There are several reasons why someone may consider using a high powered pressure washer on a roof:
- To brighten up/clean the roof
- To remove moss that has built up on the tiles
- In preparation for a roof coating/sealant
Why You Should NEVER use a Pressure Washer on Roof Tiles
The most common roof tiles in the United Kingdom are made from concrete and have a sandy/granular finish on the exposed side of the tile. This part of the tile is designed to gradually wear off until the tile needs to be replaced, usually after 80-100 years of normal weathering.
Using a high powered pressure washer may strip the surface of the tile and shorten it’s life expectancy.
Advice From Marley:
Marley Eternit have been producing roof tiles for over 100 years, they are one of the leading tile manufacturers in the United Kingdom. They have the following advice for homeowners and roofing contractors:
The use of high powered jet washes is not recommended as it can damage the surface of the slate or tile, thus reducing its expected life considerably.
With regards to roof moss they claim that:
In normal circumstances, the (moss) growths are not harmful to the tiles or slates. However, in some extreme cases this growth may affect the drainage of water down the roof. In such cases, they should be removed to avoid possible water ingress into the roof space.
In a response to an email enquiry made in February 2014 Marley made the following statement about one of their popular concrete roof tiles(the emphasis/capital lettering is theirs):
We DO NOT recommend pressure washing your roof covering.
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Email Response From Forticrete
Forticrete are a large producer of concrete building products including roof tiles. They responded to an email enquiry with the following statements:
As a manufacturer of concrete roof tiles we would not advocate power washing or recoating tiles as this would invalidate any guarantee.
Power washing the tiles will remove fines in the concrete which will affect the lifespan of the tiles.
Advice From NFRC (National Federation of Roofing Contractors)
The NFRC was established over 120 years ago and in 2012 they issued this statement:
Most ‘techniques’ that we have come across to remove moss, are often achieved by ‘violent’ means such as power-washing, which may cause damage to the roof, especially when combined with unnecessary foot traffic often leading to cracked or broken roof tiles.
They go on to add:
While NFRC is a dedicated roofing trade association that has been around for over 120 years we have never recognised the practice of moss removal or roof painting as a true roofing discipline. The reason being, in most instances, moss on roof tiles is rarely a problem and only needs to be removed if it is blocking gutters, outlets and other drainage points on the roof.
Some more common sense advice from the NFRC:
The removal of the moss is often followed up by painting the roof tiles. There are claims that this can increase the thermal efficiency of roofs as well as extending the life of the roof tiles, but unless the products carry credible UK based, third party accreditation we would urge members of the public to view any claims with extreme caution. Our advice, therefore, to property owners who are concerned about the condition of their roof is to contact a reputable roofing contractor for a proper assessment and, if necessary, to obtain at least three quotes for any works, using conventional roofing methods.
Dudley Council Warning
Dudley council have issued a press release and a seperate pdf help leaflet highlighting mis-selling within the roof coating industry and also reiterating that jet washing a roof can damage the tiles:
These claims are largely unsubstantiated and independent experts say that lichen, mould and similar growths although unsightly are not generally detrimental to the longevity of a roof tile and that coating paints do not have a significant benefit in reducing heat loss. (taken from the press release)The following is taken from their pdf leaflet:
Roofing experts warn that roof tiles should not be subjected to high-pressure
washing or jet washing for many reasons which include: Removal of Lichen and moss by jet washing will invariably remove the granular surface finish and potentially damage the structural integrity of
the tile, this process followed by a surface treatment application would
invariably invalidate any tile manufacturer’s guarantee. Interlocking roof tiles are not designed to be walked upon, foot traffic
can cause these interlocks to break opening the roof up to water
ingress. Inappropriate pressure washing can potentially force water up between
the tiles and roofing felt into the roof space. Bedded mortar and
flashings can also be blasted off using this method, potentially leading
to further costs for rectification.
When using a high powered pressure washer there is a risk of flooding to the loft space. While a typical roof is designed to withstand the elements, it isn’t designed to cope with water being ejected from a nozzle at such high speeds.
The water may penetrate past the overlaps in the tiles and enter the roof.
That is exactly what happened to a property that was featured on BBC 1’s Rogue Traders programme.
BBC Rogue Traders
The roofing company featured on the BBC programme didn’t check the condition of the felt before they blasted the roof tiles with a pressure washer. The water not only penetrated past the tiles but leaked through the old felt and puddled on the loft insulation.
Luckily this insulation prevented the water from getting onto the ceiling where it could have dripped into the house and onto electrics such as light fittings.
Read about this story on the BBC website (opens in new window)
If you are a homeowner looking to remove roof moss or otherwise brighten up your roof then there is another option. You will be pleased to know it doesn’t involve any pressure washing and there is no flood risk at all.
Read more on how to remove roof moss without pressure washing.
You may wish to explore these pages:
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(Page updated February 2014)