I-Joist Special Precautions

• Mixing I-Joists and sawn lumber in a floor or roof system should never be done without a careful analysis of the potential consequences.

• I-Joists are manufactured at very low moisture contents (5% or lower) and to a high degree of dimensional accuracy. Sawn lumber products, on the other hand, are sized and sold at moisture contents often in excess of 16%. When used together on a job site, the I-Joist has a tendency to expand due to increased moisture content while the sawn lumber is subjected to shrinkage as it dries out. In situations where lumber and I-Joists are used together in floor or roof systems, this differential shrinkage can lead to situations that must be considered by the design professional.

• The use of sawn lumber blocking or rim boards in conjunction wood I-Joists in a floor system is a classic example. In such situations blocking and rim board materials are used, at least in part, to assist the wood I-Joist in distributing vertical loads through the floor system into the structure below. As the building materials in the structure reach an equilibrium moisture content with their surroundings, sawn lumber blocking and rim boards shrink while the I-Joists do not. As a result, lumber components are not "available" to carry the applied vertical load that they were designed to carry, thus overloading the I-Joists.

• This is one of the reasons that I-Joists are manufactured in depths different from nominal sawn lumber products. A nominal 47 x 250 mm solid lumber joist may not easily be used to block a 9-1/2" deep I-Joist. Sawn lumber is not effective for such applications and should never be used without careful consideration.

• There are applications, such as diaphragm blocking, "squash" blocks, backer or filler blocking where sawn lumber is acceptable for use in conjunction with engineered wood products.