Karcher K4 vs Nilfisk C 135 for Cleaning a Moss-Covered Sandstone Patio

Moss on sandstone is not one problem, it is two. There is the green surface film that a garden hose and a stiff brush will shift, and there is the black-green stuff wedged into the pointing and into the open pores of the stone itself. The K4 and the C 135 handle the first easily. The second is where the difference shows, and where people wreck the stone.

Start with what actually matters on soft paving: not peak bar, but how the pressure arrives at the surface. A K4 with the standard vario lance held at 30 cm, angled at maybe 45 degrees, lifts moss in strips. A C 135 with the same distance and angle does the same job, marginally slower on the deep pointing, because the Nilfisk’s flow rate through the standard nozzle is a touch lower. Neither number on the box tells you this. You find it out on the third slab.

The dirt blaster nozzle is the thing that ruins patios

Both machines ship with a rotary nozzle. Karcher calls theirs the Dirtblaster, Nilfisk the Powerspeed or Tornado depending on the year. It spins a single pencil jet in a cone. On concrete or engineering brick it is brilliant. On sandstone it is a router.

Hold a spinning jet 10 cm off Yorkstone for four seconds and you get a circular scour mark, lighter than the surrounding stone, permanent. Riven sandstone hides it a little because the surface is already irregular. Sawn or honed sandstone shows every pass. The moment you overlap two circles you get a figure-of-eight ghost that catches the light at low sun and never leaves.

So the rotary nozzle gets used only on the pointing joints, quickly, and never held still. For the stone faces you switch to the fan lance and open it to the widest spray the moss will tolerate. On the K4 that is the vario lance twisted toward the low-pressure end. On the C 135 it is the Click&Clean fan nozzle, which is fixed-angle, so you control intensity purely by distance. That fixed nozzle is genuinely the C 135’s weak point here, because you cannot dial pressure down at the lance, only step back.

Which one, if you only buy one

For a moss-covered sandstone patio specifically, the K4. The vario lance and the water-cooled motor tip it. The C 135 is the better machine for a car and a set of wheelie bins.

Motor, duty and the reality of a whole patio

A 40 square metre patio is not a five minute job. It is closer to two hours if the moss is established, longer if the joints are lime mortar and half washed out already. This is where the motor type stops being a spec-sheet line.

The K4 uses a water-cooled induction motor. It runs cooler, quieter, and it tolerates the stop-start rhythm of patio work, where you are constantly releasing the trigger to move a kneeler or shift the hose off a bed of hostas. The C 135 uses an air-cooled universal motor. Lighter, cheaper, whinier, and it heats up faster on a long session. Neither will die on a single patio afternoon, but the Nilfisk asks for a break sooner and the pump sounds like it means it.

Water supply is the quiet killer. Both machines want a decent flow in, not just pressure. If you are drawing from a water butt for rainwater harvesting rather than the mains, check the intake spec before you assume it will work. The K4 needs positive pressure at the inlet; it will not reliably self-prime a lift from a butt sitting at ground level, and running it dry chewing air is how you cook a pump seal in about ninety seconds. The C 135 is the same. A butt gives you free soft water with no chlorine, which is kinder to the stone, but you need a small feed pump or a butt raised on blocks with a gravity head to make either washer happy.

The other supply trap is a kinked or narrow-bore hose. A 12mm hose feeds these fine. A cheap 9mm hose on a long run starves the pump, the machine hunts and pulses, and you blame the washer. It is almost always the hose.

Run-off is the part nobody plans. Two hours of washing moss off sandstone produces a lot of black slurry, and it goes somewhere. If your patio drains toward a lawn edge or a border, that slurry silts the soil surface and pools. Sweep the bulk of the loosened moss off dry first with a stiff yard broom before you ever pull the trigger. Less to wash, less to flood.

Angle, distance and the passes that actually work

Keep the fan lance at a shallow angle, 30 to 45 degrees off the surface, never square on. A jet hitting sandstone at 90 degrees drives water and grit into the pores and can spall the surface, lifting thin flakes that then hold more moss next year. At a shallow angle the jet works under the moss and peels it off the way you would lift wallpaper.

Work in the direction of the fall, pushing slurry toward the drain, and work the joints last. Blasting the joints first floods the stone faces with dirty water you then have to redo. On the K4 the vario lance lets you drop pressure for the pointing so you do not carve out sound mortar. On the C 135 you back off by distance, standing up straighter and letting the fan spread.

Let the stone dry fully before you judge the result. Wet sandstone looks uniform and clean. Dry, you see the passes you missed and the faint tracks where you lingered. Two moderate passes beat one aggressive one every time on soft stone.

After the wash, the moss comes back

Pressure washing removes moss. It does not stop it. The spores are in every gutter and on every north-facing damp slab, and within a season the film returns, faster now because the washed surface is slightly opened up.

A weak solution treatment after the stone dries slows the regrowth. Sodium hypochlorite based path cleaners work but can lighten sandstone and scorch any planting the run-off touches. Benzalkonium chloride cleaners are gentler on the stone and on nearby beds. Either way, apply it to dry stone on a still day and keep it off the lawn.

The longer game is drainage and light. Moss thrives where water sits and sun does not reach. A slab that never dries out will re-green no matter how hard you blast it. If a whole corner of the patio stays dark and damp, the washer is treating the symptom on a loop.

The verdict nobody puts on the box

Between these two, the K4 is the safer tool for sandstone because the vario lance gives you a low-pressure setting you can trust near soft pointing, and the induction motor shrugs off a two hour session. The C 135 cleans just as well on the stone faces and costs less, but its fixed fan nozzle means every pressure adjustment is a step backward on your knees, and that gets old fast on a big area.

What neither machine changes is why the moss was there. If your patio slopes the wrong way, or a downpipe discharges across it, or a hedge keeps half of it in permanent shade, you will be back with the lance next autumn regardless of which brand is in the shed. So before you spend the afternoon choosing between 130 and 135 bar, it is worth asking what is keeping that stone wet in the first place, and whether the answer is a washer at all or a spirit level and a length of channel drain.