Illustrative comparison; not a verified customer job.Common places black staining appears
Check below windowsills, near downpipes, under eaves and on shaded walls. The pattern can help a professional decide what to test, but appearance alone is not a reliable diagnosis.
What could be responsible?
Biological growth is one possibility, but grime, metallic staining, old sealant residue or damage beneath a coating may create similar marks. Some dark spotting is persistent and needs specialist evaluation.
Why aggressive jets are not the answer
Concentrated high-pressure water may strip superficial dirt yet leave deeper marks and risk damaging the texture. A render specialist will choose a method based on the actual substrate and stain.
What to show the contractor
Supply a close-up of the black marks, a full-wall shot and one view showing the gutter or roof detail above them. Mention how long the marks have been there, whether they change after rain, and any recent repair work.
Helpful answers from manufacturer and UK industry guidance
Short answers with links to check the supporting guidance.
What causes black streaks on K Rend?
Black marks may come from biological growth, rainwater runoff, dirt or pollution deposits; colour alone cannot identify the cause. Look at where the streaks start, whether gutters leak and whether the surface is cracked. A contractor may need a small test area before recommending cleaning.
Is every black mark on render mould?
No. Algae, weathering, traffic pollution, runoff and other deposits can all look dark. K Rend describes different natural green, black and red discolouration on exterior façades. Identifying the likely cause first helps avoid choosing a treatment that does not address the mark.
Can black render stains come back after cleaning?
Yes. If shade, moisture or runoff persists, fresh deposits or biological growth can reappear. Cleaning may improve the appearance, but it does not automatically fix blocked gutters, cracks or underlying dampness. Ask for written aftercare advice rather than an unsupported permanent guarantee.
Frequently Asked Questions
Are all black K Rend spots mould?
No. Black marks can have several causes, and a visual inspection or small test may be necessary.
Does a biocide remove black staining?
A suitable biocide may address biological growth but will not remove every non-biological stain.
Can a cleaner guarantee a perfect result?
It is more sensible to ask for a tested method and realistic expectations based on your surface.