We receive a lot of enquiries for cleaning old roofs, typically with plain concrete tiles built around the 1940s, close-boarded construction so no roofing membrane. These roofs are suffering from a build-up of moss which is falling from the roofs blocking guttering and falling onto conservatories, patios, and driveways.

There is a common misconception which is promoted by builders and roofers, that under no circumstances roof tiles should be pressure washed. The reason they give is that the pressure washing removes the ‘protective coating’ or finish of the tiles, and the tiles will no longer be guaranteed. What they fail to mention or accept is roof tiles on properties of this age have already lost their ‘protective’ finish by the weathering process and due to their age they are no longer under warranty! As most manufacturers’ warranty is for 30 years. The vast majority of concrete tiles from this age are not colour-fast, as they do not have a colour pigment through the body of the tile. So when the surface naturally wears away it reveals a lighter colour. The tiles will be thinner and as they have lost the surface protection are more prone to hold the water and eventually become porous.

At this stage, we would recommend to the owner if they can afford to replace the roof tiles, this would be the best action. However, this can be a substantial cost!

If replacement isn’t an option due to the cost and depending on the condition of the roof, tiles, and close-boarding, both need to be suitable we would recommend cleaning the roof by pressure washing. Carrying out any necessary repairs, local tile replacement, and re-pointing as required. Once dry apply a roof coating.

The reason for this choice is once the roof tiles have been coated, the tiles will be stronger and re-coloured with a protective weatherproof coating. The roof coating will extend the lifespan of the roof making the complete process cost effective.

Removing the moss via scrapping and then treating the roofs with a biocide ‘soft washing’ (Chemically cleaning) will remove the moss, but it adds zero surface protection to the tiles. The tiles will continue to weather due to the lack of surface protection and eventually fail causing leaks.