How Long Does Roof Cleaning Last, and Is It Safe for Shingles?

Two questions come up at almost every roof visit in Charleston. Will this hurt my shingles? How long will it stay clean? Here are honest answers. You will also learn what the black streaks are, and when a roof needs a roofer instead of a wash.

What are the black streaks?

The dark streaks running down Lowcountry roofs are not dirt or age. They are algae, and algae is alive. It settles on the roof and feeds on moisture and the minerals in shingles. It shows up first where the roof stays damp longest. That means north slopes and anything under trees. Charleston's damp air feeds it almost all year. The spores ride the wind from roof to roof.

The key point is that it is growth, not a stain sitting on top. Force takes off the top layer and leaves the rest. That is why a roof that was blasted clean can look streaky again in one season.

Why is high pressure wrong for shingles?

Asphalt shingles have a coat of small mineral granules on top. These granules shield the shingle from sun and weather. A pressure washer knocks them loose. Once they are gone, problems follow:

• Bare shingle is left out in the sun.
• The shingle dries out and ages faster.
• The roof does not last as long.
• The warranty is often voided.
• Water can be pushed under the shingles.

The warranty point matters. Most shingle makers say pressure washing is not an approved way to clean. So damage from it falls on the homeowner, not the warranty. The safe way is low pressure. We put on a cleaning solution gently so it can kill the algae where it grows. Then we rinse with less force than a hard rain. The solution does the work, not the machine.

Do metal roofs need the same care?

Yes. Metal roofs look tougher, and the metal is strong. But the part you see is a factory coating. High pressure can scratch it, dull the shine, and push water under panels, seams, and flashing. Metal roofs grow the same algae. Near the water, they also get a salt film. They get the same low pressure method.

How long do the results last?

There is no set number of years. Be careful with anyone who promises one. The answer comes from the roof and the lot, not the cleaning. Here is what matters most:

• How much direct sun each slope gets.
• Trees and limbs that hang over the roof.
• How well air moves around the house.
• How steep the roof is, and which way it faces.
• How close you are to the marsh or open water.

A roof in open sun with good air flow stays clean much longer. The same roof under live oaks will streak sooner. One thing is sure. Killing the growth at the root keeps a roof clean longer than a rinse. It will come back someday, because the spores are in the air. A maintenance plan puts the next cleaning on a schedule, so you do not have to remember.

Why do some stains only fade?

Growth responds very well to treatment. But not every mark on a roof is growth. These marks may fade instead of going away:

• Rust that runs down from flashing or nails.
• Spots where the granules are already worn off.
• Old sealant or patch repairs.
• Shadow lines left by moss that sat for years.
• Damage from an old high pressure wash.

Timing plays a part too. A roof looks much better the day we treat it. It often keeps getting better for a few weeks. The dead growth breaks down, and rain carries it off.

How do we protect your plants?

Everything we put on a roof comes off the roof. It runs down the slopes and onto whatever is planted below. So we plan for your plants before we start. We wet plants before and rinse them after. We manage and thin out the runoff while we work. If you have special plants near the roof edge, tell us. We will plan the runoff around them.

Does it need a cleaning or a roofer?

A roof that needs a cleaning has dark streaks, green growth on the shady slopes, or moss in the valleys. The roof itself is sound. The surface is just dirty.

A roof that needs a roofer looks different. Watch for these signs:

• Shingle edges that curl or lift.
• Cracked or missing shingles.
• Loose granules piling up in the gutters on their own.
• Nail heads showing, or failed sealant at the flashing.
• A roof line that sags, or soft spots underfoot.
• Water stains on ceilings inside.

If a roof is at the end of its life, cleaning will not fix it. It can even make old damage easier to see. When we see that, we tell you before we start.

Roof cleaning FAQ

Is roof cleaning safe for asphalt shingles?

Yes, when it is done with low pressure. Shingles have a coat of mineral granules that protect them, and a pressure washer knocks them loose. A low pressure cleaning uses a solution to kill the growth, then a gentle rinse. Nothing scrapes the surface. We never put a pressure washer on a roof.

How long does roof cleaning last?

It depends on the roof, so we will not give you a set number of years. Shade, trees, air flow, how steep the roof is, and which way the slopes face all change how fast growth comes back. A roof in open sun stays clean much longer than one under live oaks.

Can you clean a metal roof the same way?

Yes. Standing seam and panel metal roofs grow the same algae and coastal grime. They get the same low pressure method. Metal looks tougher than shingles, but the finish is a coating. High pressure can scratch or dull it and push water under panels and flashing.

Will every stain come off my roof?

No, and we would rather tell you up front. Growth responds well to treatment. Rust from flashing, spots where granules are worn off, old sealant patches, and shadow lines from old moss may only fade. We point those out before we start.

What happens to my plants during a roof cleaning?

Everything we put on a roof runs down the slopes, so we protect your plants as part of the job. We wet plants before we start. We manage and thin out the runoff while we work. Then we rinse everything again at the end.

How do I know if my roof needs cleaning or a roofer?

Streaks, green growth, and moss on shady slopes mean it needs a cleaning. Curling or missing shingles, soft spots, nails showing, and water stains on ceilings inside mean it needs a roofer. If we see signs that call for a repair instead of a wash, we tell you.

Related