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Background. The effect of temperature on properties can be separated into reversible and permanent effects. The National Design Specification (NDS) provides factors ( C t) for reducing properties for reversible effects but provides little guidance on permanent effects. Objectives. The primary objective of this paper is to evaluate the effect of prolonged heating (permanent effect) on the flexural properties of solid-sawn and composite lumber products exposed at 66°C and 75 % relative humidity (RH) and at 82°C and 30 % RH. A second objective is to determine how to estimate total effects. Procedures. Solid-sawn lumber, laminated veneer lumber (LVL), and laminated strand lumber (LSL) were heated continuously for up to 6 years. After each exposure period, the lumber was con-ditioned to room temperature at the specified RH and then tested on edge in third-point bending. Some lumber was also tested hot at 66°C after 48 h of exposure and after 3 years of exposure. Results. After 3 years of continuous exposure at 66°C and 75 % RH, solid-sawn Spruce-Pine-Fir (SPF) and Douglas-fir retained about 72 % of their original modulus of rupture (MOR) and southern pine about 47%. For the first 2 to 3 years of exposure, changes in MOR of LVL wer