Knowledge Base

Oh, now it makes sense…

Welcome to our knowledge base. A collection of wood words, timber terms, definitions, technical data, explanations and elaborations all on the subject of wood and trees. Come on in and grow your wood knowledge…

Moisture content

The term ‘moisture content’ is our way of talking about the water present in wood. All solid wood has a moisture content and it’s measured by the percent of saturation. Moisture content is measurable to within a few percent using a moisture meter – a handheld electronic device with electrodes (spikes) that penetrate the surface of the wood to get an internal reading using the conductivity of the water within. Moisture meters are available in varying types and levels of accuracy.

The moisture content of a timber changes due to a natural rate of drying once sawn, Drying is the evaporation of water from wood fibres, usually a natural air drying process, a response to atmospheric conditions. Depending on use, it’s possible to mechanically dry the timber even further in a process of kilning.

Guidelines for levels of moisture content are given in BS 1186-3 as follows:

Exterior Joinery – all
13 – 19%
Interior Joinery – Buildings with intermittent heating
13 – 17%
Interior Joinery – Buildings with continuous heating to room temperatures of 12º-19º
10 – 14%
Interior Joinery – Buildings with continuous heating to room temperatures of 20º-24º
8 – 12%

Read more TRADA: Moisture in Timber information sheet for more in depth information

Moisture movement

Movement is the dimensional change across the width and thickness of boards when the moisture content of timber changes in response to atmospheric conditions (statement TRADA).

Moisture movement is a relative term and species have been given the broad classes of Small, Medium or Large movement.

Rule of Thumb: within the moisture content range 5-30% the across the grain dimensions change by the following classes:

Small
1% movement
for every 5% change in moisture content
Medium
1% movement
for every 4% change in moisture content
Large
1% movement
for every 3% change in moisture content

To illustrate: a board of 150mm width Oak (Medium) at 25% moisture content will come down to 148.5mm at 21%, 147mm at 17%, and 145.5mm at 13%. Greater width boards are therefore more liable to larger movement.