What Moisture Content Actually Means
Moisture content (MC) is the amount of water in wood expressed as a percentage of the weight of the dry wood. A piece of timber with a moisture content of 30% contains water equal to 30% of its own dry weight. A freshly felled tree can have an MC of 50–120% depending on species — the sapwood of some species holds more water by weight than the dry wood itself.
There are two forms of water in wood: free water, which sits in the cell cavities and is relatively easy to remove, and bound water, which is chemically held within the cell walls and requires more energy to drive out. The point at which free water has been removed but the cell walls are still saturated is called the fibre saturation point, approximately 28–30% MC for most species. Below this point, the wood begins to shrink as the cell walls lose water — and this is where most of the dimensional change in timber occurs.
What Happens When Timber Changes Moisture Content
Wood is not a static material. It continuously exchanges moisture with the surrounding air, absorbing water when the humidity rises and releasing water when the humidity falls. The moisture content at which wood is in equilibrium with the surrounding air is called the equilibrium moisture content (EMC), and it varies with both temperature and relative humidity.
In Sri Lanka's interior environments — air-conditioned spaces at 60–70% relative humidity — wood reaches an EMC of approximately 12–14%. Outdoors in the shade, the EMC is higher, typically 16–19%. Timber that is processed or installed at a moisture content significantly above or below its EMC will move as it adjusts — and that movement causes warping, joint failure, surface cracking, and fitting problems.
- Below fibre saturation point (~30% MC): wood shrinks as MC decreases
- Above fibre saturation point: wood does not change dimensions significantly
- Interior (air-conditioned, Sri Lanka): EMC approximately 12–14%
- Outdoor shaded (Sri Lanka): EMC approximately 16–19%
- Movement is not uniform — tangential shrinkage is roughly twice radial shrinkage
How Moisture Content Is Measured
There are two reliable methods for measuring timber moisture content: the oven-dry method and resistance-based electrical meters.
The oven-dry method is the reference standard. A sample is weighed, dried in an oven at 103°C until the weight stabilises, then weighed again. The MC is calculated as (wet weight minus dry weight) divided by dry weight, expressed as a percentage. This method is accurate to within a fraction of a percent, but it destroys the sample and takes several days, making it impractical for routine production use.
Resistance meters — commonly called moisture meters — measure the electrical resistance between two probes pushed into the timber. Dry wood has much higher electrical resistance than wet wood, and the meter converts this resistance to an MC reading using a calibration curve for the specific species. Resistance meters are fast and non-destructive, but they measure only the MC in the immediate area of the probes — typically the outer 5–25mm depending on probe depth. For thick sections, capacitance meters (which measure the whole cross-section) or pin meters with deep probes should be used to check for MC gradients between surface and core.
Target Moisture Content for Different Applications
The correct target MC depends on the application and the final environment. Using timber at the wrong MC is one of the most common — and most expensive — mistakes in construction and manufacturing.
- Furniture (interior, air-conditioned): 12–15% MC
- Joinery (doors, window frames, interior): 12–15% MC
- Structural timber (roofs, floors, framing): 15–18% MC
- Export packaging (ISPM 15 heat-treated pallets): typically 15–20% MC
- Outdoor furniture and decking: 16–20% MC (higher EMC environment)
- Parquet and strip flooring: 10–12% MC (low EMC in finished interiors)
Why You Cannot Rely on Air-Dried Timber for Finished Products
In Sri Lanka's climate, air-drying in open or covered storage rarely brings timber below 18–20% MC — and it does so slowly and inconsistently. Timber on the outside of a stack may be at 18%, while timber in the centre is still at 30%. Air drying also cannot be controlled for speed, which means the surface can dry much faster than the core during periods of low humidity, causing checking and surface cracking.
For construction timber where dimensional precision is not critical — rough roofing battens, temporary formwork, scaffold planks — air-dried timber may be acceptable. For furniture, joinery, finished flooring, and any application where movement after installation is not acceptable, kiln drying to a controlled target MC is the only reliable approach.
St. Xavier Timber operates an industrial kiln that brings timber to a specified target MC with humidity and temperature monitoring throughout each cycle. We provide batch-level moisture content data with every order. Contact us to discuss drying requirements for your timber species and dimensions.
How to Verify Moisture Content When Buying Timber
Ask your timber supplier for the current MC of the batch and how it was measured. A reputable kiln drying facility will have batch records showing the drying cycle, final MC readings, and the date of drying. If your supplier cannot provide this information, the timber has not been dried to a controlled specification.
When taking your own readings with a moisture meter, test multiple pieces in the batch and multiple locations on each piece — at least the face, edge, and one cut end. A good kiln-dried batch will show consistent readings across pieces; poorly dried timber will show significant variation. Any reading significantly above the stated target MC is grounds for rejecting the batch or requesting re-drying.