Martin Sapec, 54, of Edwardsville, Treharris suffered a broken pelvis and ankle in the incident at Bryntirion, Mountain Ash, on August 15 2011.
Pontypridd Magistrates heard that Sapec was helping delivery driver Wayne Ford to unload the scaffolding material while standing on the back of the lorry.
The vehicle was fitted with a crane operated by Ford, who failed to extend a stabilising outrigger from the vehicle designed to provide balance while the crane is in operation.
Because the crane was used without the extended outrigger, the lorry became unstable and tipped over, throwing Sapec off.
As he landed, some of the scaffold boards and tubes from the lorry fell on top of him.
Sapec has been unable to work since the incident because of his injuries.
An HSE investigation found Ford was responsible for the lorry, and was at fault for not using the crane and extending outrigger in the correct, safe manner.
Ford, of Nelson, pleaded guilty to safety breaches and was fined £1,000 and ordered to pay £1,000 in costs.
After the hearing HSE Inspector David Kirkpatrick said: “Mr Sapec sustained serious injuries in this incident, and could easily have been crushed and killed by the lorry when it toppled over.
“Operators of vehicle mounted cranes must ensure the safety of people they are working with by using the equipment in the proper manner.
“Had the extending outriggers been use for their intended purpose then the incident could have been avoided. It was wholly preventable on that basis.”