Creep is the time-dependent deformation of loaded member undergoing elastic deformation.

The National Design Specification for Wood Construction (NDS) addresses creep in section 3.5.2-Long Term Loading. Under long term loading, the expected (average) deflection would be 1.5 times the initial deflection for seasoned lumber and 2.0 times the initial deflection for unseasoned lumber. Long term loading will cause a permanent set of about 1/2 the creep deflection.

The creep deflection varies anywhere from zero to twice the initial deflection. This means that the total deflection can vary from the initial deflection to as much as three times the initial deflection.

Forest Products Laboratory’s Wood Handbook – Chapter 4: Mechanical Properties of Woo