Pressure washing — what it is
Pressure washing uses water at 1,500–3,000 psi to physically blast dirt and growth off hard surfaces. It's loud, fast, and effective on the right surface.
Good for: block paving, concrete patios, tarmac drives, paths, decking, brick (with care).
Bad for: roof tiles, K-Rend, silicone render, painted masonry, soft sandstone, anything with a coating.
Soft washing — what it is
Soft washing uses low-pressure water (under 500 psi, often under 100) plus a biocide solution that does the cleaning chemically instead of mechanically. The biocide kills the algae, moss and lichen at the spore level so it doesn't grow straight back.
Good for: roofs, render (including K-Rend and silicone), painted masonry, conservatory roofs, fascias and soffits, soft natural stone.
Bad for: heavily oil-stained driveways (no abrasion to lift the oil), heavy moss on paving (still needs scraping or pressure to lift it).
When the wrong method costs you money
Pressure-washing a concrete tile roof strips the protective coating, exposes the aggregate, and accelerates tile failure. A new tile coating is £1,500–£3,000.
Pressure-washing K-Rend voids the manufacturer warranty and removes the silicone surface — the entire wall needs re-rendering, £3,000–£8,000.
Soft-washing block paving alone won't lift heavy oil or sunk-in moss — you'll pay twice when a proper pressure clean is needed afterwards.
What a proper professional does
Matches the method to the surface — soft wash on roofs and render, pressure wash on hard horizontal surfaces, often both on a single visit.
Tests a small area first if the surface or coating is unknown.
Includes a biocide on any cleaned surface that's prone to re-growth — that's what makes the result last more than 12 months.