Oil and grease stains are one of the most common causes of paint failure on metal roofs — if you paint over them, the coating simply won't bond and will peel within months. The good news is that with the right products and a methodical approach, you can get your roof surface clean and ready for a long-lasting paint job.
Why Proper Degreasing Matters
Metal roofs — whether corrugated steel, Colorsteel, or aluminium — have a smooth, non-porous surface. Any oil, grease, or contamination acts as a barrier between the metal and the primer. Even a thin film of grease left behind will cause adhesion failure, leading to bubbling, flaking, and costly repaints down the track. This is especially important in NZ's variable climate, where roofs expand and contract significantly with temperature changes, putting extra stress on any poorly bonded coating.
Safety First
Before you climb onto any roof, take these precautions seriously:
- Never work on a wet or damp roof — metal surfaces become extremely slippery.
- Use a safety harness and anchor point rated for roof work. Falls from roofs are a leading cause of serious injury in NZ. Check WorkSafe NZ for guidance on working at height.
- Wear chemical-resistant gloves and eye protection when handling degreasers.
- Work with a buddy whenever possible, and let someone know you're on the roof.
- Check the weather forecast — avoid working in high winds or direct summer sun, which can cause degreasers to dry too quickly.
What You'll Need
| Item | Purpose |
|---|---|
| Alkaline or solvent-based degreaser | Breaks down oil and grease |
| Stiff-bristle scrubbing brush | Agitates contamination |
| Pressure washer or garden hose | Rinses residue away |
| Clean rags or absorbent cloths | Blots up pooled grease |
| Chemical-resistant gloves & goggles | Personal protection |
| Roof ladder or crawl boards | Spreads your weight safely |
Step-by-Step: Removing Oil and Grease Stains
Step 1 – Blot Up Excess Grease
If there's any pooled or heavy grease, blot it up with absorbent rags or paper towels first. Don't spread it around — work from the outside of the stain inward.
Step 2 – Apply a Degreaser
Choose a purpose-made alkaline degreaser or a solvent-based degreaser suitable for metal surfaces. Products like Sika cleaning solutions or trade-grade degreasers available from Mitre 10 or Bunnings are widely used. Always read the product data sheet and follow the dilution instructions.
- Apply the degreaser directly to the stained area.
- Allow it to dwell for the time specified on the label (usually 5–15 minutes) so it can penetrate and lift the contamination.
- For stubborn stains, a second application may be needed.
Step 3 – Scrub Thoroughly
Using a stiff-bristle brush, scrub the stained area in a circular motion. Pay attention to laps, ridges, and any low spots where grease can pool.
Step 4 – Rinse Completely
Rinse the area thoroughly with a pressure washer or strong hose. It's critical to remove all degreaser residue — any left behind can interfere with paint adhesion just as much as the original grease. Work downhill so rinse water flows away from cleaned areas.
Step 5 – Check and Repeat if Needed
Once dry, inspect the area. Water should sheet off evenly rather than beading up in spots. If water still beads on an area, there's likely residual contamination — repeat the degreasing process on that spot.
Step 6 – Full Roof Wash
Once spot stains are treated, wash the entire roof with a suitable roof cleaner to remove moss, lichen, dirt, and general grime before painting. This gives your primer a consistently clean surface to bond to.
Choosing the Right Degreaser
- Alkaline degreasers (water-based) are generally safer to handle and environmentally friendlier. They work well on most grease and oil contamination.
- Solvent-based degreasers (such as mineral turpentine or proprietary solvent cleaners) are more aggressive and better suited to heavy industrial grease or bitumen-based contamination. Use with good ventilation.
- Always check that your chosen degreaser is compatible with your intended primer and topcoat. Paint manufacturers like Resene and Dulux publish technical data sheets specifying approved surface preparation methods — follow these to keep your paint warranty valid.
Priming After Cleaning
Once the roof is clean and fully dry, apply a metal primer recommended for your roof type and topcoat. For Colorsteel or galvanised iron, a zinc-based or epoxy primer is typically specified. Check with your paint supplier — Wattyl, Resene, and Dulux all have dedicated roof coating systems with specific primer requirements.
When to Call a Professional
If the grease contamination is extensive, the stains keep returning (suggesting an ongoing leak from a flue, exhaust, or mechanical unit), or the roof surface is damaged or heavily corroded, it's worth getting a qualified roof painter to assess the job. A professional will have access to commercial-grade degreasers, pressure washing equipment, and the experience to identify underlying issues before they become expensive problems. You can find trusted local roof painters on tradedirectory.co.nz.
Also note: if your roof work involves any structural repairs, replacing roof cladding, or flashings, check whether a building consent is required with your local council. Some work may qualify as exempt under Schedule 1 of the Building Act, but it pays to confirm — visit MBIE Building Performance for guidance.
Master Painters NZ member
Licensed Building Practitioner (LBP)
Roofing Association of NZ (RANZ)
