Commercial jet washing guide
Can you jet wash pebble dash?
Standard high-pressure jet washing will physically dislodge the pebbles from pebble dash and damage the mortar underneath, leaving bald patches and opening the render to water ingress. The correct cleaning method is softwashing, low-pressure biocidal approach that cleans thoroughly without touching the structural integrity of the finish.
Pebble dash is one of the most common exterior wall finishes on mid-20th century properties across London. The finish is created by throwing small stones, gravel or aggregate onto a freshly applied coat of mortar, embedding them partially into the mortar while it is still wet. When the mortar cures, the pebbles are held in place by the mortar grip around their lower half. This construction method makes pebble dash a highly practical and weatherproof finish under normal atmospheric conditions, but it also makes it uniquely vulnerable to high-pressure water. Unlike smooth render or painted masonry where the surface is continuous and bonded throughout, pebble dash consists of individual stones held at a relatively shallow depth. High-pressure water directed at the surface forces itself between the mortar and the pebble, breaking the bond and physically ejecting the stone from the wall. The resulting bald patches cannot simply be refilled — matching the original pebble aggregate, colour and distribution is extremely difficult and patch repairs are almost always visible.
No high pressure
standard jet washing at normal settings will dislodge pebbles and cause bald patches that cannot be invisibly repaired
Softwash
biocidal softwashing at garden hose pressure is the professional standard for pebble dash — it cleans without any risk to the finish
Kills at root
biocidal treatment kills algae, moss and lichen spores at the root level, providing significantly longer-lasting results than pressure washing alone
What softwashing is and how it cleans pebble dash
Softwashing uses low-pressure water (comparable to garden hose pressure) combined with a specialist biodegradable biocidal cleaning solution to remove algae, moss, lichen, black mould and general organic staining from wall surfaces. The chemical solution does the cleaning work rather than the water pressure. The biocide penetrates the biological growth and kills the organisms at their root level, including the spores that drive rapid recolonisation after pressure washing. Once the growth is killed, it releases from the surface and can be rinsed off at very low pressure or left for rain to wash away naturally over several weeks.
The key practical difference between softwashing and pressure washing for pebble dash is that softwashing relies on chemistry rather than force. The water pressure involved is so low that it poses no risk to the pebble bond. Because it kills the organisms at the root level rather than simply blasting away the visible growth, softwashing also provides significantly longer-lasting results. Pressure washing alone removes visible organic growth but leaves the root systems and spores in the surface, allowing rapid recolonisation within weeks or months. Softwashing kills the spores and in some systems provides an ongoing residual biocidal effect that delays regrowth for 12 months or more.
The softwash process for pebble dashHow pebble dash is professionally cleaned
1. Inspect the render for damage and loose sections
Before any cleaning, the pebble dash is inspected for sections where pebbles have already been lost, areas where the render is cracked, delaminating or bulging, and any areas where water may be ingressing behind the finish. Damaged sections should be repaired before cleaning begins to prevent water penetration during the treatment process.
2. Apply biocidal cleaning solution at low pressure
A biodegradable biocidal solution containing sodium hypochlorite or a specialist render cleaner is applied to the entire surface using a pump sprayer or low-pressure application system. Surrounding plants, planting and ground-level features are protected from runoff before application. On heavily contaminated surfaces a second application may be necessary.
3. Allow dwell time for the solution to work
Dwell time is critical. The solution must remain wet on the surface for sufficient time to kill the biological growth at root level. Depending on the product and contamination level, this is typically 30 to 60 minutes. The wall must not be allowed to dry before the dwell period is complete. On dry or sunny days a second application to keep the surface wet may be needed during this period.
4. Rinse at garden hose pressure only
Once the dwell time is complete, the surface is rinsed using only garden hose pressure or a pressure washer with the pressure reduced to a very low setting with the widest available fan nozzle held well back from the wall. This removes the dead biological material and cleaning product residue without risking pebble dislodgement. Remaining dead moss and algae will continue to weather away naturally over the weeks following treatment.
Commercial jet washing London
Pebble dash softwashing across London by Cloud Nine
Cloud Nine uses professional softwashing methods to clean pebble dash and rendered surfaces across London. We never use high-pressure washing on pebble dash. Our biocidal treatment kills organic growth at root level for results that last significantly longer than pressure washing alone. Contact us for a free quote.
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