Seasoning defects and moisture limits

Defects caused by drying

• Checks are surface cracks that form when the outside of a piece shrinks while the inside is still wet. They occur more often at the ends, which dry faster. A deep check can hold water and encourage rot.
• Warping (bow, cup, twist) happens when different parts of a piece shrink by different amounts. Tangential shrinkage is larger than radial, so the way a board is cut from the log affects how it distorts.
• Splits and honeycombing are internal or through cracks. Seasoning defects are a main cause of them.

How to limit them

• Dry slowly enough that the surface does not shrink far ahead of the core. Drying schedules for kilns balance temperature, humidity and time for each species and thickness.
• Seal or protect the ends of valuable pieces during air drying.
• Stack with evenly spaced stickers so air moves through the pile and the weight keeps boards flat.

Moisture limits in use

• A maximum of about 19 % moisture content is commonly recommended for untreated timber in service.
• Decay fungi need moisture content above roughly 25 % (dry-weight basis) to attack wood. Established decay can continue at around 22 to 24 %.
• Wood that stays below those levels can remain sound for very long periods.

Moisture content figures here are on the dry basis; see the article on moisture content for the difference to the wet basis.