Introduction to kiln dried wood

Utilising timber with the correct moisture content for the intended end-use is probably the single most important factor in ensuring satisfactory long-term performance of wood-based products and is one of the least well understood by specifiers.

Timber shrinks as it dries and swells again if wetted. If dried to a moisture content close to that which it will reach in service it will not swell or shrink significantly.

For regulatory and grading purposes, timber for structural uses is considered dry if it has an average moisture content of 20% or lower. By comparison, recently felled trees and 'green' timber can have moisture contents as high as 200% of the weight of the dry wood.

The moisture content of timber will eventually reach equilibrium with its surroundings. Consequently, if stored in dry conditions wood will air dry naturally. Whilst effective, air drying can take weeks or months.

In the UK during the summer, timber may reach moisture contents below 20% but in winter months, even under cover, it is more likely to stabilise at 22 - 23%. Kiln drying allows faster drying under controlled conditions to minimise potential degrade of the timber.

The most common method is to heat the wet timber in an enclosed kiln to a temperature of 60°C-70°C and extract the moisture from the warm air. The heating regime may last several days and is determined by the species of timber and target moisture content. Timber can be kiln dried to moisture contents lower than can be attained with air drying.

Softwood for structural uses which has been designed on a dry basis must be strength graded and installed at an average moisture content of 20%. Such timber must be marked DRY or KD (kiln dried) within the structural grade stamp.

However, even at this level, structural timber in heated buildings will still lose moisture and shrink in cross-section. Structural softwood which has been strength graded and installed at higher moisture contents must be marked WET or GRN (green).

Kiln dried wood specifications

There are many British Standards containing timber moisture content requirements for different end uses. Correct specification and material supply is important for the following reasons:

Dimensional stability

For most practical purposes, shrinkage and swelling is taken as 1% change in thickness for every 5% change in moisture content. In centrally heated buildings, the moisture content of timber can be as low as 8-10%, meaning that if a 200mm deep joist were to be installed with a moisture content of 30%, a reduction in depth of around 4%, or 8mm, would occur.

The detail design of the building should take such anticipated movement into account. The dimensional stability, or 'movement', of timber varies with species. Timbers with small movement characteristics may be selected for uses such as decorative flooring.

Distortion

Timber dried under controlled conditions and installed at a moisture content close to that which will achieve in service is less likely to distort in use.

Strength

Strength increases as timber dries.

Durability

Fungi and moulds cannot live on dry timber (below 20% moisture content). Wet wood is also usually more susceptible to insect attack than dry.

Machining

Dry timber produces a smoother surface.

Finishing

Finishes such as paints, varnishes and stain perform less well if applied to wet wood.

Preservative treatment

Pressure treatments do not penetrate wet wood effectively.

Gluing

Glues do not stick to wet wood.

Insulation

Dry timber is a better insulator than wet.

Fastenings

Metal fasteners may corrode in contact with wet wood.

Transport

Dry wood does not weigh as much as wet, so handling is easier and transport costs are reduced.

A realistic kiln drying specification of moisture content could include:

1. the average moisture content of the batch

2. the tolerance limits on the average moisture content of individual pieces within that batch

3. a limitation on the variability of moisture content within the individual pieces, either at different depths (ie moisture gradients) or at different positions along the length, or both

4. the method of measurement.

The appropriate British Standard will usually give the information necessary to specify 1. Items 2 and 3 will depend on the end use and whether variation from piece to piece or within pieces is critical.

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