In fall protection, the most terrifying and least understood hazard isn't a harness breaking or a rope snapping; it is a phenomenon called "roll-out." Roll-out occurs when a snap hook or carabiner is connected to an anchorage point, the gate is closed and locked, yet the hardware spontaneously disconnects from the anchor under the force of a fall. It is a terrifying, lethal failure that has killed numerous workers who thought they were securely tied off.
Roll-out is a physics problem caused by incompatibility. It happens when a snap hook is connected to a D-ring, bolt, or eye bolt that is too small or has the wrong geometry. Imagine a worker clipping a large, heavy snap hook onto a small wire eye bolt. Because the eye bolt is so small, the snap hook can rotate freely inside it. During the course of a shift, as the worker moves around, the heavy snap hook rotates upside down. The worker's own body weight now rests entirely on the gate of the snap hook.
Standard snap hook gates are only rated to hold about 350 pounds of side load before they pop open. If the worker falls, the sudden, violent shock load presses the gate against the small eye bolt. The gate bursts open, the snap hook rolls over the eye bolt, and the worker plummets to the ground.
To prevent roll-out, you must strictly follow the rule of compatibility. A snap hook or carabiner must never be connected to an object that will allow the hook to rotate, or to an object that puts a side load on the gate. You must never connect two snap hooks together; they will twist and roll each other out under load. You must never tie a knot in a lanyard and clip the snap hook back onto the lanyard itself.
Always ensure the anchorage point is large enough that the snap hook sits snugly and cannot flip upside down. More importantly, always use auto-locking carabiners-ones that require at least two or three distinct manual movements to open the gate-rather than standard spring-loaded snap hooks that can be opened by bumping against a beam. Before you climb, grab the connector, try to twist it, and pull it aggressively in every direction. If it can flip and rest its weight on the gate, it is incompatible and you must find a new anchor point immediately.
