The time and stress dependent mechanical behavior of reinforced and unreinforced plastic lumber is described using Schaperys single integral constitutive equation for nonlinear viscoelastic materials. Viscoplastic behavior is accounted for using a modification of the plasto-viscoelastic model proposed by Lai and Baker. In this way, both creep and recovery behavior of viscoplastic materials investigated can be successfully described. Applying the correspondence principle, a solution of the elasticity equations allows for prediction of time-dependent composite material behavior based on creep data of the base polymer. A creep testing fixture with heating chamber and data acquisition system was build to allow for midpoint deflection measurement of full-sized specimens under controlled temperature (24, 40 and 60°C) and constant loads. Creep experiments were performed over a duration of 20 hours followed by a recovery period of 200 minutes. Experimental results for a continuously reinforced composite showed good agreement with model predictions over the entire time range studied. Furthermore, the continuously reinforced samples exhibited better creep behavior at times exceeding 20 h than the short fiber composite having a glass fiber content twenty times greater. Finally, applicability of time-temperature-superposition as an accelerated testing technique could not be proved due to material variations of investigated samples which were more dominant at longer testing times.