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Timber & MoistureOne of the most important factors affecting the performance of timber is its moisture content (MC). It is estimated that approximately 80% of in-service problems that get reported are related to the MC. These can include splits occurring, distortion, joints opening up and various others. Timber needs to be looked after, before, during and after installation.

Looking after Timber

It is good practice to store timber somewhere that is close to the conditions that it will be in during service. This means keeping internal joinery where possible in a heated dry store to maintain the correct MC. The same care needs to be taken with external joinery which should be stored under cover and kept as dry as possible with little dampness and not left open to the elements. However when covered it still needs to be sufficiently well ventilated to prevent condensation.

Panel products such as plywood need to be treated similarly to timber especially if being used externally, or in situations that could lead to moisture take up, in which case it is good practice to seal the edges using Paint or Wood Preservative. Panel products also need to be stored correctly with bearers or “sticks” every 10-15 boards to allow ventilation. All sticks/bearers need to be in line to prevent distortion of the boards.

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Initial Levels of Moisture in Timber

Imported timber usually comes as “shipping dry”. This means a MC of between 18-22%, but this will change in response to the prevailing weather conditions. When it is cold and wet the MC is likely to be at the upper level, when it is very hot and dry then the MC may fall to much lower levels.

“Fresh Sawn” timber will generally have a very high MC and will need to be dealt with slightly differently when trying to achieve “conditioning”. It is desirable to change the temperature slowly which will allow a slower release of moisture, this is helped by occasionally wetting the timber. It may seem like hard work and time consuming, but when done correctly this means it is less likely for the timber to dry out too quickly and therefore preventing distortion and/or spliting.

maximum600mm

max150mm

Correct flat storage of thicker, over 6mm panels

In Service Levels of Moisture in Timber

Any changes in the range of temperature and humidity of the air that surrounds the timber during its service life, will mean that it will continually respond to these changes in the atmospheric conditions. Picking up or losing moisture will lead to the timber “moving”. This means that if the timber has a low MC and it is put into an area that has cool damp air, it will absorb moisture (i.e. Swell), and vice versa if it has a higher MC and is put into an area that has warm dry air, it will release moisture (i.e. Shrink). Timber will generally reach a point where there is very little change taking place, this is sometimes referred to as “conditioning“ or that the timber has been “conditioned” or that the timber has reached its “Equilibrium Moisture Content” (EMC). This does not mean that the timber will not continue to change, as slight movement will occur during changes in the seasons, however it must be stressed that should conditions alter drastically then this could undo everything that has already been achieved. The fact that timber will “move” is important when you finally come to externally fit timber to buildings, such as claddings. If fitted during the winter months sufficient consideration needs to be taken to shrinkage as the timber dries during the following summer, this is even more important to south facing walls/surfaces as these are exposed to more sunlight which also means more heat and can lead to the MC falling as low as 10% (or even lower), with levels being similar to the in-service MC expected in a centrally heated room. Because of this movement it is advisable to paint the external cladding prior to fitting. This prevents naked timber showing if the cladding does shrink.

When dealing with exterior use timber that has been preservative treated the MC will be increased if the High Pressure Treatment (Osmose Naturewood) is used and therefore after being treated the timber will require time to dry and come down to a more usable MC, this may take several weeks to achieve and is wholly dependant on the time of year and climatic condition. This is less of a problem when using the Low Pressure Treatment (Osmose Clearchoice) but a period of drying may still be required to make sure the timber has reached a suitable level of MC.

For information here is the relative in-service MC:Out of Doors (not in ground contact) = 15% - 19% (Note: This can be considerably lower on South facing elevations.)

Indoors (Unheated buildings) = 14% - 17%Indoors (Heated {Room temperatures of 12-21 degrees}) = 10% - 14%Indoors (Heated {Room temperatures over 21 degrees}) = 6% - 8%

CE Marking

What is CE Marking? Since July 1st 2013 CE marking of construction products became compulsory in the UK. The new Construction Products Regulation (CPR) will affect products such as Solid Timber Flooring, External Claddings and Strength Graded Carcassing plus panel products including plywood and OSB. The introduction is in an effort to have a harmonised technical specification across the European Economic Area (EEA) allowing for the ease of trade between countries within the EEA. By definition a construction product is anything that is produced for incorporation in a permanent manner in the construction (or parts of construction) and therefore has an effect on the performance of the overall construction.

What have we done?Ridgeons standard stock Timber Flooring and Cladding machined by our mill at Herringswell is produced under a ‘Declaration of Performance’, (DoP), which permits us to sell these products as conforming to the exacting standards required under the “Conformité Européene” (CE). Part of our DoP requires use to make four MC checks per pack at the point of machining, if these readings are outside of the required parameters then that pack is not used for these products.

We are continuing to source Machine Strength Graded Carcassing direct from our trusted supply partners’ timber mills in Sweden and Latvia, who are providing CE marked pre graded carcassing. If you are concerned whether the new regulations apply to you, or products you are purchasing, please contact Ridgeons directly and we will do our utmost to help you.

Our DoP is available via the following link: www.ridgeons.co.uk/about-us/forest-stewardship-council-fsc/

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