How do I get the green stuff off the side of my house?

Wet the wall with a garden hose, apply a mild cleaner — 30 percent white vinegar in water for light algae, or a detergent and diluted bleach mix for heavier mildew on vinyl — let it sit 10 to 15 minutes without drying, scrub with a soft brush on a pole from the bottom up, and rinse from the top down. Match the cleaner and pressure to your siding material, because fiber cement, wood and stucco each need different handling.

Why does green mold grow on siding?

The green is usually algae rather than mould, and it grows where a wall stays damp and shaded: north-facing walls, surfaces behind shrubs and trees, walls a sprinkler hits every day, and stripes below overflowing gutters. It feeds on the thin film of dust and pollen on the surface, not on vinyl or fiber cement itself, which is why it returns unless the moisture and shade are changed.

What kills green mold naturally?

A solution of about 30 percent white vinegar and 70 percent water kills and loosens light algae and mildew on most siding; let it sit 10 to 15 minutes, scrub and rinse. Oxygen bleach, which breaks down into water, oxygen and soda ash, handles heavier growth and is gentler on plants than chlorine bleach. Neither prevents regrowth if the wall stays shaded and damp.

How do you remove mold and mildew from cedar siding?

Use an oxygen bleach solution rather than chlorine bleach, which damages the wood surface. Wet the siding, apply the solution, keep it wet for 10 to 15 minutes, scrub along the grain with a stiff synthetic brush, and rinse thoroughly. Avoid pressure washing, which cuts soft cedar and pushes water behind the boards. If the finish is worn, plan to refinish once the wood is dry.

Can I pressure wash green algae off vinyl siding?

CertainTeed, one of the largest vinyl siding manufacturers, says it does not recommend power washing vinyl siding because it can cause moisture intrusion, damage and discolouration. A garden hose, a soft brush and a proper cleaner remove algae without that risk. If a pressure washer is used anyway, keep it at low pressure with a wide fan tip and spray level or downward, never up under the laps.

Does bleach damage vinyl siding?

Diluted household bleach used in the industry mix — one quart of bleach to three quarts of water with detergent — is the standard mildew treatment for vinyl, and CertainTeed notes that stronger concentrations may damage the siding. Never mix bleach with ammonia-based cleaners, rinse plants and the wall thoroughly, and do not let the solution dry on the surface.

How often should siding be cleaned?

Once a year is typical for most houses, and twice a year for shaded or humid-climate walls where algae returns quickly. A yearly gentle wash is much easier than removing established growth, and it is part of what keeps siding within its expected lifespan.

How to Clean Green Mold and Algae Off Siding

Independent research · Last reviewed September 2026 · Your siding manufacturer's care guide takes priority

The short answer

Wet the wall, then scrub green growth off with a soft brush and a mild cleaner: 30% white vinegar in water for light algae, or the vinyl-industry mix of detergent, TSP substitute, one quart of bleach and three quarts of water for heavy mildew. Work bottom to top, rinse top down with a garden hose, and skip the pressure washer.

What the green actually is — and what it tells you

The green film on siding is almost always algae; black or grey speckles are mildew. Neither eats vinyl or fiber cement. They grow on the thin layer of dust, pollen and sap that settles on every wall, in places that stay damp long enough — which means where it grows is a clue to why.

What you see	Usually	What it points to
Green film, worst on north walls or behind shrubs	Algae	Shade and slow drying. Cleaning plus more light and airflow.
Black or grey speckles that spread	Mildew or mould	The same moist conditions. Cleaning plus airflow; mildewcide in the mix helps.
A green or black stripe directly below a gutter joint or corner	Growth fed by water running down the wall	A drainage problem. Fix the gutter or it returns within a season.
Growth only where the sprinkler reaches	Algae watered daily	Re-aim or re-time the sprinkler.
Staining with soft or swollen boards, or a musty smell indoors on that wall	Possibly water behind the siding	Not a cleaning job. Probe and inspect — see siding problems.

The last row matters most. Surface growth on an otherwise sound wall is a cleaning job. Staining that comes with soft spots, swelling or a smell on the inside of the same wall can mean water is getting behind the siding, and scrubbing the outside will not touch that — see siding problems before you clean.

Clean it the way your siding needs

Most cleaning guides are written for vinyl and quietly assume every house is vinyl. Fiber cement, wood, engineered wood and stucco each have their own weak points, and two of the most common pieces of advice — pressure wash it, and use an acid or bleach cleaner — are exactly what some manufacturers tell you not to do.

Siding	Cleaner	Tools	Pressure washing	Avoid
Vinyl	Light growth: 30% white vinegar in water. Mildew: 1/3 cup detergent, 2/3 cup TSP or TSP substitute, 1 quart household bleach, 3 quarts water.	Soft cloth or soft-bristle brush on a pole; garden hose	Not recommended by CertainTeed. If used at all: low pressure, wide fan tip, spraying level or downward.	Stronger bleach mixes, solvents, abrasive pads, spraying upward into the laps
Fiber cement (Hardie and similar)	Mild detergent in water; a siding or house-wash cleaner for mildew, used per label	Soft to medium non-metal brush; garden hose	James Hardie advises against it. If unavoidable: under 1,500 psi, wide tip, at least 6 ft away.	Acid cleaners (they etch the surface), wire brushes, abrasives
Wood and cedar	Oxygen bleach (sodium percarbonate) solution; rinse well	Stiff synthetic brush along the grain	Avoid. Pressure cuts soft wood and drives water behind the boards.	Chlorine bleach (it damages the wood surface), wire brushes, cleaning a finish you are not ready to redo
Engineered wood (LP SmartSide)	Mild detergent and water	Soft brush or cloth; garden hose	Low pressure only, per the manufacturer's care guidance.	Harsh solvents, abrasive pads, water forced into cut edges
Stucco	Oxygen bleach or a masonry-safe detergent; pre-wet the wall first	Soft to medium brush; garden hose	Low pressure only, and never on cracked or patched areas.	Letting cleaner soak into dry stucco, high pressure, muriatic acid

Vinyl mix and pressure-washing position from CertainTeed's published cleaning guidance; fiber cement limits from James Hardie's care guidance as summarised by its installers. TSP is restricted in some states — a TSP substitute works in the same mix. Check your own manufacturer's care guide, especially if the siding is under warranty.

Step by step

Pick a cool, overcast day or work on the shaded side. Cleaner that dries on a hot wall stops working and leaves streaks.

1. Protect plants and fixtures

Soak the planting beds with plain water and cover delicate plants with sheeting you can lift off within the hour. Close windows, and cover outdoor outlets and light fixtures.

2. Wet the wall

Rinse a section about 10 feet wide with a garden hose on a shower or fan setting, spraying level or downward. A wet wall absorbs less cleaner and rinses cleaner.

3. Apply the cleaner from the bottom up

Use a pump sprayer, or a soft brush on an extension pole dipped in a bucket. Starting at the bottom stops cleaner running down over dry siding and leaving pale streaks that are hard to blend.

4. Let it sit — but never dry

Ten to fifteen minutes is enough for most growth. Mist it with more solution if it starts drying. This dwell time does the work a pressure washer would otherwise do by force.

5. Scrub gently

Light passes with a soft brush, following the lap lines on horizontal siding and the grain on wood. Heavy scrubbing scuffs vinyl's sheen and the factory finish on fiber cement.

6. Rinse from the top down

Rinse thoroughly with the hose from the top of the section down, until no suds or colour run off, then rinse the plants again. Move to the next section with a small overlap.

Which cleaner to reach for

Water and dish soap — dust and pollen, and a light green film caught early. Start here on any material.
White vinegar, about 30% in water — light algae and mildew, the “natural” option. Weaker on established growth.
Oxygen bleach (sodium percarbonate) — heavier growth on wood, stucco and most other surfaces; gentler on plants than chlorine bleach.
The vinyl mildew mix — detergent, TSP or substitute, diluted household bleach. Vinyl only, at the stated strength; stronger mixes can damage it.
Commercial house-wash cleaners — convenient and often hose-end. Check the label lists your siding material.

Never combine chlorine bleach with ammonia or acidic cleaners — the fumes are dangerous — and do not use acid cleaners on fiber cement or stucco.

What manufacturers say about pressure washing

Pressure washing is the default advice online and the thing siding makers are least keen on. CertainTeed states that it does not recommend power washing vinyl siding because it can cause moisture intrusion, damage and discolouration. James Hardie's guidance for fiber cement is likewise to avoid it, and if it cannot be avoided, to stay under 1,500 psi with a wide tip at least six feet from the wall.

The risk is not only the surface. Horizontal siding is designed to shed water running down it, not water driven up under the laps from below. Water pushed behind the panels wets the sheathing and can void a warranty, which is a bad trade for a job a hose and a brush can do.

Why it comes back — and how to slow it

Cleaning removes the growth; it does not change the conditions that grew it. A wall that went green in two years will go green again in two years unless something else changes.

Cut back vegetation so shrubs sit at least a foot or two off the wall and sun and air reach it.
Fix gutter overflow at the joints and corners where stripes start.
Re-aim sprinklers so they water the lawn, not the siding.
Wash yearly — a light wash every spring, twice a year on shaded walls in humid climates, before growth establishes.

Houses in hot, humid climates fight this constantly, and material choice makes a difference over the long run — see the best siding for hot, humid climates. If the siding is also faded and chalky after cleaning, painting may be the next step: can you paint siding.

Safety

Work from the ground with an extension pole wherever possible; a wet wall and a ladder are a poor combination. Wear eye protection and gloves with any bleach or oxygen-bleach solution, keep children and pets away until everything is rinsed, and keep water away from outlets and light fixtures. For second-storey walls, a professional soft-wash service is often the safer answer.

Frequently asked questions

How do I get the green stuff off the side of my house?: Wet the wall, apply a mild cleaner, let it sit 10–15 minutes without drying, scrub bottom-up with a soft brush, rinse top-down. Match the cleaner to your siding material using the table above.
Why does green mold grow on siding?: It is usually algae, growing where walls stay damp and shaded — north walls, behind shrubs, in sprinkler range, below gutter leaks — and feeding on surface dust, not the siding.
What kills green mold naturally?: About 30% white vinegar in water for light growth; oxygen bleach for heavier growth. Neither stops it returning if the wall stays damp and shaded.
How do you remove mold and mildew from cedar siding?: Oxygen bleach, not chlorine. Keep it wet 10–15 minutes, scrub along the grain with a synthetic brush, rinse, and skip the pressure washer. Refinish once dry if the finish is worn.
Can I pressure wash green algae off vinyl siding?: CertainTeed does not recommend it — it can force water behind the panels and discolour them. A hose, soft brush and cleaner do the job without the risk.
Does bleach damage vinyl siding?: Diluted as in the standard mix (1 quart bleach to 3 quarts water, plus detergent), no — stronger concentrations can. Never mix bleach with ammonia.
How often should siding be cleaned?: Once a year for most houses; twice for shaded walls in humid climates. A light yearly wash beats scrubbing off established growth.

Keep going

Siding problemsWhen staining means water behind the wall, not just algae.Can you paint siding?The next step for faded, chalky siding.Siding for humid climatesMaterials that resist growth over the long run.Siding repair costIf cleaning turns up damaged panels or boards.

Cleaning guidance summarises manufacturer and industry recommendations as published; your siding manufacturer's care instructions and warranty terms take priority. SidingWise is not a contractor or a product retailer.