Sculptform Timber Leaching

Timber leaching is often the elephant in the room when it comes to using timber externally, however, disaster can be avoided with the correct techniques.

We are regularly asked about tannin leaching, a common problem for anyone working with timber, and we’re hoping to deliver some answers and demystify some theories and problem-solving techniques. Specifically, we are talking about the leaching of tannins (polyphenols) from timber products, leaving an unsightly stain on the surface below. Before we get into specifics of how to prevent or remove tannin stains, let’s establish how it occurs in the first place.

Sculptform Timber Leaching

A piece of timber is made up of what looks like a cluster of straws. The tannins are stored within the walls of these “straws.” Moisture will generally penetrate timber through the ends of the boards. Tannins are water-soluble extractives, which are present in some species of timber, and moisture or water permeating through the timber will bring these tannins to the surface, causing black or brownish stains. Leaching and discolour occurs when these stains are washed onto concrete, render or pavers.

It’s important to note that when staining of this nature occurs, the stains are due to tannin, not to an applied oil coating ‘washing off’. Once a coating is fully dry – whether it’s water based or oil based – it cannot be washed off with water. If you’re not sure, a simple way of testing for tannin stains is to wipe over the affected area with a wet white cloth. The cloth will discolour if the stains are tannin, as they are water-soluble.

Let’s work through some common question around timber leaching, along with some simple solutions and troubleshooting:

How do I prevent or control timber leaching?

Prevention is always better than any cure and at Sculptform, we suggest using a reliable mechanical seal (aluminium angle with sealant) on our Tongue and Groove Cladding timber to keep moisture and tannins in and unwanted moisture out.

It’s important to note that some timber species are more likely to leach tannins than others and alternatively, some including Burnt Ash, are resistant to any leaching. Australian eucalypts such as Stringybark and Blackbutt and tropical hardwoods, including Merbau, are more susceptible to tannin staining but the good news is the level of extractive runoff will reduce overtime, decreasing the amount of tannin staining.

For a full rundown on timber species touch base with our expert team on 1800 008 828.


tongue and groove cladding

How do I remove tannin stains?

The best way to remove tannin stains from concrete, render or pavers is by washing affected areas with water and scrubbing with a stiff bristle brush or broom. Severe staining could require a bit of ‘elbow grease’ and hard work, including multiple washes. For small areas, a wet cloth can be used and remember to remove the ‘wash’ water and rinse off quickly for all horizontal surfaces, to avoid the re-depositing of tannins.

Timber leaching doesn’t have to be the elephant in the room for timber lovers with some basic knowledge and at Sculptform we are here to support you with any issues relating to timber products.