Timber leaching is often the elephant in the room when using natural timber in external applications. Yet with the right understanding and proactive techniques, it’s entirely manageable – and often preventable.

At Sculptform, one of the most common questions we get is about tannin staining. This refers to the leaching of tannins – naturally occurring polyphenols – from within the timber. When exposed to moisture, these compounds are drawn out and deposited onto adjacent surfaces, leaving dark tannin stains on concrete, render or paving.

In this article, we’ll explain what causes timber leaching, which species are most susceptible and offer practical guidance on how to stop timber leaching. You’ll also find practical advice for cleaning and preventing hardwood bleeding and preserving the look of your project.

How Does Timber Leaching Occur?

What Causes Timber Leaching?

To understand tannin staining, it’s helpful to picture the cellular structure of timber like a bundle of drinking straws. Tannins are stored within the walls of these ‘straws’. When timber is exposed to water, particularly at the end grain, these water-soluble extractives are drawn out and carried to the surface. As water runs off, the tannins are deposited onto surrounding materials, creating dark, reddish-brown or black stains.

This staining can be mistakenly attributed to finishes or coatings washing off. However, once timber oils or water-based sealers are fully cured, they are not water-soluble. A quick test for tannin stains is to wipe the affected surface with a wet white cloth – if it discolours, it’s tannin.

Timber Species Prone to Leaching

Some timbers have higher tannin levels, making them more prone to timber leaching than others. Species to watch for include:

• Merbau – A dense tropical hardwood known for its high tannin content and characteristic reddish-brown staining.
• Blackbutt – A popular Australian eucalypt that can leach heavily in the first few months after installation.
• Stringybark – Another high-tannin species requiring careful sealing and detailing.
• Spotted Gum – Common in external applications; less prone to leaching than Merbau or Blackbutt, but still benefits from proper sealing, especially in high‑visibility areas.

For architects and specifiers, understanding how to stop hardwood bleeding begins with species selection, especially in high-visibility areas.

How to Prevent Timber Leaching and Tannin Staining

Prevention is always better than a cure. If you’re designing or installing timber cladding walls, decking or screening systems, consider the following best practices to minimise tannin staining before it starts.

Best Sealants to Prevent Timber Leaching

Choosing the right sealant is key to locking in tannins and protecting adjacent surfaces.

• Use penetrating oil-based sealers formulated for high-tannin hardwoods. These soak into the timber, reducing leaching at the source.
• End-grain sealers are essential for capping cut ends, where moisture penetration is highest.
• Water-repellent preservatives (WRPs) can also be applied pre-installation to block tannin migration.

Always apply sealants following the manufacturer’s recommendations and allow sufficient curing time before exposure to weather. Explore our Comprehensive guide to timber coatings for further information on this topic.

Installation Tips to Minimise Tannin Bleeding

Good detailing reduces the risk of timber leaching. Consider:

• Aluminium angle trims with sealant help create clean junctions and can act as a barrier to moisture entry in timber applications, where tannin staining may occur.
• Adequate drainage and falls – ensure water doesn’t pool around timber interfaces or drip onto stain-prone surfaces.
• Off-ground installation – raise the base of timber screens and cladding above paving to avoid splashback.
• Pre-weathering – some installers allow leaching to occur in a controlled environment (e.g., on-site staging) before final installation.

How to Clean Timber Leaching and Remove Tannin Stains

Even with precautions, tannin staining can still occur, especially during the first weathering cycle of newly installed timber. Here’s how to handle it effectively.

Best Methods for Cleaning Timber Leaching Stains

• Hose down the affected area as soon as staining appears. Early response reduces long-term discolouration.
• Use a stiff-bristle brush and water to scrub off deposits from concrete or pavers.
• For more stubborn stains, try a mild oxalic acid solution – a natural compound that breaks down tannins. Always rinse thoroughly.
• Avoid harsh cleaners like bleach, which can damage surfaces or react with timber oils.

Remember to remove run-off water to avoid redistributing the tannins onto clean surfaces.

DIY vs. Professional Cleaning – Which Is Better?

• For small areas or early-stage stains, DIY cleaning is often sufficient.
• Larger or heavily stained areas — especially on porous materials like render — may benefit from professional pressure washing or chemical treatments.
• Always test cleaning products in an inconspicuous spot first.

If in doubt, contact us for expert advice tailored to your timber species and installation environment.

Preventing Future Stains After Cleaning

Once you’ve cleaned tannin stains, prevent recurrence by:

• Applying a quality sealer (if not already done).
• Installing drip edges or flashings to redirect water away from sensitive zones.
• Resealing cut ends or fastener penetrations after maintenance.
• Scheduling seasonal inspections – especially after heavy rain.

Ongoing care ensures your timber cladding walls continue to perform and look their best over time.

Manage Timber Leaching with Confidence

Timber leaching doesn’t need to deter you from using natural hardwoods. With the right timber selection, protective treatments and installation detailing, you can confidently enjoy the warmth and beauty of real timber, without the stains.

Whether you’re looking for advice on how to stop timber leaching, reduce hardwood bleeding or clean up existing tannin stains, Sculptform can help.

Browse our full range of architectural timber systems or contact us for tailored advice. You can also explore our product solutions for stain-free detailing, including our Click-on Cladding and timber cladding walls systems.