Quick Answer
The best way to stop a timber deck from rotting is a combination of annual cleaning, oiling every 1–2 years, and ensuring good drainage and airflow under the deck. In Melbourne’s climate — wet winters followed by hot, dry summers — merbau and treated pine decks need oiling before the dry cracks set in. Budget $80–$200 in materials for a typical 20m² deck, and plan a weekend in autumn or spring when the timber is dry but not baking hot.
Why Melbourne Decks Rot Faster Than You Think
Decks in south-eastern Melbourne cop a punishing annual cycle. June–August brings sustained rain, and clay soils under the deck stay saturated — keeping the subframe in a damp, fungus-friendly environment for months. Then November arrives and the same timber bakes at 35–40°C, opening surface cracks that funnel the next winter’s moisture deep into the board.
Merbau is the most common decking timber in Melbourne and it holds up well — but it still needs maintenance. Treated pine (H3 above-ground) is cheaper and rot-resistant to a point, but once the surface coating fails, water penetrates quickly. The subframe timbers (bearers and joists) are usually H4 or H5 treated and much more rot-resistant, but poor drainage can defeat even these.
Annual Maintenance Checklist
| Task | Frequency | Time Required | Cost (DIY) |
|---|---|---|---|
| Clean deck surface (sugar soap + scrub) | Annually (autumn) | 2–3 hours | $15–$30 |
| Check and clear drainage gaps between boards | Twice yearly | 30 mins | $0 |
| Apply deck oil or penetrating sealer | Every 1–2 years | Half day | $50–$120 |
| Inspect subframe for rot and termites | Annually | 30 mins | $0 |
| Tighten or replace loose screws and fixings | Annually | 1 hour | $10–$20 |
| Sand rough or badly cracked boards | Every 3–5 years | Full day | $50–$150 |
How to Clean and Oil a Deck (Step-by-Step)
Step 1: Clear and Sweep the Deck
Remove all furniture, pot plants, and mats. Sweep debris from between boards using a stiff broom — leaf litter and soil trapped between boards is one of the fastest ways to accelerate rot, as it holds moisture against the timber surface for weeks. If you have pot plants directly on the deck, use pot feet to allow airflow underneath.
Step 2: Clean with Sugar Soap or Deck Cleaner
Mix sugar soap at 1:10 with water (or use a proprietary deck cleaner like Flood or Cabot’s Deck Cleaner). Wet the deck surface first, then scrub with a stiff deck brush working along the board grain. For green algae or heavy grime, let the cleaner dwell for 5 minutes before scrubbing. Rinse thoroughly with a garden hose — not a pressure washer, as the high pressure can raise the grain and damage the surface.
Step 3: Allow to Dry Completely
The deck must be completely dry before oiling — usually 24–48 hours in Melbourne’s autumn weather. Test with your hand: the timber should feel dry, not cool-damp. Oiling wet timber traps moisture inside the board and can cause the oil to peel within months.
Step 4: Apply Deck Oil or Penetrating Sealer
Choose the right product for your timber. Merbau takes oil well — Cabot’s Australian Timber Oil, Feast Watson Deck & Exterior Oil, and Intergrain UltraDeck are all commonly available and suitable. Apply with a wide brush or roller working along the grain, then back-brush to work the oil into the surface. Don’t over-apply: a second light coat after 2–4 hours is more effective than a thick single coat.
Improving Drainage and Airflow Under the Deck
The subframe is the most expensive part of a deck to replace — bearers and joists that rot out can cost $3,000–$8,000 to replace on a mid-sized deck. Protecting the subframe costs almost nothing:
- Clear any leaf litter or soil build-up under the deck annually (use a leaf blower or long-handled rake).
- Ensure ground under the deck slopes away from the house — accumulated water sitting under a low deck will rot the subframe within 3–5 years.
- Remove any plants growing up through or against the deck structure.
- If the deck is close to the ground (under 300mm clearance), consider laying weed mat and coarse gravel under the deck to improve drainage.
Identifying Rot Early
| What You See | What It Means | Action |
|---|---|---|
| Grey, dry surface with surface cracks | UV weathering — surface only | Clean, sand lightly, oil |
| Black discolouration in cracks | Fungal growth beginning | Clean with deck cleaner, treat with timber preservative, then oil |
| Soft or spongy board under foot pressure | Active decay in the core | Replace affected boards |
| Screwdriver penetrates board easily | Significant rot — board compromised | Replace board; inspect subframe beneath |
| Joist feels soft when you push through a gap | Subframe rot — urgent | Call a builder or licensed fencer for assessment |
When to Call a Professional
Surface maintenance and board replacement are excellent DIY tasks. However, if probing reveals rot in the bearers or joists — the structural frame — call a licensed builder for an assessment. Reframing a deck is building work that may require a permit from your local council, depending on the deck’s size and height. In Victoria, decks over 1m above ground level generally require a building permit.
Tips and Gotchas
- Oil in the shade or on overcast days: Applying deck oil in direct summer sun causes it to skin over before penetrating properly. Work in the early morning or on a mild, overcast day.
- Match oil to timber type: Using a product designed for hardwood on treated pine (or vice versa) can give poor penetration or blotchy results. Check the tin before buying.
- Don’t use paint on a deck: Film-forming products like exterior paint peel on horizontal surfaces subject to foot traffic and UV. Use penetrating oils or specific deck coatings only.
- Re-coat before the previous coat fully wears: Waiting until the timber is visibly grey means the protection has already failed. Oil when the water-bead test shows water soaking in rather than beading.
- Pot plants = rot risk: Terracotta and plastic pots sitting directly on timber trap moisture underneath. Use pot feet — they’re cheap and save the deck board below.
- Check between boards: Gap debris is the hidden rot accelerator. Clear it twice a year with a blunt scraper or thin stick — never a metal tool that could scratch.
- Termites love deck subframes: If you see mud tunnels or damaged wood under the deck, call a licensed pest controller. Don’t disturb the mud leads — this destroys the inspector’s evidence trail.
- Composite decking still needs cleaning: If you have composite or PVC decking, it doesn’t rot but still needs annual cleaning to prevent mould and surface staining.
Local Melbourne Resources
- Bunnings — Deck oils, cleaners and sealers
- Cabot’s — Australian Timber Oil product range
- Victorian Building Authority — Do I need a building permit?
- Mitre 10 — Decking oil and maintenance products
- Australian Environmental Pest Managers Association — Find a pest controller
FAQ
How often should I oil a merbau deck in Melbourne?
Every 12–18 months in Melbourne’s climate. The wet winters and hot, dry summers accelerate UV and moisture damage. Do the water-bead test: if water soaks in rather than beading, it’s time to oil. Don’t wait until the timber is visibly grey and cracked.
Can I use a pressure washer to clean my deck?
Use with caution. Keep pressure under 1000 psi and the nozzle at least 300mm from the surface. High pressure can raise the grain, leaving a rough surface that holds moisture and ages faster. A stiff deck brush and sugar soap gives comparable results with less risk.
My deck boards are grey and cracked — is it too late to oil them?
Grey is recoverable with a light sand (80-grit) to open the grain before oiling. Deep cracks that you can fit a coin into mean the board is past treatment — replace those boards before oiling around them. Always clean thoroughly before sanding or oiling.
Can I replace individual deck boards myself?
Yes, for standard decking boards screwed to joists. Remove the screws (use a good quality driver bit — Torx head screws strip easily), pry out the damaged board, and cut a new board to length. Predrill screw holes to avoid splitting, and leave a 5–6mm gap between boards for drainage and expansion.
What is the best product to treat deck rot before oiling?
For surface-level fungal staining, use a deck brightener or oxalic acid-based cleaner before oiling. For early-stage soft rot in a board you want to preserve short-term, a penetrating timber hardener (like Selleys Liquid Sugar Soap or Rot-Stop) can stabilise the area, but it’s a temporary fix — replace the board within the next season.