuals they enlist to clean their roof (or any other exterior surface) are adequately insured and has the correct equipment and training to do so.