Why Old Hardwood Decks Start to Rot – and What to Do About It

By Hobart Building Inspections · October 4, 2025

Tasmania’s climate is tough on timber. With wet winters, coastal exposure, and older detailing, rot can creep in unnoticed. Here’s how to spot decay early, why it happens, and what to do before it becomes a structural safety risk.

Key warning signs your deck is deteriorating

• Green staining or algae — timber is holding moisture.
• Splits and checks — repeated wet/dry cycles open the grain.
• Soft or spongy areas — probe bearers/joists; softness suggests decay.
• Sagging or bounce — weakened joists/bearers or failed connections.
• Rust on fixings/railings — coastal exposure accelerates deterioration.

Why hardwood decks rot

Hardwoods resist decay better than softwoods, but they’re not immune. Once protective coatings fail and water penetrates, fungi attack the fibres. In Tasmania, common accelerants include:

• Poor drainage/ponding — water sits on boards or on top of joists and bearers. Joists are often the first to rot, usually from the top, and early signs can be hard to detect.
• Low ventilation — decks close to the ground trap moisture.
• Unflashed ledgers — water tracks into the wall/deck interface.
• Coastal conditions — salt air speeds corrosion and fibre breakdown.
• Untreated end-grain & cut edges — fastest points for moisture uptake.

Special note on Tas Oak decks: Tas Oak isn’t rated for outdoor exposure. Even with coatings, it breaks down rapidly in Tasmania’s climate. Capping joists may help preserve it longer, but decay still occurs early.

Why deck rot is a structural safety risk

Decay in structural members (bearers, joists, ledger) compromises load-bearing capacity and balustrade anchorage. Failures often occur during gatherings when loads peak. If you notice movement, gaps at connections, or decayed fixings, treat it as a structural hazard and avoid using the deck until assessed.

Maintenance checklist that actually works

• Inspect quarterly — look underneath; probe suspect areas.
• Clean & keep dry — clear leaf litter; prevent gutters/downpipes discharging onto the deck.
• Re-seal exposed timber — apply exterior oil/coat every 12–24 months (more often on the coast).
• Ventilate — create airflow under low decks; add vents or gravel strips.
• Upgrade fixings — use stainless near the coast; replace corroded brackets/screws.
• Detailing & joist capping — flash ledgers, cap posts, seal cut ends, and use joist caps or flashing tape to stop moisture from above.

Repair or replace?

Localised, shallow decay may be remediated (cut-back, treat, scarf/new member). If decay affects ledgers, bearers, or joists, or spans are inadequate, a partial or full rebuild is usually safer and more economical in the long run.

Pro tip: End-grain sealing and proper flashing at the house ledger prevent most early failures we see in Tasmanian decks.

FAQ: Hardwood deck rot