Two employees were installing a 6-inch test T in a vertical 6-inch cast iron storm riser. The T was to be placed approximately 2 feet above floor level and required cutting and removing a section of the 6-inch storm riser. The two employees placed a riser approximately 5 feet above the section they intended to remove and utilized a 5-foot piece of 2" cast iron to support the riser clamp and ultimately the 6-inch cast iron pipe above the section being removed. They then cut and removed the 12-inch section of pipe. Employee 1 began installing the rubber no hub gasket on the cast iron riser coming through the floor while Employee 2 was installing the rubber no hub gasket on the cast iron riser coming down from the floor above. As Employee 2 was installing the rubber no hub gasket on the upper section of pipe the riser clamp slid off the 2-inch cast iron. This resulted in the upper section of pipe falling when the no hub band holding it in place failed to support the weight of the section of pipe. The upper section of pipe fell on Employee 2's right thumb smashing it between the two sections of pipe. The employee's thumb was fractured and lacerated requiring surgery to repair.