Pressure washing a driveway: moss, block paving, concrete and tarmac

In Kerry's damp, mild climate a driveway can go from clean to green in a single winter. Moss settles in the joints of block paving, algae turns concrete slippery, and leaves and tyre marks build up on tarmac. A pressure washer is the obvious tool, but used carelessly it can strip the surface, blast out jointing sand and send dirty water where it should not go.

This guide explains how to prepare, which settings and techniques suit each surface, and what to do afterwards so the driveway stays cleaner for longer.

Before you start

• Check the surface type. Block paving, poured concrete, imprinted (pattern) concrete, tarmac and gravel all need a different approach.
• Treat moss and algae first. Pressure washing alone removes the visible growth but often leaves roots in joints and pores, so it grows back quickly. Apply a moss and algae treatment made for hard surfaces a few days or weeks beforehand, as the label directs, and let it work.
• Sweep and weed. Brush off loose debris and scrape out weeds and moss from joints with a weeding knife or wire brush.
• Pick your day. Choose a dry, mild day. Washing in frosty weather is a bad idea, because water left in joints and cracks can freeze and lift the surface.
• Protect the surroundings. Move cars, close windows and doors, cover nearby electrical outlets and protect plants and render from spray.

Safety

A pressure washer jet can cut skin and drive debris into eyes. Wear safety glasses, sturdy footwear and old clothes; never point the lance at people, pets or yourself; and keep children well away. With an electric machine, use an outdoor socket protected by an RCD and keep connections off wet ground. Petrol machines must be used outdoors only, away from open windows, because of exhaust fumes.

Technique that protects the surface

1. Start gentle. Begin with a wide fan nozzle and the lowest effective pressure on a small, out-of-sight patch. Increase only as needed.
2. Keep the lance at a low angle, roughly 30 degrees to the ground, and hold it a consistent distance from the surface. Pointing straight down concentrates the jet and can etch concrete and blast out joints.
3. Use a surface cleaner attachment if you have one. These rotary heads give an even, stripe-free finish on large flat areas and throw far less spray around.
4. Work downhill, from the house outwards towards the road or drain, overlapping each pass slightly so dirt is not washed back over cleaned areas.
5. Rinse as you go so loosened dirt does not dry back on.

Surface by surface

Block paving

Block paving cleans up well, but the sand between the blocks is what locks them together. Use a fan nozzle at a shallow angle and do not aim the jet along the joints. Expect to lose some sand anyway. Once the paving has dried completely, brush kiln-dried sand into the joints, working it in from several directions until they are full, then sweep off the excess. Kiln-dried sand flows into narrow joints because it is completely dry; ordinary building sand will not. Re-sanding also makes it harder for weeds and moss to take hold again.

Plain concrete

Brushed or plain concrete is fairly robust but can be etched by a narrow jet held close, leaving visible stripes. A surface cleaner gives the most even result. Oil stains rarely shift with water alone; use a degreaser made for concrete and scrub before rinsing.

Imprinted or sealed concrete

Pattern-imprinted driveways are usually coloured and sealed. High pressure can damage a worn sealer and lift colour. Use lower pressure and a wider nozzle, and consider whether the driveway needs resealing once it is clean, following the sealer manufacturer's advice.

Tarmac

Tarmac (asphalt) is softer than concrete, particularly on warm days, and loose stone can be pulled out by a powerful jet. Use low pressure and a wide fan, or simply scrub with a stiff brush and a suitable cleaner and rinse with a hose. Avoid strong solvents and degreasers, which can soften the binder.

Gravel

Do not pressure wash loose gravel. Rake it, remove weeds by hand or with a suitable treatment, and top up with fresh gravel where it has thinned.

Where does the water go?

Most driveways drain into a road gully or a surface water drain. In many areas these carry rainwater straight to a local stream or river rather than to a treatment plant. Clean water is not a problem, but detergents, degreasers, oil and moss treatments should not be washed into them. Use products sparingly and as directed, choose biodegradable cleaners where possible, and soak up oil spills with absorbent material before washing. The Environmental Protection Agency and your local authority are the bodies responsible for water quality if you want to know more about how drainage works in your area.

Keeping the driveway clean

• Sweep regularly, particularly in autumn. Leaves left to rot feed algae and stain paving.
• Improve the light and airflow. Driveways shaded by overhanging trees and hedges stay damp and green. Cutting back growth, within the legal hedge cutting season, makes a noticeable difference.
• Fix drainage. Puddles that sit for days point to sunken areas or blocked channels, which encourage moss and frost damage.
• Consider a sealer for block paving or concrete, which can make it easier to keep clean. Follow the manufacturer's guidance on preparation and drying time.
• Treat little and often. A light moss and algae treatment once or twice a year is easier than a heavy clean every few years.

How often should you pressure wash a driveway?

For most homes once a year is plenty, ideally in late spring when the worst of the winter growth can be removed and the surface has the summer to dry out. Washing too often wears surfaces and empties joints. In between, sweeping and spot-treating moss is usually enough.

Patios need a slightly gentler approach, especially natural stone; see our guide on how to clean patio slabs. If you would prefer someone else to do it, power washing is one of the handyman services in County Kerry provided by Alpine Home Improvements.