How often should you pressure wash your house?

Short answer

Wash the siding once a year. The shady or north-facing wall mildews sooner, so wash that side twice a year. The job is less about pressure than people think: a house wash detergent, a garden hose or a pressure washer on its gentlest setting, and a soft brush do the work without forcing water behind the siding. It takes 120 to 240 minutes and costs $0 to $100, the top end being a day's pressure washer rental. A professional house wash runs $200 to $500.

Why it matters

Siding collects a film every year: dust, pollen, cobwebs, the spray from the lawn, and on the shady side, mildew. The mildew is the part that matters. It's a living thing that feeds on the film and spreads, and on painted siding it works into the paint until the paint fails. On vinyl it stains, and the stain gets harder to remove each season. A wall that's black-green in the corners and under the eaves is a wall whose finish is being eaten.

A yearly wash takes the film and the mildew off before either does harm. It also gives you a close look at the whole exterior, which is how you notice the cracked caulk, the loose piece of trim and the peeling patch of paint that would otherwise go unseen until they'd let water in. Clean siding lasts longer, holds paint longer, and makes the house look cared for, which is what it is.

When to do it more often

The index gives one modifier: the shady or north-facing wall. It gets less sun, stays damp longer after rain, and grows mildew while the sunny sides stay clean. Wash that side twice a year, in spring and fall, and the rest once.

A wall behind thick shrubs, or one that gets hit by a sprinkler, behaves like a shady wall and gets the same treatment. And after a wildfire season or a construction project next door, the whole house may need a wash outside the schedule.

What you need

A pressure washer on its lowest-pressure tip, or a garden hose with a long-handled siding brush
An extension wand, so you can reach the top of the wall from the ground
Safety glasses, since the spray bounces back with grit in it
House wash detergent, the kind with a mildew killer, mixed as the label says

Steps at a glance

Close the windows, cover outdoor outlets and light fixtures, and move anything that can't get wet.
Wet the plants along the wall so detergent doesn't burn them, and rinse them again at the end.
Working from the bottom up, apply detergent to one wall at a time with the washer's soap tip or a pump sprayer.
Let it sit for the time on the label without drying, and scrub mildew patches with the brush.
Rinse from the top down with the hose or the washer's widest tip, keeping the spray pointed down and away from seams, vents and windows.
Move to the next wall and repeat, then rinse the plants and the ground.

You're done when every wall is the same color from top to bottom and no green shows in the corners or under the eaves.

Signs it's overdue

Green or black mildew in the corners, under the eaves and on the shady side.
Dirt streaks running down from under the windows.
Cobwebs in every corner and around the lights.
Chalky paint that comes off on your hand, which is the finish breaking down under the film.

DIY or pro

This is a moderate job, and the moderate part is restraint. A pressure washer on a strong tip, pointed up at the wall, drives water behind lap siding and into the sheathing, strips paint, and etches wood. Use the lowest pressure that moves the dirt, keep the spray angled down, and let the detergent and the brush do what the pressure shouldn't. A garden hose with a siding brush is enough for a single-story house.

A house wash company charges $200 to $500 for an average home and uses a low-pressure soft wash that's kinder to the siding than most rental washers. That's the better choice for a tall house, for stucco, for cedar shakes that pressure can shred, and for anyone who'd rather not spend a day on it. If you're having the house painted, a professional wash first is part of a good paint job.

Safety

Pressure: a pressure washer can cut skin at close range and will drive water into any gap in the siding. Keep the wand pointed away from people, pets and yourself, wear the safety glasses, and keep both feet on the ground. Don't spray up under lap siding, into vents, or at windows and electrical fixtures.

FAQ

Can you pressure wash vinyl siding?

Yes, on a low-pressure setting with the spray angled down. High pressure forces water behind the panels and can crack them. Detergent and a brush do most of the work.

How do you get rid of green mold on siding?

A house wash detergent with a mildew killer, left to sit as the label says and then scrubbed and rinsed. The green comes off with the film it was living on.

Is it better to pressure wash or soft wash a house?

Soft wash, which is low pressure with a cleaning solution, is gentler on siding, paint and caulk. It's what professional house washers use and what the hose-and-brush method amounts to.

When is the best time to wash the outside of a house?

Spring, after the pollen has dropped, and before any painting or staining. A mild, overcast day keeps the detergent from drying on the wall.