Black Roof Streaks in Alexandria — Fixed Properly
How to tell if it's black roof streaks
Four tells we look for on every Alexandria survey before quoting.
- Dark vertical streaks running from ridge to eaves, worst on north- or east-facing pitches
- Streaks that look worse after rain and appear to 'dry lighter' — that's the algae rehydrating
- Neighbouring houses on the same row showing similar staining
- A slightly rough or matte texture on tiles that used to look smooth
Why it happens on Alexandria properties
Once a colony establishes, spores travel airborne to neighbouring properties — which is why streaks tend to appear in clusters along a street rather than one house at a time.
Uncleaned gutters spill water back onto tile edges, keeping the underside of ridge courses damp and accelerating streak formation from the top down.
In Alexandria specifically, vale of leven catches heavy westerly rain off loch lomond — gutters fill fast and slate moss is the dominant roof issue — which is the single biggest reason this issue is more stubborn here than in a drier inland town.
Our Alexandria fix — step by step
- 1Roof survey
Ground and drone-level inspection to confirm tile type, pitch, condition and any slipped or cracked tiles that need addressing first.
- 2Manual moss removal
Where moss is present alongside the algae, we soft-scrape by hand before wetting the tiles — never with a pressure washer.
- 3Biocide soft-wash
A low-pressure application of a professional-grade sodium hypochlorite/benzalkonium biocide blend, dwelled and rinsed at low pressure.
- 4Gutter clear-down
Every roof clean includes a gutter clear — algae debris will otherwise wash straight into blocked runs.
- 512-month cover
A residual biocide film keeps regrowth at bay for a season; annual top-ups are optional and cheaper than a full re-clean.
Chemistry & materials we use
- Sodium hypochlorite at controlled concentration for algae kill
- Benzalkonium chloride residual for 6–12 month regrowth cover
- Neutralising surfactant so run-off is safe for driveways and planting
- Never high-pressure water — pressure cracks the tile surface and voids most roof warranties
One day for a typical 3-bed semi, two days for large detached or steep-pitched properties.
Alexandria quote to job completed is usually under two weeks.
£320–£620
£ per typical 3-bed roof
Every Alexandria quote is fixed-price in writing — no on-the-day surprises.
Alexandria-specific considerations
Heavy ex-council stock through Tullichewan and Bonhill, sandstone terraces in the town centre, and a steady stream of modern Stewart Milne new-builds towards Jamestown.
Vale of Leven catches heavy westerly rain off Loch Lomond — gutters fill fast and slate moss is the dominant roof issue.
Off the A82 — Alexandria work combines naturally with Balloch and Dumbarton stops. Nearest motorway is the A82, with routes via A82 and A811.
What NOT to do
Common mistakes we see on Alexandria properties before we're called in.
- Do not jet-wash the tiles — it strips the mineral coating and the streaks return worse within 12 months.
- Do not use household bleach — the concentration is wrong and it will scorch adjoining render and planting.
- Do not paint over the streaks — coatings peel within a season on damp Scottish roofs.
Black Roof Streaks — questions from Alexandria customers
Is your roof cleaning insured for my property?+
Yes — £5m public liability, and West Dunbartonshire Council addresses are on file weekly. Certificate sent before the job starts.
Can you treat one pitch only?+
Yes. On many Alexandria semis the streaks are only on the north or gable-facing side, and treating one pitch alone is significantly cheaper.
Do black streaks damage the roof?+
Long-term, yes — the algae etches the mineral coating on concrete tiles, which then absorb more water. In Alexandria that matters, because vale of leven catches heavy westerly rain off loch lomond — gutters fill fast and slate moss is the dominant roof issue. Cleaned early, tiles are restored without replacement.
Will the streaks come back?+
The 12-month biocide layer prevents regrowth in that window. After that a light top-up spray every 12–18 months keeps Alexandria roofs clear — much cheaper than a full re-clean.
Free quote for Alexandria — same day
Fixed-price quote for black roof streaks on your Alexandria property. Fully insured, West Dunbartonshire Council-covered, 12-month guarantee on the work.
Other fixes we handle in Alexandria
Related pages for Alexandria
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