This paper details the impacts of overmanning on labor productivity for labor intensive trades, namely, mechanical and sheet metal contractors. Overmanning in this research is defined as an increase of the peak number of workers of the same trade over actual average manpower during project. The paper begins by reviewing the literature on the effects of overmanning on labor productivity. A survey was used to collect data from 54 mechanical and sheet metal projects located across the United States. Various statistical analysis techniques were performed to determine a quantitative relationship between overmanning and labor productivity, including the Stepwise Method, T-Test, P-Value Tests, Analysis of Variance, and Multiple Regression. The results indicate a 0% to 41% loss of productivity depending on the level of overmanning and the peak project manpower. Cross-validatbn was performed to validate the final model. Finally, a case study is provided to demonstrate the application of the model.