How to remove black staining from render
Black staining on render is not one problem. It can be organic growth, it can be dirt washing down off a surface above, and occasionally it is the render itself showing something that no clean will alter.
Working out which you have is the first job, because it decides whether cleaning is worth doing at all.
The usual causes
- Black algae and lichen, common on damp shaded elevations alongside green growth
- Dirt and traffic film running down from sills, copings and projecting details
- Streaks beneath window corners where water consistently runs the same path
- Runs below a leaking gutter joint, a blocked outlet or a discharging overflow
- Splashing at the base of the wall off paving, gravel or a driveway
- Soot and general airborne grime on walls facing a busy road
What cleans off and what does not
Organic staining and accumulated dirt respond well to soft washing. So do most run marks, once the source has been dealt with.
What will not change is colour variation caused by patched or repaired render mixed from a different batch, or areas where the surface has been eroded by previous pressure washing. A clean wall makes those differences more visible, not less, which is worth expecting before the work starts.
How it is treated
Low-pressure soft washing, full elevation at a time, with dwell time for the solution to break the staining down before rinsing. Heavily marked runs sometimes need a second application rather than more pressure.
On render, more pressure is never the answer. It removes texture and leaves permanent marks in the finish.
Deal with the source at the same time
If the staining follows a gutter, a downpipe or an overflow, that is the thing to fix. Otherwise the same marks are back on a clean wall within a year. Gutters can be cleared during the same visit; genuine leaks or failed joints may need a repair first.
Want it looked at properly?
Send a few photographs of the elevation along with the address, and Rob will tell you what can realistically be improved, what it will cost and when it can be done.