leaching of merbau timber deck

“Is my merbau timber deck bleeding?” “Why are their brown stains on the concrete/bricks/sandstone/pavers surrounding my deck?” We hear these questions regularly. It’s probably the biggest mystery in your backyard. It’s now time to lift the lid on this mystery and discuss how to handle these stains correctly.

A common assumption is that the decking oil is washing away. There’s no need to call back your decking oil contractor. Regardless of whether its oil or water based, decking oil won’t wash away. The cause of the problem lies much deeper.

The correct term is timber leaching or tannin leaching. Tannins (polyphenols) are present in many hardwood species, particularly in one of our favourite decking timbers, Merbau. Tannins are a water-soluble substance which is drawn to the surface from exposure to moisture or water1. At this stage, the tannins can easily drip onto nearby driveways, pavers, leaving a runny brown mess. Fortunately, the level of timber leaching will reduce over time as the freshly cut timber ages.

One case which left a client particularly perplexed was when their cat left paw prints on their sandstone pavers. Assuming the cat had been exploring the garden, the client went to scrub the stains but quickly realised that little whiskers had stepped in something extra adhesive.

timber leaching stains on stone

After investigating the stain and the surrounding area, we identified what had really happened. The client had a combination of merbau timber decking and sandstone pavers surrounding their backyard pool. The merbau deck became wet when the pool was in use, which caused it to leach. As you would expect, this is when the cat decided to walk across the decking and onto the pavers.

While sandstone and other masonry surfaces may feel hard and strong, most people are surprised to learn that they’re actually highly absorbent. Scrubbing, sanding or blasting the surface with a high pressure cleaner will do little to remove the stain. For sandstone in particular, an abrasive method of cleaning will cause the surface to crumble and lead to structural damage.

We were able to remove the leaching stains by using a formulation to break down the molecular structure of the tannins and draw them to the surface. At this point, the stain was safely suspended in situ and carefully removed. The surface was neutralised with gentle use of a high-pressure cleaner and all bi-product was removed with a vacuum recovery system.

We have also removed stains where leaching has occurred on a second level deck and dripped onto a concrete/paved area and the adjacent wall underneath. Stains, where merbau in a feature wall has dripped onto adjacent render or the paved surface underneath are also common.

The Stain Eaters specialise in removing surface deposits from hard surfaces, with representatives in Victoria, New South Wales, Queensland and South Australia. Read more about our stain removal services or request a quote.


1 https://www.woodformarch.com/blog/timber-leaching-elephant-room