Frequent and prolonged use of an improperly designed hand tool not only affects productivity but may also cause painful symptoms which, if left untreated, can develop into chronic musculoskeletal disorders (MSDs). This study was undertaken to assess the effects of ergonomic guidelines related to grip characteristics of a hammer stapler. Specifically, surface composition, shape, and angle of grip were investigated. Three commercially available hammer staplers were selected for this study. Tool #1 had a basic grip design, Tool #2 had a somewhat improved grip design, and Tool #3 incorporated most of the ergonomic design guidelines in terms of grip surface, grip shape, and grip angle. In a laboratory setting, 16 male participants used each of these tools on each of two simulated roof pitches at 4:12 and 6:12 inclines. Each experimental trial consisted of stapling roofing underlayment onto the simulated roof at a frequency of 1 staple per second for a two-minute duration. At a significance level of 5%, Tool #3 produced lower discomfort ratings in fingers and hand and higher favorable ratings for perceived grip comfort and protection from injury. Tool #3 also had significantly lower (p<0.05) muscle EMG in the flexor carpi ulnaris and lower ulnar deviation of the wrist angle at the instant of tool impact. The outcome of this study strongly favors implementation of ergonomic guidelines in hand tool design for non-powered, impact type hand tools. The quantitative results derived from this study would be useful in making design improvements in future hand tool design.