The real reason why some houses look like their walls are 'bleeding'

The marks will often occur on one or two sides of a property
-Credit: (Image: LancsLive)

With new housing estates springing up all across Lancashire, something else has become a little more common. The unsightly red stains seen on exterior walls.

Especially during the summer months, the red markings appear -as if out of nowhere - on properties all over the UK. Often, they cause a nightmare for homeowners who want their walls as clean as possible.

Many believe the red staining is due to silicone run-off or rust, and the marks will often occur on one or two sides of a property, the ones that usually take the brunt of the weather, wind and rain, and aren't exposed to regular sunlight.

The red colour is actually caused by a build-up of algae typically found on tree trunks. And there's a reason why you only see it on modern homes.

The bleeding stains are usually caused by a type of algae called trentepohlia, which can grow on exterior surfaces such as render, walls, walkways and the roof of a property. It is also typically at this time of the year, when the weather begins to warm up combined with periods of heavy rain.


The problem often appears where a type of wall coating made of limestone and minerals, called K-Rend, has been used on the exterior walls, but it can also be found on brickwork.

Because K-Rend is very porous, the walls remain wet for longer after periods of rain, and the problem is more prevalent on newer homes because modern insulation is so efficient and there is little heat coming from the cavity walls, which is why you also see red algae on new brick homes.

Algae build-up on properties in Darwen
Algae build-up on properties in Darwen -Credit:LancsLive

Usually, one or two sides of the building will be more affected than the rest. This is because algae, lichen and black mould grow very well in damp and murky conditions. They will usually show first on the side of the building that faces prevailing winds away from regular sunlight.

Early growth will look green in colour and is the easiest to treat. However, left to mature, the rendered surface will show , almost like the building is bleeding. This is more difficult to treat but can be done by an expert using the correct pressure and chemicals.

The later stages of growth can include black marks, usually black mould. Again, this is relatively easy to treat by a professional with the knowledge and expertise in steam cleaning, biocide selection and appropriate application.