Merbau Tannin Bleed: Why Your New Deck Stains the Paving (and How to Stop It)

A homeowner installs a beautiful new merbau deck, the first rain comes, and a red-brown liquid runs off it onto the pale paving below. The paving stains. The homeowner panics, assumes the timber is faulty or the builder did something wrong.

Nothing is wrong. This is tannin bleed, and every new merbau deck does it. Here is what is happening and how an installer who knows merbau prevents it from becoming a problem.

What tannin bleed is

Merbau, also sold as kwila, is a dense Southeast Asian hardwood that contains a large amount of natural tannin. Tannin is water-soluble. For the first 6 to 12 months of a merbau deck's life, every time it rains or is hosed, water draws tannin out of the timber and carries it off the deck as a red-brown stain.

It is most intense in the first few weeks and tapers off over the following months. It is a property of the timber, not a defect, and it is the single most common thing that catches new merbau owners by surprise.

What it stains

The tannin runs off the deck and onto whatever is below and around it. The surfaces most at risk are pale ones: light-coloured paving, concrete, render, light brickwork, and pale pool surrounds. On a dark surface the tannin barely shows; on a white paver it leaves a visible red-brown mark.

Left unmanaged on a deck built right against a light-paved patio or a white pool surround, tannin bleed can leave staining that is difficult to remove later.

How an experienced installer manages it

This is where a builder who genuinely knows merbau differs from one treating it like any other board. There are several things a good merbau installer does.

They seal the cut ends of the boards, and sometimes the full board, before fixing - tannin leaches most from exposed end grain. They advise you to hose the new deck down several times in the first weeks to flush the loose surface tannin off deliberately, in a controlled way, rather than letting the first storm do it onto your paving. They warn you to protect adjacent pale paving and render during the bleed period. And they time the first oil coat correctly - oiling too early traps tannin under the finish, so the deck needs to weather and be flushed first.

A builder who does none of this, and just fixes the boards and leaves, has set up a tannin-staining problem you will discover with the first rain.

It is temporary, and merbau is still excellent

None of this makes merbau a bad choice. It is the most popular decking timber in Adelaide for good reason - dense, durable, naturally rot and termite resistant, handsome, and well priced against the premium hardwoods. Tannin bleed is a known, temporary stage, and a competent installer manages it as a matter of course.

The lesson is to make sure your builder treats merbau as merbau. When you compare 3 quotes, ask each builder how they handle tannin bleed and how they time the first coat. The one who answers without hesitating is the one who has installed merbau before.