Using timber externally and applying clear coatings, or even leaving the timber unfinished and allowing it to weather, can provide an authentic natural effect but there are a few things to consider before specifying this;
It maybe your desire for external joinery to be left unpainted so that it maintains its natural appearance which, of course can be incredibly effective used for the right application. However, the effects of the weather and sunlight can be harmful to the timber damaging the surface and leading to undue movement. Careful consideration is needed to ensure that the required purpose and application of the timber is not compromised if to be left exposed to the elements.
Timber exposed to the weather and not protected by a suitable finish will, quite rapidly, bleach or 'grey' giving the timber a neglected appearance. Further exposure can lead to surface checking and splitting due to rapid changes in the moisture content of the timber at the surface, which can differ greatly to the moisture content at the core.
Directly after rainfall the moisture content of the timber surface can exceed fibre saturation point of 25% to 30%. In dry conditions heat from the sun and the drying effects of the wind can rapidly dry the surface of the timber, while the core will dry more slowly. This cyclic action and the rapid changes at the surface can cause stresses to be induced in the timber, leading to splits and checks. This may well be the desired affect and that is fine. The affects can however affect the performance of the product due to excessive movement (and the forces here can be dramatic – we have seen large oak, structurally supporting, posts and beams physically move masonry (damaging walls and the structural integrity of buildings) and sheering steel supports and fixings.
All timber moves, when exposed to different conditions as it; ‘wants’ to be in ‘tune’ or in balance with its environment. It is hygroscopic in its nature and therefore will take on water in wet environments (expanding) and lose water when dry (shrinking) all of which causes movement, surface and even whole component damage. This is unavoidable but can be reduced and managed by a) selecting the right timber for the job and b) choosing the right decoration/treatment/coating and on-going maintenance. For more on the specifics please click on these useful links;
Using oak for external applications great – it is environmentally sustainable, it’s readily available and it looks great. For external use it should be European Oak and not American. This due to the fact that American Oak is particularly dry – some say due to the way the American’s dry it, rapidly, often at sea/in transit, but others consider it’s the nature of the wood, either way it is not good for external use in the UK.
Oak needs to be looked after – think twice if specifying clear finishes or no finishes at all.
Oak is not as resistant to weathering as oily hardwoods and, therefore, needs better protection when used outdoors. The open pores provide a route for water-ingress. Blue-stain mould spores are likely to be present on any piece of timber and will be activated by moisture. Another consideration with Oak is the high tannin content, if not properly sealed water (rain) can wash the tannin out of the wood leaving a patchy colour and staining to the surrounding areas (bricks, patio, etc.) and this can be significant. It is therefore essential to use a coating that will give a high-build, seal the pores effectively and so provide a barrier to water. The items should not be exposed without adequate protection. Clear treatments will not provide protection from UV rays (but will obviously allow the beauty of the Oak to be displayed).
Colour changes of unprotected timber may be partly due to the leaching of water soluble extractives, which contribute to the colour of the timber, and partly to bleaching by Ultra Violet light (UV). For example, oak contains extractives known as tannins which are water soluble and are leached from the surface by rainwater. The bleaching of the timber is solely caused by the effect of sunlight on lignin, one of the chemicals in timber, which is photosensitive and degrades and changes colour on exposure to UV.
Movement of timber is due to changes in moisture content. Distortion can occur as tangential movement (in the direction of the growth rings) can be up to twice the amount of radial movement (across the growth rings) and because the grain in timber is rarely perfectly straight. Modern paints and finishes are classified by their ability to affect the amount of movement in timber. For joinery, finishing systems should be classified by the BS EN 927 series of standards as suitable for stable end-use. That is, the coating should be formulated to keep the movement of the timber to a minimum. The coatings protect the surface of the timber from UV and direct wetting from rain and they permit water vapour to pass through allowing the timber to change moisture content. The stability of the timber is achieved by making the coating less vapour permeable causing the changes in moisture content to be slow. Uncoated timber does not benefit by this slowing effect and subsequently a greater amount of movement can occur and this can be detrimental to the joinery.
To maintain the appearance of the timber and to limit problems due to surface degradation and movement, a suitable coating should always be applied. As a minimum, joinery should be primed or stained with a basecoat and finishing coats should be applied as soon as possible.
If you have any queries on the above or require any specific information on what wood is best for which applications or what coatings to use please get in touch.
Posted by Paul Hayman on Monday 26 June 2017 at 10:46