São Paulo. September 3, 2012.
Wood is a hygroscopic material, i.e. it exchanges moisture with its surrounding environment. As a piece of wood loses moisture it shrinks and as wood gains moisture back it swells.
Dimensional changes due to moisture exchange in wood can lead to defects, such as warping, checking or splitting, that may compromise the performance of the wood piece or product. Hence, most of the challenges of utilising wood in architecture or engineering involve understanding wood-moisture relationship and its influence in wood properties.
This work intends to provide a more comprehensive form for visualising how different climate conditions influence different wood species regarding their dimensional stability. Nevertheless, it also aims to provide a design tool for most wood-workers (from architects to cabinet-makers) that could quickly estimate the behavior of some wood species in a specific location along the year and, then, implement necessary design modifications in order to accommodate dimensional changes.
When sheltered from direct sun and rain, a given piece of wood will attain a specific Moisture Content (MC), depending on relative humidity and air temperature values. Wood's MC is expressed as percent and measured as the ratio of the weight of water in the piece to the weight of wood when it is completely dry [1,2].
The following diagram shows values for temperature, relative humidity and MC by month in different cities.
Green wood refers to a condition in which the cell wall fibers are fully saturated with water and it starts about 30% MC for most wood species. This condition is usually seen in freshly sawn wood or wood that has been directly exposed to water. Any increase in MC over 30% will not affect wood dimensions. However, under the conditions in which wood is commonly used, its MC drops bellow 30%; as wood loses moisture bellow fiber saturation point it starts shrinking .
The amount of shrinkage will vary according to the wood species and, due to wood's anisotropic behavior, direction. Basically, there are three main directions: perpendicular to the growth rings (radial direction), transverse to the growth rings (tangential direction) and parallel to the fiber orientation (axial or longitudinal direction). For longitudinal direction dimensional changes are very little, at most about 0.1% [1,2], they are left out.
The following diagram shows the absolute the amount of linear shrinkage as a percent of Green Wood's dimensions by month in different cities.
After a certain time wood will reach a state of equilibrium with its surroundings. In this situation wood is neither gaining or losing moisture to the environment . Nevertheless, this equilibrium is dynamic: whenever the weather condition changes wood will try to reach achieve a different equilibrium MC value. As a given piece of wood loses moisture it shrinks and as wood gains moisture back it swells.
The following diagram displays the amount of dimensional changes (swell, shrinkage) by species in different cities, within a selected month and the maximum amount of changes along the year from that month.
 FOREST PRODUCTS LABORATORY (FPL). The wood handbook: wood as an engineering material. United States Department of Agriculture, Forest Service: Wisconsin, 2010.
 HOADLEY, B. R. Understanding wood: a craftsman guide to wood technology. The Tauton Press: Newtown: 2000.
 HERZOG, T.; et al. Timber construction manual. Birkhäuser: Munich, 2008.
 IPT - Instituo de Pesquisas Tecnológicas. Madeira: uso sustentável na construção civil. IPT: São Paulo, 2003.
Temperature and humidity data were retrieved from weatherBase.com. The equilibrium moisture content (EMC) was calculated using EMC formula . The relative and absolute dimensional changes were calculated using wood data from [1,2,3,4].