1800 156 455 
What is Timber Leaching and How Can it Be Prevented
Why Does Timber Leach and How Can It Be Prevented
Timber leaching, often called tannin bleed, is when water-soluble compounds in timber, mainly tannins, are washed out by rain or moisture. The run-off can leave reddish-brown stains on nearby concrete, pavers, render, tiles and painted walls.
Natural timber is used in design for many sustainable reasons, so leaching is worth understanding and planning for rather than avoiding timber altogether.
Here’s why timber leaches, which species are most prone to it, and how to prevent and clean up staining.
Natural Extractive
Tannins and Resins: Many types of wood contain natural extractives such as tannins, oils, and resins.
These compounds can leach out, especially when the timber is exposed to moisture.
Tannins, for instance, are water-soluble polyphenolic compounds that can cause discoloration when they leach out of the wood.
Pigments: Some woods have natural pigments that can leach and cause staining, particularly in contact with moisture.
Exposure to Moisture
Water Solubility: Timber’s exposure to rain, humidity, or other sources of moisture can dissolve and wash away water-soluble compounds present in the wood. This process is more pronounced in freshly cut or untreated wood.
Absorption and Desorption Cycles: Timber absorbs moisture from the environment and then releases it as conditions change. During this process, soluble compounds within the wood can leach out.
Chemical Treatments
Preservatives and Fire Retardants: Treated timber, which has been infused with preservatives or fire retardants to enhance its durability and safety, can also leach these chemicals.
Over time, these treatments can be washed out by rain or other sources of moisture, especially if the treatment was not properly applied or if the timber is continuously exposed to harsh environmental conditions.
Copper-based Treatments: Treated wood often contains copper-based preservatives which are highly effective against rot and insects. However, these can leach out and cause staining on adjacent surfaces.
pH Levels and Environmental Factors
Acidic or Alkaline Conditions: The pH of rainwater or soil can influence the leaching of certain compounds from the wood. Acid rain, for example, can accelerate the leaching of tannins and other extractives.
Environmental Pollution: Pollutants in the environment can interact with the timber and enhance the leaching process, either by breaking down the compounds in the wood or by altering the wood’s surface properties.
Wood Species
Different wood species have varying amounts and types of extractives.
Species like oak, cedar, and redwood are known for their high tannin content, making them more prone to leaching, especially when freshly cut or untreated.
This is particularly important to consider when using these timbers in exterior cladding applications.
Which Timbers Leach the Most?
| Leaching tendency | Examples |
|---|---|
| Higher | Merbau, Blackbutt, Western Red Cedar, oak |
| Lower | Spotted Gum, Ironbark, Cumaru, Cypress Pine |
How to Prevent Timber Leaching
You can’t stop timber from containing tannins, but you can reduce how much leaches and where it ends up:
Proper seasoning and drying: Allowing timber to dry properly before use can reduce the amount of leachable compounds.
Pre-washing: Cleaning timber with an oxalic acid-based timber cleaner before installation helps draw out excess tannins.
Sealing and coating: Sealants, stains and paints create a barrier that reduces moisture getting into the timber and minimises leaching. Pre-coated timber gives protection from day one, as long as the coating is maintained.
Using quality treated timber: Where treated timber is needed, choosing timber treated with stable, high-quality preservatives can minimise chemical leaching.
Choosing low-leach species: Using species less prone to leaching is the simplest fix where staining would be a problem.
Detailing run-off: Design drips, gutters and falls so water running off timber is directed away from vulnerable surfaces.
Interested in Using Western Red Cedar Cladding in Your Next Project?
With its balance of performance, versatility, and timeless appeal, Western Red Cedar continues to be a leading choice for architectural cladding.
Contact the Modinex team today to discuss your project requirements and discover how this premium timber can bring your design vision to life.
Article By Joel Leitch | 19 December 2025
Joel Leitch
Project Consultant
Joel is an integral team member of NSW projects division with over a decade as a consultant to architects and builders. With extensive experience as a head of projects, Joel Leitch combines deep knowledge of natural timber, aluminium, and concrete to provide innovative and inspiring design solutions.
