Managing director of Southern Property Maintenance, Richard James, 37, was cleared of causing the death of Shane Offer, 21, from Lancing, in June 2009.
But he and the firm were each found guilty at Lewes Crown Court of breaching their duty to an employee.
The case was adjourned until 5 May for sentencing at the same court.
James was remanded on bail while reports are prepared.
Offer fell through a skylight at Rosebys in Bognor Regis while he was working on the roof.
He was flown to the Royal London Hospital in east London, but died from his injuries.