Wood Shrinkage Calculator

The Wood Shrinkage Calculator is a quick and easy tool to estimate the amount of shrinkage the structure may experience as the wood member loses moisture content after it is framed and in service. The calculator estimates the shrinkage of each wood member in the wall and floor framing assembly and provides a graphical summary to help understand the global impact of shrinkage of individual elements in the wall system. The summary can then be used to determine the best approach to handle the shrinkage in the system. Each member of the wall and floor assembly is listed in the results table along with its calculated shrinkage amount which may be printed and maintained with the project.

It is important to note that wood is a complex building material and that shrinkage is a variable property. There are many variables that will affect moisture content and the amount of shrinkage that will occur in a given wood member. In order to simplify calculations general shrinkage coefficients are used on certain species groups that are commonly used in light frame construction.

Step 1 – Select Initial and Final Moisture Content

All wood is composed of solid material and moisture which will be gained or lost depending on the environment. Place dry wood in a moist environment and it will gain moisture and swell; place wet wood in a dry environment and it will loose moisture and become smaller. The moisture content of wood is a measurement, expressed as the percentage, of the weight of water in a piece of wood to weight of wood completely dry (oven dry). Although the moisture content of lumber can vary significantly, most lumber used in construction will be 19% or less after it is seasoned. Furthermore, in most cases the moisture content at the time of construction (Initial Moisture Content) will be greater than the moisture content when in service (Final Moisture Content) causing the lumber to shrink.

In order to estimate the shrinkage of wood a reasonable estimate of the initial and final moisture content must be established and the following information provides some guidance that might be useful:

• The Initial Moisture Content is considered to be the moisture content at the time of building enclosure and is to some extent defined by specification for the lumber. Dry lumber is defined as having a moisture content of 19 percent or less at the time it is manufactured and will be grade stamped as S-DRY (surface dry) or KD (kiln dried). Conversely, unseasoned or green lumber has a moisture content that is greater than 19% and is grade stamped S-GRN (surfaced green). A number of variables can impact the moisture content of lumber such as atmospheric conditions and the handling of lumber while on the job site however the specification by the designer for dry lumber means the moisture content should be 19% or less at the time of use.
• The Final Moisture Content, also know as the equilibrium moisture content, is the moisture content reached after the building is in-service. Affected by atmospheric conditions, the moisture content range in most buildings in the US is between 7% & 14%. The following moisture content information of wood structures in various locations in the US, as reported in Table 1.3 of Wood Structures: A Design Guide and Commentary, ASCE 1975, provides useful values which may be used to estimate the Final MC:
  • Dry Southwestern United States (Eastern California, Nevada, Southeastern Oregon, Southwestern Idaho, Utah and Western Arizona):
    • Average: 9%
    • Range: 7-12%
  • Remainder of United States:
    • Average: 12%
    • Range: 9-14%
• The following are several resources available which may provide valuable information on this topic such as:
  • Design of Wood Structures – ASD, Breyer et al
  • Wood Structures: A Design Guide and Commentary, ASCE 1975
  • Multistory Wood-Frame Structures: Shrinkage Considerations and Calculations, Rummelhart and Fantozzi, 1992

Step 2 – Input Global Wall & Floor Data

The user is asked for all key wall and framing information necessary to perform the calculation. Depending on initial input information subsequent input values might be required. Additionally, some input information is asked as global questions however the user is able to change any framing member at the next stage. For example, the user is asked for the typical plate height regardless of the number of stories but is allowed to change the plate height at each level at the next stage.

• EWP stands for Engineered Wood Product and is assumed to be dimensionally stable and assumed not to shrink.
• First Floor Data refers to the foundation type supporting the first wood framed level which may coincide with the first level or might be an elevated level supported by a wood or steel beam.
• Typical Plate Height may be adjusted at each level at the next stage.
• Upper Floor Data may be changed at each level at the next stage.
• A floor system that is not present is one where the top of the wall frames to the underside of the floor deck above and the floor system runs parallel to the wall.

Step 3 – Optional Parameter

The user has the ability to exclude the shrinkage of the studs from the calculation since shrinkage along the grain of wood is much less than across the grain. However, the longitudinal shrinkage of the studs can be calculated and is often comparable to a single 2x plate since it occurs over the entire length of the stud.

Step 4 – Confirm Input Data

After the user has input the global data for the wall and floor framing they have the opportunity to visually see the configuration and change any member as required to replicate their condition.

Step 5 – Run Calculation

After all input data has been confirmed the shrinkage calculation is ready to run and a summary graphic is presented which provides shrinkage values at key locations for a continuous rod restraint system and traditional holdown/strap restraint system. The user is able to directly compare the effects of cumulative shrinkage on the continuous rod system to the incremental shrinkage confined at the floor framing area of the holdown/strap system. The following options are available after the calculation has run:

• Summary table of the calculated shrinkage of each component may be viewed by choosing the 'Calculations' button.

Step 5 – Summary Calculation

The summary graphic and table may be printed or a hard copy can be created.