July 26, 2015
This week we moved half of the cladding we had previously been storing internally onto the scaffolding outside the house so that it could acclimatise to the external temperature and humidity.
The cladding was kiln dried so the moisture content of the wood when we received it was in the region of 16-18% (+/- 2%) - since it has been stacked indoors for quite a while now we need to take it back outside but under cover for a few weeks before we start to fix it to the building.
The moisture content of wood is commonly expressed as a percentage, and is calculated as the difference between the weight of a sample of 'wet' (green) wood and the weight of the same sample after oven drying (to remove all moisture), divided by the oven-dry weight, all multiplied by 100. Got it?! So, for example, a piece of wet timber weighing 750g, which then weighs 650g when fully dry, will have a moisture content of c.15% (i.e. [(750-650) / 650] x 100].
With an open rain screen cladding in particular, and to some extent all timber cladding, it is important to try to maintain consistency of moisture content (as far as possible of course!) before installation as the timber boards will swell across the grain (which is the width of the board); during periods of heavy rain or extreme humidity the gaps between the boards can alter by several millimetres. Whilst this might not sound significant the difference to the eye between a 6mm and an 8mm gap (for example) is perceptible. As the gaps between the boards are there to allow for a complete drying cycle it is important that they are consistent. So once the cladding arrives on site it should be installed as soon as possible. If this isn't possible for some reason then it should be stacked horizontally with spacers between each board to promote ventilation, and under cover so that it is not subjected to repeated drying and wetting cycles, which can lead to extensive staining.
The strength of timber is also affected by its moisture content: the strength increases as the moisture content reduces and vice versa, so it is important to use kiln dried timber for structural purposes, dried to the correct moisture content for it's purpose / location so that it is dimensionally stable.
Siberian Larch is a naturally durable timber (Class 3 according to BS EN 350-2), which is why it is often chosen as a cladding board. The resin and other properties of the wood create a high resistance to decay and rot, and even though it is a softwood, this makes Siberian Larch one of the toughest and most durable softwoods. It also has a high density which makes it more difficult for decaying organisms to penetrate the wood. All of this gives Siberian larch cladding a lifespan of 50 -100 years when it is subjected to general outdoor exposure, without any surface treatment.
So, in another few weeks we can begin the (slightly epic) task of hand nailing our cladding!