A pergola is usually several materials at once — timber beams, steel posts, polycarbonate or Colorbond roofing, sometimes a climbing plant through the whole thing — and each wants a different approach.
Cleaning it as one surface is how pergolas get damaged.
Check the structure first
Before anything else, look at where the posts meet the ground and where the beams meet the house.
Post bases are where pergolas fail. Timber rots and steel rusts at the point where the post meets the slab or the soil, because that is where water sits. Press with a screwdriver. If it gives, that is a structural job and not a cleaning one.
The ledger — the beam bolted to the house — is the other failure point. Water getting behind it rots the fascia or the wall framing, and a pergola pulling away from a wall is a serious problem.
Bolts and brackets, particularly on the coast, where corrosion does the work quietly.
Worth a proper look while the structure is accessible, and worth reporting rather than washing over.
Work top down
Roofing first, then beams and rafters, then posts, then the floor below. Doing the floor first means doing it twice.
The roofing
Polycarbonate is the one most often ruined. It scratches easily and goes brittle with UV. Soft cloth or sponge mop, mild detergent, no abrasives, no solvents, no pressure washer. An older sheet can crack under pressure, and sheets cost more than the clean.
Clean both sides where you can reach. The underside collects dust and cobwebs and cuts the light noticeably.
Colorbond — mild detergent, soft brush, low pressure, gentle if chalked.
Shade cloth or a sail over the frame — see how to clean a shade sail.
And check the gutter if it has one, plus where the water actually goes. A pergola gutter discharging at the base of a post is how that post rots.
Timber beams and rafters
Usually treated pine, hardwood or occasionally Merbau.
Low pressure only. Pressure raises the grain on any timber, and on a structural member that means a rough surface that holds water and grows mould faster than before.
Use a purpose-made timber cleaner rather than a pressure washer, work along the grain, and rinse. Grey weathered timber can be brought back with an oxalic acid brightener.
If it is Merbau, expect tannin to run onto whatever is below — here.
Let it dry properly before any oil goes on, which takes longer than people allow — here.
Climbing plants
Wisteria, star jasmine, ornamental grape. Beautiful and the main reason the timber underneath is rotting.
A climber holds moisture against the structure permanently and hides the condition of it. If you have one, pull it back periodically and actually look at the beam underneath.
Deciduous climbers also drop into gutters and onto the roof below.
If you are using any biocide on the structure, the plant will be damaged. Soak it first, cover what you can, rinse afterwards — and accept some leaf burn.
The floor
Whatever is under it — paving, concrete, decking — done last, with the method for that surface. See patio cleaning.
Lighting and fans
Pergolas increasingly have lighting, fans, speakers and heaters. All of it needs isolating and protecting before water goes anywhere near — the electrical check.
Outdoor fans in particular collect a surprising amount of grime on the blade tops and are easy to damage with pressure.
How often
Annually for the structure, more often for the roofing if light transmission matters to you. Spring is the sensible time, before the entertaining season.
For the structure and surrounds as one job, see patio cleaning.
