White deposits on your deck?

Noticed white deposits on your pressure treated timber decking, particularly around knots? These marks are usually one of two things: surface mould or resin bleed. Identifying the cause will help you determine the best way to remove them and prevent future occurrences.

Surface mould

If the white deposit is soft and easily wiped away, it’s likely surface mould. This type of mould grows on damp wood and, while it doesn’t cause structural damage, it can affect the deck’s appearance.

Causes of surface mould:

• Poor ventilation or drainage through poor installation practices
• Trapped moisture from pots placed directly on the deck

How to prevent & remove surface mould:

• Prevention: Ensure proper ventilation and drainage. Elevate pots to allow airflow beneath them.
• Removal: Clean with a mild detergent and water. If mould returns, it may indicate an underlying issue with the deck’s construction that needs attention.

Resin bleed

If the white deposit is hard and crusty, it’s likely resin bleed. This occurs when natural resin in the wood seeps to the surface, often around knots. Resin bleed is more common in cool, damp conditions, where the resin crystallises into a white deposit.

How to remove resin bleed:

There is no easy fix unfortunately.  You could try:

• Scrape off the bulk of the crystals.
• Dissolve any remaining residue with methylated spirit.
• Allow the wood to dry completely before recoating.

Alternatively, simply leave it to weather naturally - it will become less noticeable over time.

By identifying and addressing these white deposits, you can keep your decking looking its best.