Your stone has turned green and black: what is growing on your house and how to remove it

The house looked white when you bought it

The stone on the north side has gone gray-green. There are black streaks running down from the window sills and under the eaves. The patio and the pool deck are slick after a rain, the driveway has a dark shadow where the trees hang over it, and the roof has dark stripes you never noticed before. None of it is dirt in the ordinary sense; you have hosed it and it did not move. The house is a decade or two younger than it looks, and you are not sure what to do about it.

We see this every week at Park Cities Power Wash. Since 2016 we have cleaned homes across Dallas, with a concentration in Highland Park, University Park, Preston Hollow and Lakewood, and the single most common call is porous stone that has gone dark with algae. The second most common is a homeowner who rented a pressure washer first, blasted a test patch, and now has a clean spot with a damaged surface around it.

This guide explains what is actually growing on your house, why it comes back, what you can safely do yourself, where the real risk lies, and how a professional soft wash works from quote to rinse.

What is growing on your house and why it likes North Texas

The green film is algae. The black streaks on the roof and under the eaves are also algae, a type that forms a dark protective coating as it grows. The gray-black blotches on stone and concrete that look like soot are usually a mix of algae, mildew and lichen living in the pores of the material. All of them need the same things: moisture, shade and a surface they can hold onto. Austin stone, limestone, brick, stucco and concrete are porous, which means millions of tiny places for growth to anchor and for water to linger.

Our climate gives them most of what they want. Humid stretches, heavy rains, irrigation that hits the house every morning and long shaded walls on the north and east sides of a home keep surfaces damp. Trees that have grown over the driveway and roof drop organic debris and block the sun that would otherwise dry things out. In our experience the staining is almost always worst where these conditions line up:

• North- and east-facing walls that stay in shade most of the day.
• Below window sills, downspouts and roof edges where water runs repeatedly.
• Stone and concrete under tree canopy, including driveways and pool decks.
• Areas sprayed daily by irrigation heads aimed too close to the house.
• Roof slopes that face away from the afternoon sun.
• Low spots in patios and walkways where water pools after rain.

What you can safely check and do yourself

Start by walking the house after the sprinklers run and after a rain and noting where water hits the walls and where it stands. Redirecting an irrigation head away from the stone and trimming a branch that shades a wall are the two cheapest things you can do to slow regrowth, and they cost nothing but an hour. Clean your gutters, because overflowing gutters feed the streaks under the eaves.

For the growth itself, you can test whether it is organic by applying a little household cleaner to a hidden spot and watching whether the color lifts. What we ask homeowners not to do is point a rented pressure washer at stone, stucco, mortar, a roof or painted wood. High pressure does not kill algae; it tears off the surface the algae is sitting on. On soft limestone that means pitting and etching, on brick it means blown-out mortar joints, on shingles it means stripped granules and a shortened roof life. The damage is permanent and the algae comes back.

• Walk the property after irrigation and rain to find where water is hitting and pooling.
• Redirect sprinkler heads that spray the house, and reduce run times on shaded sides.
• Trim branches that keep walls, roofs and driveways in constant shade.
• Clear gutters so water is not running down the walls.
• Spot-test a hidden area with a mild cleaner to confirm the staining is organic.
• Leave stone, stucco, brick, roofs and painted surfaces alone when it comes to high pressure.

When it can wait, and when it should not

Light green film on a concrete walkway in a spot you rarely use can wait for a convenient time; it is not hurting anything yet. A faint shadow on the stone that bothers you only when you look for it is also not urgent. If the budget or the calendar is tight, fixing the moisture sources first and cleaning in a few months is a reasonable plan.

It should not wait when surfaces you walk on are slick, because a wet, algae-covered patio or pool deck is a genuine slip hazard, especially for children and older family members. It should not wait when the roof has heavy dark streaking, since the growth holds moisture against the shingles. And it should not wait if you are putting the house on the market: the exterior is the first photograph and the first impression, and a clean facade, driveway and windows change how a buyer reads the whole property.

Once you have decided to have it cleaned, the question becomes who does it and how, and that is where the method matters more than the price.

How a soft wash actually works, from quote to final rinse

Soft washing is the opposite of what most people picture. Instead of force, it relies on a cleaning solution applied at low pressure that kills the algae, mildew and lichen at the root, followed by a thorough low-pressure rinse. Our technicians adjust pressure and solution strength to each surface, so the approach on soft limestone is different from the approach on a concrete driveway, and different again on a roof. We say we pioneered a process for cleaning large, unique properties safely, and that process begins with the quote.

You contact us by call, text or online form and we gather the surface types, approximate square footage and access. New customers and complex homes get an in-person estimate; simple jobs can be quoted from satellite imagery or your photos. The quote comes by text and email, priced by the square foot, and includes labor, water and supplies with no hidden fees. On the day, your technician texts when on the way and, if you are home, walks the property with you before starting. Plants are pre-soaked and rinsed before and after, pet-safe detergents are used, sensitive items are covered, and you receive post-cleaning instructions. If anything is not right, we return within twenty-four hours.

• Contact by call, text or form; we collect surfaces, square footage and access details.
• In-person estimate for new customers and complex properties, or a quote from maps and photos for simple jobs.
• Square-foot pricing by text and email, including labor, water and supplies.
• Technician texts en route and walks the property with you before starting.
• Plants pre-soaked and protected, pet-safe detergents, sensitive items covered.
• Low-pressure application of the cleaning solution, dwell time, then a thorough low-pressure rinse.
• Digital invoice, follow-up, and a return within twenty-four hours if you are not satisfied.

What decides the price of an exterior cleaning

Our typical exterior cleaning runs from 250 to 1,250 dollars depending on the service and the size of the property, and veterans receive a ten percent discount. Within that range, the main driver is square footage, which is why we quote that way. The second is the surface: porous stone and stucco take more time and solution than painted siding, and roofs require their own approach. The third is the severity and age of the growth; a decade of accumulation needs longer dwell times than last season's film.

Access and height matter, since a three-story facade or a steep roof takes more time and care than a single-story ranch. Bundling services on the same visit, such as house, driveway, windows and gutters, is usually more economical than scheduling each separately, and recurring plans keep the growth from ever reaching the heavy stage again. Commercial properties are priced on the same square-foot logic, with night and weekend scheduling where a business needs it.

Choosing a company that will not damage your house

The question to ask any exterior cleaner is simple: how will you clean my stone, and at what pressure? If the answer is a pressure number and nothing about solution, dwell time or surface type, keep looking. Ask whether they are insured, including workers' compensation for the people on your roof and ladders, and ask to see it. Ask what they do to protect plants and pets, because the cleaning solution that kills algae will also stress a flower bed if nobody rinses it.

Ask how the quote is built and whether it includes everything. Ask what happens if a spot is missed or a problem appears afterward. Reviews are useful when they mention specific results, such as Austin stone brought back to its original color or a surface cleaned after another company failed, rather than general praise. And trust the first contact: a company that explains the process before you ask is usually the one that follows it on your property.

• Soft washing described specifically for stone, stucco, brick and roofs.
• Full insurance including workers' compensation, shown on request.
• A clear plant and pet protection routine.
• Square-foot pricing that includes labor, water and supplies.
• A written satisfaction policy with a defined response time.
• Local history and reviews that describe actual results on surfaces like yours.

Mistakes we are called to fix

The most expensive one is the rented pressure washer. We are regularly called after a homeowner or a handyman has etched limestone, blown out mortar or stripped a section of roof while trying to clean it. There is no way to put that surface back. The second is bleach from the hardware store sprayed without protecting the landscaping, which leaves dead plants along the foundation and a wall that is streaked rather than clean.

The third is hiring on price alone and getting the "show up and spray" treatment: no pre-soak, no walkthrough, no dwell time, and a result that lasts a season. The fourth is ignoring the moisture sources, so the same wall is dark again within a year. And the last is leaving the roof out. Roof algae is the most visible staining from the street and the one most homeowners do not realize can be removed safely.

Getting on the schedule, and what to expect that day

Call, text or use the form, and tell us what you are seeing and roughly how large the house is. We will either schedule an in-person estimate or, for a straightforward job, quote from imagery and your photos. The quote arrives by text and email, by the square foot, with everything included. If you are preparing to list the house or have an event on a set date, tell us and we will work to that.

On the day, your technician texts before arriving, walks the property with you, protects the plants and your belongings, and cleans each surface with the pressure and solution it calls for. You get a digital invoice and simple aftercare instructions, and if anything is not right we come back within twenty-four hours. We are veteran-owned, locally operated, and we work within about thirty miles of Dallas, on homes as well as storefronts, parking garages, apartment communities and HOA common areas. The stone you fell in love with is still under there.

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