Jet-washing the patio every spring

Does it work?

Supported

Supported rather than disputed, because the manufacturer permits the technique explicitly. What the record carries is the set of conditions the permission depends on, which is the part the popular version drops. A claim that is right about the method and wrong about how it is performed is a different kind of record from one that is simply wrong, and the evidence model can express the difference.

This is a verdict on whether the technique is effective. Whether it is safe on a particular surface is a separate question, answered below.

Where not to use it

Why it works

A pressure washer accelerates water through a small orifice, and the kinetic energy of the stream strips loosely bound material from a hard surface. No chemistry is involved, which is genuinely its advantage. The mechanism's limitation is that kinetic energy does not distinguish between the deposit and the substrate: jointing sand, weathered mortar, the surface laitance of a cast paving unit and the softer summer wood of a timber deck are all removed by the same stream that removes algae. The manufacturer's angle instruction is a mechanical remedy for that — a shallow angle transfers energy along the surface rather than into the joint.

Background

This is unusual among the claims in this register in that the manufacturer permits it, in writing, and then bounds it precisely — which makes the interesting question not whether to do it but how. A paving manufacturer's guidance is that power washing used to excess damages the paving and shortens its service life, and it gives five specific instructions. Sweep first. Use a low-pressure setting. Do NOT direct the lance down onto the paving, because that costs jointing material. Hold it at a shallow angle not greater than thirty degrees across the diagonal rather than parallel to the joints. And test first, because some machines mark wet cast paving. Then the step almost nobody performs: inspect afterwards to ensure the joints are full, because empty joints are detrimental to the pavement's performance. That last instruction answers the annual cycle people find themselves in: a patio jet-washed with the lance pointed down loses its jointing sand, which is why weeds return the following year. The manufacturer's own routine for ordinary soiling involves no machine at all — brushing, and where colour is masked, warm water and washing-up liquid worked with a bristle brush.

Conditions

• Low pressure setting
• Lance at no more than thirty degrees across the diagonal, never pointed down at the paving
• Test first on cast paving
• Joints inspected and refilled afterwards

What it does not do

• It removes the growth and not the conditions that produced it, which are shade, overhanging trees and inadequate fall.
• It removes jointing material along with the dirt unless the lance angle is controlled.
• Repeated at excess it shortens the service life of the paving, in the manufacturer's own words.

What to use instead

• Nylon brush — Mechanical action delivered into texture that a flat cloth cannot reach. The bristle is softer than most surfaces and harder than several worth protecting.
• Neutral cleaning — The default that four separate trades converge on independently: a neutral detergent, warm water, a soft tool and a thorough rinse.
• Moss and lichen — Three different organisms treated as one green problem. Where they grow tells you more about the fix than any product does.
• Moisture control — The treatment for mould that is not a cleaning product. Every authority in this register puts it first, and it is the only one that stops the problem recurring.