Moisture Content Measurement and Control

Building codes specify that structural wood building products should be at dry at time of installation. Dry is defined as 19% moisture content (MC) on a dry weight basis. This number has been derived from past experience and has been used by the construction and wood products industry over the last 100 years. It is intended to ensure that wood will perform well in construction, under varying end use conditions.

Measurement of moisture content of wood products can be difficult, particularly if done in variable site conditions. Guidelines should be followed to measure and interpret results to correctly assess whether wood products are dry at installation time. For example, when measuring the moisture content of a piece of wood the following factors affect the individual result.

• Type of test (oven dry is most accurate)
• Type of meter (dielectric, DC resistance)
• Product type
• Temperature
• Species
• Variation of wood (wet pockets)
• Frequency, location and depth of sampling to correctly represent the entire piece

When assessing whether a structure composed of different types of wood products will perform satisfactorily under given end use conditions, the following factors should be considered

• Moisture distribution
• Where moisture will accumulate
• Number of storeys
• Location in structure
• Construction type and orientation
• Sampling and analysis of individual results

Practices to help control moisture content during construction phase

• Proper storage of building products (off the ground and covered)
• Early close-in of walls and roof
• Allow venting as long as possible but balance with local climate
• Sequence of weather-resistant barrier installation
• Sequence of flashing installation
• Sequence of cladding installation
• Sequence of vapour barrier installation

Further information will be provided in the near future.

For related info, see www.durable-wood.com or www.forintek.ca or www.cmhc-schl.gc.ca.

Shear Values

Shear Design Values for Lumber

Shear design values for lumber are approved by the American Lumber Standard Committee, in accordance with ASTM Standard D 245, Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber. To obtain more information on lumber shear design values, contact any of the following agencies:

• American Wood Council
• Southern Pine Inspection Bureau
• West Coast Lumber Inspection Bureau
• Western Wood Products Association

Design provisions, including requirements for shear design of lumber, are published by the American Forest & Paper Association (AF&PA) in the National Design Specification^® for Wood Construction (NDS^®), an ANSI national consensus standard.

Horizontal Shear Values for Canadian lumber used in the U.S.
Applicable to all grades.

For more information on the new shear design provisions for lumber, contact the American Wood Council (AWC) Helpdesk at 202/463-4713 or email awcinfo@afandpa.org.

Molds

Good information on molds can be found in the FAQ section of www.durable-wood.com Answers to the following questions can be found there.

• Are all molds bad?
• Do molds grow only in buildings?
• Are all discolored areas mold?

Because Forintek has often been asked to investigate such questions a fact sheet has been prepared to assist buyers and users of wood in understanding the nature of wood discolorations and deciding whether or not action is required.

The fact sheet can be downloaded in PDF format from the FAQ page of www.durable-wood.com.