In the construction industry, workers falling to a lower level has been the primary cause of fatalities according to the Bureau of Labor Statistics Census of Fatal Occupational Injuries database. From 2006 to 2010, an average of 353 construction workers died annually as a result of falling to a lower level. An average of 126 workers (36%) died when falling from unguarded roof edges, and through roof and floor holes or skylights. The National Institute for Occupational Safety and Health evaluated the strength of job-built guardrail structures around an opening. The study focused on a 2 x 4 opening typical of residential skylights. Nine full-time residential carpenters built guardrails for strength testing. Guardrails were constructed with 2" x 4" lumber and 16- d duplex nails. The strength test determined if each guardrail could support a 200-lb loading on the top rail as required by the OSHA Fall Protection Standards (Subpart M). A quantitative pull test was then done to measure the strength and integrity of each guardrail. All nine guardrails passed the 200-lb drop test, and the strength test results ranged from 161 to 575 lbs. Three of the nine test subjects were randomly selected to construct similar guardrails using 3-inch all-purpose screws instead of nails. An average of 75 screws were used per guardrail compared to an average of 85 nails per guardrail. The strength of the structures built with screws was more consistent, and had a 67% increase in overall strength with respect to the nail structures and the strength test results ranged from 395 to 470 lbs. The overall strength and integrity of the structures was directly related to the construction techniques used by each subject. The successful construction techniques were determined to be the following: the orientation of vertical support posts relative to the applied loading, anchoring the vertical posts inside the opening, the overlapping of the rails of the structure, and the number, type, and orientation of the fasteners.