Mold on a Roof Is a Symptom, Not the Problem
Mold does not cause a wet roof. A wet roof causes mold. Which means cleaning it off without finding out why it is there guarantees it comes back — usually in the same place, usually within a year.
Houston gives mold everything it needs. Humidity for most of the year, warmth for nearly all of it, and organic material in the form of decking, debris and the paper backing on insulation. Where those three meet standing moisture, growth follows in 24 to 48 hours.
So the useful question is not "how do I get rid of it". It is "what is wet, and why". That is a roofing question, and it is the one this page answers.
What we do
Fix the cause
- Find the leak — which is rarely above the stain
- Repair flashing, penetrations and failed seals
- Replace decking that has been wet long enough to soften
- Correct the ventilation, which is the cause more often than the leak is
- Document all of it, photographed, so you have a record
What we do not do
Mold remediation
In Texas, mold assessment and mold remediation are licensed occupations under Occupations Code Chapter 1958, administered by TDLR.
We do not hold that licence, so we do not offer that work. What we will do is tell you clearly when you need a licensed remediator, and how to check that the one you hire actually is.
A roofer offering to remediate mold is worth a question about their TDLR licence.
The rule worth knowing before anyone quotes you
The 25 Contiguous Square Feet Threshold
Texas draws a line at 25 contiguous square feet of mold contamination.
- Below 25 sq ft
- You may hire any contractor to do the work, or do it yourself.
- 25 sq ft or more
- The work must be performed by a licensed Mold Remediation Contractor, following a written protocol from a licensed Mold Assessment Consultant.
- Your own home
- A residential owner remediating their own property is exempt, whatever the size.
- Rental or commercial
- That exemption does not apply. If you are a landlord, licensed professionals are required.
And the part most people get wrong: TDLR has clarified that the 25 square feet is not limited to what you can see. The entire area requiring cleaning, removal or treatment counts. Visible growth on an attic rafter is often the smallest part of the affected area.
You can verify any mold licence directly at tdlr.texas.gov. A legitimate remediator will expect you to.
Four Reasons a Houston Roof Grows Mold
In order of how often we find them. The first one surprises people, because it is not a leak at all.
Ventilation that does not work
The most common cause we find, and the one nobody expects. A poorly ventilated attic traps warm, moist air. That air condenses on the underside of the decking overnight, and the decking is damp every morning without a single drop of rain getting in.
It usually means exhaust without intake — ridge vents or turbines with no soffit venting to feed them, or soffits packed shut with insulation. Air cannot leave if none is coming in.
How to spot it: mold spread evenly across the underside of the decking rather than concentrated in one place. Even distribution means condensation, not a leak.
A leak you have not found
Water entering at a flashing, a penetration or a failed seal, running along the deck, and keeping one area permanently damp.
How to spot it: a concentrated patch, usually darker at the center. The entry point is often several feet uphill of it.
Debris holding water on the roof
Pine needles and leaves packed into valleys or behind a chimney hold moisture against the shingles for days after rain. Under canopy this is constant rather than seasonal.
Pine needle removal from $149 — and it is a different job from gutter cleaning.
Shade and humidity
North-facing slopes and areas under heavy tree cover dry slowly, which is why the dark streaking you see on Houston roofs is almost always on the same side of the house.
Worth knowing: most of that streaking is algae (Gloeocapsa magma), not mold. Ugly, largely cosmetic, and often mistaken for something worse by a contractor who wants to sell you a roof.
What most people actually have
Those Black Streaks Are Almost Certainly Algae
If your roof has dark streaks running down the slope — particularly on the north side, or under trees — that is Gloeocapsa magma, an airborne blue-green algae. It feeds on the limestone filler in asphalt shingles and forms a dark protective coating as it grows.
It is cosmetic. Algae does not damage the structural integrity of standard asphalt shingles. It affects how the roof looks, not how long it lasts.
We are saying that plainly because it is regularly presented as something worse. If a contractor points at black streaking and recommends a full replacement, get a second opinion.
Do not
Pressure wash it
High-pressure water strips the protective granules off the shingle surface — the granules that block UV and decide how long the roof lasts. It voids most manufacturer warranties, and it turns a cosmetic problem into a real one.
This is the single most common way homeowners damage their own roof.
The recommended method
Soft wash
The Asphalt Roofing Manufacturers Association recommends a gentle 50/50 mix of water and household bleach, applied at low pressure.
- Wet nearby landscaping thoroughly first
- Apply the solution gently — no pressure
- Let it dwell 15 to 20 minutes
- Rinse at low pressure with a garden hose
- Wet the landscaping again afterwards
One caution. All of that means being on a roof, with a wet, slippery surface and a bleach solution. Half of non-fatal ladder injuries happen below ten feet. Before you go up.
And the permanent fix, if it keeps coming back: algae-resistant shingles. They carry copper-containing granules that inhibit growth, and most major manufacturers offer a stain warranty on them. Worth specifying next time the roof is replaced rather than cleaning it every few years.
Why it matters
What Sustained Moisture Actually Does
Set aside the health question for a moment. The structural case is enough on its own, and it gets more expensive the longer it waits.
Decking softens, then fails
Plywood and OSB lose structural integrity once they have been repeatedly wet. Soft decking will not hold a fastener, which means it cannot hold a new roof either — so it has to be replaced before anything else happens.
$180 per repair area · $90 a sheet during a replacement
Insulation stops insulating
Wet insulation compresses and loses most of its R-value, so the attic gets hotter, the air conditioning works harder, and the roof above cooks faster.
And it does not recover when it dries
It reaches the ceiling
Once moisture is through the decking it finds drywall, paint and trim. That is when a roofing problem becomes an interior renovation.
$500–$2,000+ in drywall and finishes
Your claim gets harder
Insurers cover sudden damage, not gradual deterioration. Moisture that has been present for months is very difficult to attribute to a storm.
On the health question. The EPA and CDC both state that damp indoor environments are associated with respiratory symptoms, and that people with asthma or allergies may be more sensitive. We are roofers, not doctors — if anyone in the house has symptoms you think are related, speak to a physician. What we can tell you is whether the roof is why the house is damp.
Roof Mold Questions
Do I need a licensed contractor to remove mold in Texas?
It depends on the area. Below 25 contiguous square feet you can hire any contractor, or do it yourself. At 25 square feet or more, the work must be done by a licensed Mold Remediation Contractor following a protocol from a licensed Mold Assessment Consultant. A homeowner working on their own home is exempt whatever the size — but a landlord is not.
Does the 25 square feet include mold I cannot see?
Yes. TDLR has clarified that the threshold covers the entire area requiring cleaning, removal or treatment — not only visible growth. Visible mold on an attic rafter is frequently the smallest part of the affected area, which is why the rule catches people out.
Does JC&C Roofing do mold remediation?
No. We do not hold a TDLR mold licence, so we do not offer remediation or assessment. What we do is find and fix the roofing cause — the leak, the flashing, the failed ventilation — and tell you honestly when a licensed remediator is needed.
What is the black streaking on my roof? Is that mold?
Usually not. Dark streaking on Houston roofs is almost always algae — Gloeocapsa magma. It is unsightly and largely cosmetic. It is also frequently misidentified as mold by contractors selling a roof replacement, so it is worth a second opinion before you accept that diagnosis.
Why does mold keep coming back after I clean it?
Because cleaning removes the growth, not the moisture. Mold does not cause a wet roof; a wet roof causes mold. Until the leak or the ventilation problem is fixed, the conditions that produced it are still there, and in Houston humidity regrowth takes 24 to 48 hours.
Does insurance cover mold damage?
Rarely, and usually only in limited amounts. Policies cover sudden damage from a covered peril, not gradual deterioration — and mold that has been developing for months is difficult to attribute to a storm. Many Texas policies also carry a specific mold sublimit. How Texas claims work
How do I safely clean black streaks off my roof?
Never pressure wash. High pressure strips the protective granules off the shingles and voids most manufacturer warranties — it turns a cosmetic problem into a real one.
The Asphalt Roofing Manufacturers Association recommends a soft wash: a gentle 50/50 mix of water and household bleach, applied at low pressure, left to dwell 15 to 20 minutes, then rinsed with a garden hose. Wet the landscaping before and after.
If it keeps returning, algae-resistant shingles with copper-containing granules are the permanent answer, and most manufacturers offer a stain warranty on them.
Find out why it is wet
Free roof inspection with attic access where possible, thermal imaging and moisture readings where they help, and photographs of everything we find with photographs — whether or not any work follows.
If what you have needs a licensed remediator, we will say so and tell you how to check theirs.
How we find leaks · 33 repairs, priced · Pine needle removal · Maintenance from $149