Timber Cladding & The Reasons Why it May Cause Staining to Other Materials

Timber Cladding & The Reasons Why it May Cause Staining to Other Materials

The following is a summary of the reasons for possible staining from timber cladding.  It has been compiled with reference to information from TRADA, The Timber Decking and Cladding Association and various coating manufacturers.

The information applies to most species of timber either pressure treated or untreated.

As the boards on a timber cladding facade weather, they can leach out “extractives”.

So what is an “extractive”.

Timber is an organic material. As such it contains a wide range of natural chemicals. Because of their ability move as a liquid these chemicals are collectively referred to as “extractives”.

“Extractives” tend to be more prevalent in the heartwood of a species where their concentration can be a principal cause of the darker colour of the heartwood.

Many “extractives” are partially water soluble and when the surface of the timber is wetted, some of these coloured “extractives” may partially dissolve.

One of the most common “extractives” found in many timber species is Tannin and this can be leached from cladding by rain.

These “extractives” such as Tannin may run off and stain other surrounding surfaces like concrete, walls, tiles, pavers and render. The Tannin “extractive” in particular is likely to react with alkaline surfaces such as concrete to form an unsightly red/brown stain.

Special care should be taken when using timber cladding above absorbent materials, as stains may be difficult to remove if left for a period. It is advisable to protect these surfaces from the run off during the initial weathering process. This process is temporary and should stop once the boards have begun to weather down.

Timber  Cladding that is to be fixed in a wet environment where it would be undesirable for leaching to occur, e.g. above a light coloured render, may benefit from a pre-clean prior to coating with a sealant. While it is impossible to guarantee that leaching or staining will not occur after this process, experience has proven that such an event is more unlikely, and if it does occur is of a significantly reduced nature.

Of course once a sealant is applied to any timber cladding this will have a service life.  Once a sealant is applied then an on going maintenance programme will be required as the sealant will need to be renewed every time it reaches the end of its service life.

Alternatively the prospect of possible leaching from a timber facade can be considered at design stage incorporating some design to channel stained water away from other materials on the building where staining could be an issue.

On many projects “extractives” leaching from timber is not an issue as it is only a temporary thing and it will not effect the other surrounding material used on the building unduly.  The main problems arise where a timber facade is used above a light coloured render or similar product and any developer/architect should be aware of the possible consequence if a timber cladding facade in this situation is not sealed or they do not include some design feature to channel stained rain water away from the building materials below.

If staining does occur, affected surfaces can be cleaned with dilute bleach, household detergent or a pressure spray and additionally, chemical cleaning and removal of these stains is usually simple and easy process and there are products on the market specifically designed for this purpose. In both cases testing a small area first is recommended and cleaning should be carried out soon after the staining has happened to ensure it does not get “grained” into the affected material.

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