Why Pressure Washing Ruins Tarmac

Mark Cave

You know, I, I, I still see it happening far too often in this industry. A contractor turns up to a site with a heavy duty pressure washer, and they see a black driveway. And, er, instead of actually inspecting what's under their boots, they just assume, well, it's a driveway, right? I'll just, I'll just crank the pressure right up and blast it clean like it's standard block paving. And then, within ten minutes, they've turned a dark, smooth tarmac surface into an absolute unraveling bed of loose aggregate.

Mark Cave

It, it, it makes my stomach turn because it's completely avoidable if you understand basic chemistry. Tarmac is not concrete. It's not a rigid solid stone. Tarmac relies on a petroleum based bitumen binder to literally glue all that aggregate and gravel together. So when you hit tarmac with a high pressure jet washer, or you go slapping aggressive solvent degreasers all over it, what are you actually doing? You're breaking down that bitumen binder. According to technical guidance from SoftWash UK, using excess pressure on tarmac dislodges that bitumen binder, leaving loose stones and a patchy, scarred appearance.

Mark Cave

And look, it's not just about high pressure either. If you go using neat or ridiculously strong sodium hypochlorite mixes on weathered asphalt, or you hold a high pressure lance two inches from the surface, you're stripping away that fine top layer of bitumen. You're exposing the raw aggregate underneath. And once that gravel is exposed, water gets in, freeze thaw cycles happen, and the weathering process accelerates by ten times. You haven't cleaned the driveway, you've just destroyed its structural integrity.

Mark Cave

I remember a case not long ago. A contractor was called out to clean an aged tarmac driveway covered in moss. He pulled out a 250 bar lance, started blasting away at full throttle, and by the time he finished, half the driveway was lying in loose piles at the bottom of the slope. The homeowner handed him a five thousand pound replacement claim on the spot. Five thousand pounds. All because he didn't take two minutes to identify the surface before pulling the trigger.

Mark Cave

So, how do we clean tarmac safely without destroying the property owner's investment? It comes down to a proper two stage softwash workflow. Stage one is physical, but gentle. You take a specialized scrape or soft broom and you manually remove the bulk moss and thick organic growth. You're not gouging the asphalt, you're just lifting the heavy biological growth off the surface.

Mark Cave

Stage two is where the chemistry does the heavy lifting. Instead of blasting, you apply a low pressure spray of a residual biocide, specifically something like DDAC, didecyl dimethyl ammonium chloride. A proper DDAC biocide sits on the surface, kills the biological spores at root level, and does not attack or degrade the bitumen binder at all. It's completely safe for the asphalt.

Mark Cave

Now, the key here is managing client expectations from day one. You have to explain to the homeowner that biocide treatments work progressively. Rain, wind, and natural weathering work with the product over weeks and months to self cleanse the surface. When you educate the customer on why this gentle process protects their driveway from five thousand pounds worth of damage, you don't just win the job, you set up a recurring annual maintenance contract.

Mark Cave

Of course, site control matters too. Before you apply anything, inspect the site for pre existing oil saturation, because oil already weakens bitumen. Check your drainage routes. You need to control run off so chemical wash water doesn't flow onto sensitive lawns, flowerbeds, or into freshwater drains. Simple site checks protect you, the client, and the environment.

Mark Cave

Professional exterior cleaning isn't about brute force or blasting everything in sight. It's about knowledge, compliance, and using the right chemistry for the right surface. If you want to build a reputable, highly profitable softwashing business, come and take a look at our SoftWash UK CPD Certified Softwash Training Course and join our Approved Softwash Applicator network at softwashing.uk. Get the knowledge first, protect your business, and do the job right.