Timber stairs rotting

Cause

Moisture is trapped

Repair

  • if the stairs are constantly wet, address this if possible. For example, if this is the result of very thick tree cover or heavy planting, consider pruning to allow light and wind to dry the steps. If it is the result of a damaged roof gutter overhead, fix the gutter

  • if the steps have closed treads, replacing with open treads may help. Open treads can dry easier and are less likely to accumulate leaves and other debris that can slow drying. For safety, the open vertical space between treads must not allow a 100 mm diameter sphere to pass through

  • replace damaged timber with SG8 (wet in use) Pinus radiata, treated to hazard class H3.2 minimum. If damage is extensive, it may be better to replace the whole stairs

  • ensure the gaps between the treads are open – it is common for the gaps to become blocked and prevent drying

  • always check the ends of the treads where they abut the side stringer – there is a higher risk of rot due to trapped moisture where treads are housed into the stringer

Cause

Wrong level of treated timber 

Repair

  • replace damaged timber with SG8 (wet in use) Pinus radiata, treated to hazard class H3.2 minimum. If damage is extensive, it may be better to replace the whole stairs 

Cause

Stair string rotting at the lower end, base of stringer in ground contact

Repair

  • there may have been insufficient treatment of the string for ground contact – any timber in ground contact must be treated to H5

  • where the problem is the result of a build-up of soil, mulch and vegetation at bottom of stringer, remove the soil etc. Replace with H5 treated SG8 timber, or:

  • trim back bottom of stringer if rot is minor and mount on a metal bracket set into a concrete pad. If rot is extensive, replace

  • for more information about timber steps, see the Build magazine supplement on timber decks here.