Treating Moss and Algae on Soft Tarmac Surfaces

Tarmac (Asphalt) is the perfect breeding ground for moss. Unlike concrete, which is hard and smooth, tarmac is textured and porous. It holds water like a sponge, and its rough surface provides the perfect grip for moss spores to anchor themselves.

But here is the problem: Tarmac is soft. It is an aggregate held together by a flexible bitumen glue. If you attack it with the same aggression you use on a concrete patio - blasting it with a high-pressure Turbo Nozzle - you won't just remove the moss. You will blast the sand out of the binder, strip the surface layer, and cause the driveway to crumble (fretting).

You cannot "force" moss off tarmac. You have to kill it chemically. But even this approach can prevent issues. Some heavy-duty cleaners will strip the adhesive binders used in tarmac, leaving your driveway massively weakened.

 Here is the safe, low-pressure method to sanitise your drive without destroying it.

1. The Physical Prep: Scrape, Don't Blast

Before you use any chemicals, you need to remove the "canopy." Thick clumps of moss act like umbrellas, shielding the roots from any liquid you spray.

• The Tool: Use a stiff-bristled yard broom or a dedicated Wire Moss Brush.

• The Technique: Wait for a dry day. Scrub the driveway vigorously to dislodge the large cushions of moss.

• The Rule: Do not use a pressure washer yet. If you spray water now, you will just make a muddy mess and drive the spores deeper into the cracks.

2. The Chemistry: Why Water Isn't Enough

Moss has a root system (rhizoids) that digs into the bitumen. If you shear the top off, it grows back in two weeks. You need a Biocide to soak in and kill the root.

Option A: DDAC / BAC (The "Walk Away" Method)

This is the industry standard for tarmac. Products containing Benzalkonium Chloride (often sold as "Moss & Algae Killer" or "Wet & Forget" style cleaners).

• How it works: It is a neutral pH sanitiser. It kills the organism over time.

• Application: Spray it on dry tarmac until saturated.

• The Benefit: You do not rinse it. You leave it there. It creates a residual barrier that prevents regrowth for months. Over the next few weeks, the wind and rain will naturally wash the dead, brown moss away.

• Safety: It is much kinder to the bitumen binder than acid or bleach.

Option B: Sodium Hypochlorite (The "Instant" Method)

If the driveway is black with slime (Algae) rather than moss, and you want it clean today, you can use a dilute Bleach solution.

• Warning: You must be careful. Strong bleach can sometimes dry out the bitumen oils if left too long.

• The Ratio: Dilute heavily (e.g., 5 parts water to 1 part Hypo).

• The Rinse: Unlike BAC, you MUST rinse bleach off after 30 minutes.

3. The "Soft Wash" Technique

Once the chemical has done its work (the moss has turned brown/white), you can wash the surface - but you must be gentle.

Do NOT use a Turbo Nozzle. A spinning turbo nozzle focuses the water into a needle-point jet. This cuts through tarmac like a laser.

• Use a Fan Tip: Use a wide 40-degree fan tip on your lance.

• Keep your distance: Hold the lance 30cm (12 inches) away from the surface. You are rinsing, not stripping.

• Angle it: Spray at a 45-degree angle to push the debris off the drive, rather than spraying directly down into it.

4. The "Petrochemical" Warning

While we are discussing chemicals, a vital warning for tarmac owners: Never use solvent-based weed killers or degreasers. Tarmac is oil. If you spill petrol, diesel, or use a Xylene-based cleaner on tarmac, it dissolves the glue chemically. You will be left with a soft, sticky hole that never sets hard again.

• Stick to Water-Based Biocides.

5. Prevention: Seal the Sponge

Once you have killed the moss and cleaned the drive, you are left with a clean, porous sponge. If you leave it, the moss will return next winter. You need to close the pores.

Apply a Tarmac Restorer (Acrylic Coating).

• This paints the surface black (or red) to make it look new.

• More importantly, it fills the micro-cracks and pores with resin. If the water can't sit in the pits, the moss can't grow. It is the ultimate preventative measure.

Conclusion

Tarmac requires a gentle touch. If you fight moss with high pressure, the moss will eventually win because you will destroy your driveway before you destroy the roots.

• Scrape it dry.

• Soak it with Biocide.

• Leave it to die.