Green Oak Beams
Timberpride’s green oak structural timbers.
This is simply what we call oak beams, cut from round wood, wet and straight off the saw. Green Oak is what we have been building with in this country for over a thousand years. Our greatest monuments Canterbury Cathedral, Palaces of Westminster and Windsor Castle, to name few, were all built using Green Structural Oak.
Green Oak beams are sustainable, available and cut to size from the saw log here in Tetbury; meaning we are able to produce green structural timbers for your structural timber frame venture and any other projects that require green oak. All saw logs are inspected in the forest, where not only is the quality of timber and management taken into consideration but also the soil the oak has grown in.
Fresh sawn structural oak beams will at first be a warm golden honey colour, without oil in external applications they will weather to a silver grey and internal applications the colour will mellow, however oiling will alter the naturally aging colour.
Structural Oak Beams are at their straightest when they are fresh sawn. Oak is a powerful material and even when weighted or anchored into position will try to move and twist. Understanding how Oak will behave and how to work with it is so important, selecting the best orientation for a beam in service has a huge impact on the performance of a beam.
Structural Oak never fully dries out, fresh sawn beams will have a moisture content of about 80% even beams which have been in service for 300 years or more will have 25 to 35% moisture content in the centre. After several years in service, the outer 75mm of a 200mm square section green oak beam could reach equilibrium. Which means a green oak beam when first milled will look near perfect, as the outer faces start to dry the character starts to become evident as knots begin to show and splits open due to the shrinkage associated with drying.
All trees grow with tension, it may be because the tree has grown on a hillside or due to the prevailing wind, whatever the reason the tree has to hold itself upright and grow towards the light. Beams will bend as the tensions release this is not a defect it is part of the nature of Oak and doesn’t weaken the beam. In service you may hear bangs in the night, often this will be the tensions moving joints as the house settles.
Over a longer period of time drying cracks will develop, again this is not a defect or any cause for alarm. As the beams dry further the fibres will shrink and cracks will develop. If the heart is boxed the beam will crack and retain the section size better than a beam cut to the side of the heart. Splits opening reduces the effect shrinkage has on the cross sectional area of the beam.
Moisture content of Green Oak
Green oak is typically at 80% moisture content, higher in summer felled wood, lower in winter felled. This means 80% of its weight is water or sap, not to be confused with rain water.
Fibre Saturation Point
Whilst the moisture content is above 35% there is water loose in the timber, below this level moisture is only found inside the fibres, this is called the Fibre Saturation Point. FSP is significant because Oak shrinks as it dries, most of the shrinkage occurs as the timber drops below Fibre Saturation Point because the water or sap inflates the fibres.
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