Metal roofs last 40–70 years, but only with proper maintenance — and cleaning is the most important part. Moss, algae, lichen, and rust stains don’t just look bad; they trap moisture against the metal and accelerate corrosion. The right cleaning method depends on your roof type: standing seam, exposed fastener, corrugated steel, or metal shingles. This guide covers safe cleaning methods, the correct cleaning solutions, and what to avoid to keep your metal roof’s protective coating intact.
What You’ll Need
Tools
- Garden sprayer or pump sprayer
- Soft-bristle brush on an extension pole
- Garden hose with adjustable nozzle (low pressure)
- Pressure washer — maximum 1,200 PSI, 40-degree tip (optional, for stubborn areas only)
- Safety harness, non-slip footwear, and ladder rated for roof work
- Rubber gloves and eye protection
Materials
- Oxygen bleach (sodium percarbonate — Oxiclean, Bio-Shield, or similar) — safest for metal roofs
- OR dilute chlorine bleach solution (1 part bleach to 10 parts water) — effective but must be rinsed thoroughly
- Zinc sulfate or copper sulfate solution (for long-term moss and algae prevention)
- Commercial roof cleaner specifically rated for metal roofs (Spray and Forget, Wet and Forget)
- Rust converter or rust-inhibiting primer (if rust spots found)
Safety Precautions
- Never work on a wet or damp metal roof — metal becomes extremely slippery when wet. Work only on dry days and allow morning dew to fully evaporate before accessing the roof.
- Use a safety harness attached to a roof anchor point for any work on a pitched metal roof. Falls from metal roofs are especially dangerous due to the slick surface.
- Protect all plantings below the roofline before applying any cleaning solution — bleach and chemical cleaners that run off into garden beds kill plants. Wet surrounding vegetation thoroughly before cleaning and rinse again after.
- Never use a pressure washer above 1,200 PSI on a metal roof — higher pressure can force water under panel overlaps, damage sealants, and strip protective coatings.
- Check your roof warranty — some metal roof manufacturers void warranties if unauthorized cleaning chemicals are used. Verify acceptable cleaning products in your warranty documentation before proceeding.
Identify What You’re Cleaning
| Growth/Stain Type | Appearance | Best Treatment |
|---|---|---|
| Algae (Gloeocapsa magma) | Black or dark grey streaks running down slope | Oxygen bleach or dilute chlorine bleach solution |
| Moss | Green/brown fuzzy mat, retains moisture | Oxygen bleach + soft brush; zinc/copper treatment for prevention |
| Lichen | Grey-green crusty patches | Commercial lichen killer (wet and forget type); requires patience — may take weeks to fully die |
| Rust staining | Orange-brown streaks or spots | Oxalic acid-based rust remover for metal; treat then seal with rust-inhibiting primer |
| Tannin staining (from trees) | Brown-tan streaks from overhanging trees | Oxygen bleach solution; no pressure needed |
| White efflorescence | White chalky streaks | pH-neutral cleaner or dilute vinegar (check coating type first) |
How to Clean a Metal Roof Step by Step
Step 1: Safety Setup and Pre-Rinse
Set up your safety equipment. Wet all plantings, shrubs, and lawn areas below the roofline with a garden hose — this dilutes any runoff from cleaning solutions. Do a pre-rinse of the roof surface with a garden hose at low pressure to remove loose debris, leaves, and dirt. This prevents debris from clogging your sprayer and removes the top layer of biological material before the chemical treatment.
Step 2: Apply Cleaning Solution
For most residential metal roofs, an oxygen bleach solution is the safest effective treatment. Mix oxygen bleach powder per package directions (typically 1 cup per gallon of warm water) in your garden sprayer. Apply generously to all affected areas. Work from the top of the roof downward so the solution flows over treated areas rather than under them. Allow the solution to dwell for 15–20 minutes — don’t let it dry on the surface. For stubborn moss or algae, gently scrub with a soft-bristle brush on an extension pole while the solution is still wet. Do not use stiff metal brushes.
For a no-rinse approach: commercial “spray and forget” products (Wet and Forget, Spray and Forget) are applied and left without rinsing. Rain gradually washes away the dead biological material over 3–6 months. These are gentler on roof coatings and require no pressure washing.
Step 3: Rinse Thoroughly
After the dwell time, rinse the roof thoroughly from top to bottom with a garden hose at low pressure. Ensure all cleaning solution is washed off — especially around panel overlaps, fasteners, and gutters. Run your garden hose along gutters after roof rinsing to flush cleaning solution residue from the gutter system. Rinse surrounding vegetation again with clean water after the roof rinse is complete.
Step 4: Address Rust Spots
If you find rust spots during cleaning, treat them before they penetrate the metal. Use an oxalic acid-based rust remover designed for metal surfaces. Apply to the rust spot, let it dwell per the product instructions, then rinse. Once the rust is removed, apply a zinc-rich rust-inhibiting primer to the bare metal area. If the rust has eaten through the metal (you see pinholes), that section needs professional repair or panel replacement — surface treatment won’t stop through-rust.
Step 5: Apply Preventive Treatment
After cleaning, apply a zinc sulfate or copper sulfate solution to prevent algae, moss, and lichen from returning quickly. These solutions create an inhospitable environment for biological growth. Apply with a garden sprayer and leave to dry — no rinsing. Alternatively, install zinc or copper strips along the ridge of the roof — rainwater flowing over the metal strips creates a mild biocide that slowly washes down the roof slope, providing years of biological growth prevention.
Maintenance Schedule
| Task | Frequency |
|---|---|
| Clear debris (leaves, branches) from roof and gutters | 2x per year (spring and fall) |
| Inspect for rust spots, sealant failure, and loose fasteners | Annually |
| Full biological growth cleaning (if needed) | Every 2–5 years depending on environment |
| Preventive zinc/copper sulfate treatment | Every 1–2 years |
| Check and re-seal panel overlaps and penetrations | Every 5–10 years |
Pro Tips
- Trim overhanging trees — branches that overhang a metal roof deposit tannins, hold moisture, and accelerate moss growth dramatically. Keeping a clear gap of at least 6 feet above the roof slows biological growth significantly.
- Clean gutters at the same time — a blocked gutter backs up water against the roof edge, one of the most common causes of rust at the eave line on metal roofs.
- Work from a ladder where possible — for lower-slope sections of metal roof, an extension pole with a soft brush and sprayer lets you clean effectively from the ladder without walking on the roof at all.
- Painted vs. Galvalume/bare metal matters — painted metal roofs have a PVDF or polyester topcoat that requires gentle pH-neutral or oxygen-bleach cleaners. Bare galvanized or Galvalume panels are more tolerant of dilute chlorine bleach. Check your roof type before selecting a cleaner.
Frequently Asked Questions
Can I pressure wash a metal roof?
Yes, but with strict limitations: maximum 1,200 PSI, a 40-degree wide-angle tip, and always spray parallel to the panels (never into panel laps or seams at an angle). Higher pressure or improper angles force water under overlaps and can strip paint coatings. For routine cleaning of biological growth, low-pressure chemical treatment plus a garden hose rinse is preferable and safer than a pressure washer.
Will bleach damage my metal roof’s paint?
Dilute bleach (1 part bleach to 10 parts water) applied and rinsed promptly is generally safe for painted metal roofs. Concentrated bleach, prolonged contact, or repeated use without thorough rinsing can degrade paint over time. Oxygen bleach (sodium percarbonate) is the preferred alternative — equally effective against algae and moss without the paint degradation risk of chlorine bleach.
How do I clean a corrugated metal roof versus standing seam?
The cleaning process is the same for both types, but corrugated metal roofs have more surface texture where moss and algae take hold. Pay extra attention to the ridges of corrugated panels where biological growth concentrates. Standing seam roofs are smoother and easier to rinse. For both types, never spray directly into the seams or overlaps with a pressure washer.
My metal roof has rust streaks — is that normal?
Rust streaks on a painted metal roof most often come from rusting steel fasteners, not from the metal panels themselves. Check the screws and exposed fastener heads — if they’re rusting, they should be replaced with stainless steel fasteners or capped with touch-up sealant. Rust streaks from the panels themselves indicate the protective coating has failed in that area and the panel needs spot treatment and possibly professional recoating.
How long does a metal roof cleaning last?
In humid climates (Southeast, Pacific Northwest), biological growth returns in 2–4 years without preventive treatment. With zinc/copper strip installation or annual preventive chemical treatment, cleaning intervals extend to 5–8 years. In dry climates, metal roofs may need cleaning only once every 10+ years. Location, tree coverage, and roof slope all affect how quickly growth returns.
Conclusion
Cleaning a metal roof is a significant but manageable DIY project with the right equipment and safety precautions. The key is using the correct cleaning solution for your roof type, never exceeding safe pressure limits, rinsing completely, and treating rust before it progresses. A clean metal roof not only looks better — it lasts longer by preventing the moisture retention that corrosion requires to take hold.
For related exterior maintenance, see our guide on how to clean aluminum siding for the exterior walls below your roofline. If yard drainage is contributing to moisture issues around your foundation, see our guide on how to build a French drain for managing runoff at the perimeter.
