Internal and external influences determine the work output that can be achieved on any given day. One factor is external temperature, with a significant effect on the ability of workers to work outdoors. If temperatures are too hot or cold, losses of productivity occur and the labor consumption rate increases. This paper presents a survey indicating the percentage labor consumption rate increase to be factored in as a result of suboptimal external temperatures during reinforced concrete works. An example demonstrates how much the selection of the construction start date influences construction costs if the survey findings are taken into account. Both deterministic and probabilistic calculations were performed to address the uncertainties included in the equations and to illustrate the results in histograms and curves. For this purpose, Monte-Carlo simulations made it possible to assign distribution functions to individual input parameters to carry out the calculations. Potential savings were identified that result from choosing a favorable construction start date. In the event of any disruptions or delays, the potential effects on the labor consumption rate can be assessed systematically with respect to chances and risks.