Imposed loads, also called live loads or variable actions under Eurocode terminology, represent forces that change in magnitude or position over time. On commercial rooftops, the primary imposed load for structural assessment is the maintenance access load: the weight of personnel and equipment that will periodically access the roof for inspection or cleaning.
Eurocode 1 (EN 1991-1-1) defines roof imposed load categories. Category H covers roofs not accessible except for normal maintenance, with a characteristic load of 0.4 kN/m². Structural engineers assess whether existing roof structures can accommodate dead loads from a proposed PV array and imposed loads simultaneously under the relevant Eurocode load combination expressions.