Removing Mildew From Exterior Surfaces

Exterior mildew is a moisture problem wearing a stain. It grows anywhere a surface stays damp and shaded, so removing it takes two steps that go together: a cleaning solution that kills the growth, and a fix for whatever keeps the surface wet — usually a gutter, drainage, or shade issue. Skip the second step and the mildew is back within a season.

Where and why mildew grows

Mildew is not random. It colonizes the parts of a house that dry out slowest: north-facing walls, shaded eaves, the base of siding near splashback, fence panels under trees, and anything downstream of a leak or overflow. It shows up as flat, powdery black or gray spotting, sometimes with a faint musty smell.

The common thread is always moisture plus shade. Sun and airflow dry a surface quickly and mildew struggles to establish. Deep shade and poor air movement leave a surface damp for hours or days after rain, and that lingering dampness is all mildew needs. That is why you can clean one wall spotless while the shaded side keeps growing film — the difference is not how dirty they get, it is how long they stay wet.

Killing the growth: cleaning method

The cleaning itself is straightforward, and it does not require force. Mildew is killed by the right cleaning solution, not by pressure. Apply a cleaner suited to the surface, give it time to dwell so the chemistry can work, then rinse at low pressure. On most of the house, that means soft-washing — the gentle approach covered in power washing versus soft washing — because siding, wood, and painted trim are all damaged by high pressure. Blasting mildew accomplishes nothing the solution does not do more safely.

Work top-down on walls, keep a soft brush handy for stubborn patches, and let dwell time carry the effort rather than scrubbing hard. Reserve real pressure for hardscape like concrete, never for the house itself.

Fixing the moisture source

This is the step that actually solves the problem, and it is the one people skip. Mildew is a symptom; the disease is a surface that will not dry. Track down why it stays wet:

• Gutters. Overflowing or leaking gutters soak the wall, fascia, and the ground below with every rain. Mildew that keeps returning along one eave or beside a downspout is a classic tell. Learn the signs your gutters are failing and check the run above the affected area — a clog or a bad joint is often the whole story.
• Drainage. Water pooling near the foundation or splashing back onto lower siding keeps that zone perpetually damp. Improving how water moves away from the house dries the surface out.
• Shade and airflow. Overhanging branches and shrubs pressed against a wall block the sun and trap humidity. Trimming them back so light and air reach the surface is one of the most effective preventives there is.

Where mildew appears	Likely moisture source	The real fix
Below a downspout or eave	Overflowing or leaking gutter	Clear or repair the gutter run
Base of siding	Splashback, poor grading	Improve drainage away from the wall
North / shaded wall	Slow drying, low airflow	Trim vegetation, open up light and air
Fence under trees	Constant shade and damp	Increase sun exposure, reduce overhang
Near ground level	Standing water, wet soil	Redirect water, fix pooling

The pattern to notice: in every case the cleaning is the easy part and identical, while the durable fix is different because the moisture source is different. Match the fix to the source and the mildew stops coming back.

Common mistakes

• Cleaning without fixing the cause. The most common mistake by far. If the surface still stays damp, you are just resetting the clock.
• Reaching for high pressure. Force does not kill mildew, and it damages most house surfaces. The solution does the work; the rinse just carries it away.
• Ignoring the gutters. Recurring mildew under an eave is often a gutter problem in disguise, and no amount of cleaning fixes a clogged gutter.
• Leaving shade in place. Branches and shrubs against the wall keep it wet; cleaning around them changes nothing long-term.
• Skipping plant protection. Mildew cleaners can scorch plants below; pre-wet and rinse to protect them.
• Treating one spot in isolation. Where mildew appears is a map of where the house stays wet — read the whole pattern before you decide you are done.

Think of exterior mildew as a helpful indicator rather than just an eyesore. It marks exactly where your home holds moisture. Clean it off with a gentle solution, then follow the stain back to its source — a gutter, a drainage path, a patch of stubborn shade — and fix that. Do both and the surface stays clean because it finally gets a chance to dry.