Roof Cleaning in Houston
The black streaks on Houston roofs are algae, not dirt — and the single most damaging thing you can do about them is hire someone who pressure washes shingles. Here is what is actually happening and what the right job looks like.
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Those dark streaks are not dirt
The black staining that runs down a Houston roof, almost always on the north-facing slope, is a blue-green algae called Gloeocapsa magma. It feeds on the limestone filler in asphalt shingles and it spreads by spores, which is why one house on a street gets it and then the neighbours do.
It matters for two reasons beyond appearance. A dark roof absorbs more heat, which does your attic and your electricity bill no favours in a Houston August. And the algae holds moisture against the shingle, which ages it faster than sun alone would.
Why pressure washing a roof is the wrong answer
Asphalt shingles are protected by a layer of mineral granules bonded to the surface. Those granules are the shingle's sunscreen and its armour. High pressure strips them off.
You will not necessarily see the damage on the day. What you see is a clean roof. What happens over the following two or three years is accelerated ageing, brittleness and a roof that fails years before it should — and no manufacturer warranty survives it.
If a company offers to pressure wash your roof, that single sentence tells you everything you need to know about whether to hire them. Ask what pressure they use. The correct answer for a shingle roof is about the same as a garden hose.
What soft washing actually is
Soft washing applies a cleaning solution at low pressure, gives it time to work, and rinses gently. The solution kills the algae rather than blasting it off, which is why the result lasts: blasted algae grows back from what remains, killed algae has to recolonise from scratch.
| Step | What it involves |
|---|---|
| Protect | Wet down landscaping and beds below the roofline, cover anything sensitive |
| Apply | Cleaning solution at low pressure across the affected slopes |
| Dwell | Time for the solution to work — this is the part that cannot be rushed |
| Rinse | Low pressure, working downward, plus a final rinse of the landscaping |
Some streaking lifts during the rinse. Some keeps fading over the following weeks as rain carries the dead growth off. A roof that looks eighty percent better on the day often looks better still a month later.
What it costs and how often
Roof cleaning is priced higher per square foot than driveways, and reasonably so — it takes longer, it needs more care, and working at height carries real risk. In this climate a properly soft-washed roof typically holds for two to four years, depending on how much shade the slope gets and how much organic debris lands on it.
Heavily shaded roofs under pine or oak need it more often. That is the same moisture logic that makes north-facing walls go green first.
What to ask before you book
- What pressure are you using on the shingles? Low. If they quote a number in the thousands of PSI, keep calling.
- What is the solution, and what does it do to my plants? Beds need wetting before and rinsing after.
- Are you insured? Roof work is where this stops being a formality.
- Will it all come off on the day? Honest answer is usually mostly, with the rest fading over following weeks.
- Are the gutters included? Often a sensible add-on since the crew is already up there.