How often should you pressure wash your siding and driveway?

Built-up grime, algae, and moss degrade surfaces and get slippery.

Left alone, algae and lichen hold moisture against render, timber and mortar, which accelerates decay and, on paths and decks, creates a genuine slip hazard. Organic growth on north-facing walls in damp climates can hold enough moisture to cause internal damp.

1. Pick the right pressure and nozzle first. Start with a wide fan, 25 or 40 degrees, and the lowest pressure that works. Never use a zero-degree pencil jet on a building.
2. Test on an inconspicuous area. Different substrates tolerate very different pressures, and render, soft brick and timber are easily damaged.
3. Protect and prepare. Close windows, cover outlets and light fittings, move furniture, and wet down plants before and after so detergent does not sit on them.
4. Apply a cleaning solution and let it dwell. For algae and lichen, the chemical does the work and the water rinses it. Dwell time beats pressure almost every time.
5. Work top to bottom, keeping the nozzle moving. Maintain a consistent distance and overlap passes. A stationary nozzle etches a mark you cannot remove.
6. Never aim upward under siding or tiles. Angling up drives water behind cladding and under roof tiles, which is the single most damaging mistake in this job.
7. Rinse thoroughly from the top down. Leftover detergent streaks as it dries.

• Green or black growth spreading across render, siding or paving
• Paths and decks becoming slippery when wet
• Dirt streaking below windows and gutter joints
• Lichen colonies establishing on masonry

• Using too much pressure, which carves soft mortar out of joints, strips timber and forces water into wall cavities
• Pointing the lance upward beneath siding or roof tiles
• Pressure washing a roof, which strips shingle granules and voids many roofing warranties. Soft washing with a chemical solution is the correct approach for roofs
• Cleaning a surface with failed paint and then repainting over damp substrate
• Holding the nozzle too close on render, which leaves permanent striping

• Shaded and north-facing elevations grow algae far faster and may need doing annually while sunny walls last years
• Overhanging trees and nearby vegetation accelerate growth considerably
• Damp maritime climates make this a yearly job; dry inland climates make it occasional

Render, historic masonry, lime mortar and anything above ground level are worth paying for. Soft washing of roofs specifically should be professional, since the technique and chemicals matter more than the equipment.