Something Rotten: Spotting Timber Decay Before It Costs You

Behind the odd title sits a very practical topic for anyone with a deck, pergola, verandah or timber carport: rot. Wood decay is one of the most common reasons outdoor structures in Australia need repair earlier than expected. A small soft patch left alone can turn a modest maintenance job into a costly rebuild.

Why outdoor timber rots

Rot is caused by fungi that feed on wood when it stays damp for long periods. In Sydney's humid summers and wet spells, the usual trouble spots are places where water collects and can't dry out: post bases sitting in soil or concrete, the tops of joists under deck boards, beam ends exposed to rain, and joints where two pieces of timber meet. Poor drainage, blocked gutters and garden beds built up against framing all make things worse.

Signs worth checking

  • Timber that feels spongy or gives way when pressed with a screwdriver
  • Dark staining, cracking across the grain or crumbling surfaces
  • Fungal growth or a musty smell under decks
  • Loose fixings, sagging boards or posts that move
  • Paint or stain that keeps peeling in the same spot

Keeping a structure sound

Regular inspection, ideally once or twice a year, catches problems early. Keep gutters clear, make sure water runs away from footings, allow airflow beneath decks, and renew protective coatings as the manufacturer recommends. Choosing treated or naturally durable timber suited to its exposure, and using stirrups to lift posts off the ground, helps from the start.

If you find significant decay in a load-bearing part, such as a post, bearer or beam, have it assessed by a qualified builder before using the structure. For more background on outdoor structures, see what we cover or browse the FAQ.