Roofers have long suffered from low back disorders (LBDs), which are a primary nonfatal injury in construction. Ergonomic studies have identified several risk factors associated with LBDs in workplaces and developed biomechanical models for general LBD risk assessments. However, these models cannot be directly used for assessments in roof workplaces because they are designed for general tasks without considering roofers' posture variance and effects of working on slanted roof surfaces. This paper examined the relationship between roofing work-related factors and LBD risk among roofers using a laboratory assessment. A pitch-configurable wood platform was built to mimic the rooftop. The maximum trunk flexion angle and normalized electromyography (EMG) signals were measured as indicators using a motion capture system and a skeletal muscle signal recording system under different settings, i.e., different roof slopes, postures, facing directions, and working paces. The results indicated the measured factors with significant effects on the LBD development and revealed unfavorable conditions (e.g., using a stooped posture to work on low-pitch rooftops at a fast pace) where the work on rooftops needs particular attention. Such information is useful for systematic understanding of roofing nonfatal LBD developments among construction professionals and may enable development of interventions and guidelines for reducing the prevalence of LBDs at roofing jobsites. (C) 2017 American Society of Civil Engineers.