What is Leaching or Tanning Bleed?
Leaching or tannin bleed, is when weather exposed timber cladding, screening, decking or structural timbers bleeds brown extractives onto the surrounding surfaces. It is important to be mindful of the consequences of tannin bleed when planning your build.
Tannins are natural extractives present in most timbers which leech during periods of rain, and this leaching can cause unsightly stains on surrounding materials, most notably porous light coloured surfaces such as concrete or natural stone which are especially prone for discoloration.
Bleeding is only temporary. Usually after a typical rainy season the tannins will be expelled from within the timber. Some timber finishes such as oils or tannin blockers can help in the amount of tannin bleed expelled but most timbers even with this pre- treatment will still extract some tannins in the first 12 months.
Tannin stain is not only unsightly, but it can block the cell structure of the wood preventing it from absorbing timber coatings evenly, in some cases giving an overall dull and patchy look. This means that tannin rich timbers or green unseasoned timber typically needs a thorough cleaning prior to coating, such as an oxalic acid wash.
What Can You Do To Minimise Timber Leaching & Tannin Bleeding
Bleeding from timber is only temporary usually when there is alot of rainfall around. Below are some common ways to minimise timber leaching and tannin bleeding from timber.
- If you are installing timber in an exposed location try to make sure that any affected light or porous surfaces are protected until extractives have leached away. Consider the use of a protection coating such as an appropriate stone, and concrete sealer
- A commercially available tannin remover can be used however, more often this only reduces the ‘natural extractives’ in the timber rather than eliminating them completely.
- Consider during the planning and design phase that run off from the timber façade occurs over a garden bed or into an upper story gutter or install suitable flashings are installed to insist in deflecting the water away from a porous surface.
- An Oxalic acid wash or sodium hypochlorite eg common bleach is the recommended application for assistance with removal of tannin stains after they have occurred on both timber and other surfaces such as concrete or stone. We can provide Preschems Grey Deck Cleaner/Oxalic Acid on request.