New York State Certificate of Competency (CoC) Class B Pyrotechnician License Practice Exam

Two inches is the required gap. This small but important space acts as a safety buffer between buried mortars and the wall of troughs or drums, helping to absorb blast energy and debris, reduce heat transfer to the wall, and prevent the mortar from binding or rubbing against the wall as it sits or as forces act on it. Maintaining this clearance minimizes the risk of damage to the trough or drum and lowers the chance of a fragmenting event propagating to the wall. In short, the two-inch spacing helps contain the blast and protect both the mortar setup and the surrounding structure.