Two companies have been fined £105,000 over the incident at Dairy Crest premises at Davidstow near Camelford on 4 November, 2008.
Truro Crown Court heard that Michael Stone, 44, of Plymouth, was erecting a scaffold at the premises when the incident happened.
Self-employed Stone was contracting for specialist fabrication firm Dartmeet Services which was contracted to creamery owners Dairy Crest to replace the roof on the chemical store.
The building had fragile rooflights but Stone had not been made aware of this and no signs or markings were evident to indicate the danger.
The HSE investigation found Stone and his employees were not requested to sign in to gain access to the roof and no-one at the site checked his risk assessment for the work.
Stone landed on a concrete floor when he fell, suffering multiple injuries. He died in hospital seven days later.
Dairy Crest Ltd of Esher, Surrey was fined £75,000 and ordered to pay £20,000 costs for safety breaches.
Main contractor Dartmeet Services of Newton Abbot was fined £30,000 with £10,000 costs.
HSE Inspector, Barry Trudgian, said: “This is yet another tragic fatality caused by working on a roof with fragile rooflights where the risks are well known.
“In this case, no-one involved took proper control to make Mr Stone aware of the issue.
“There should have been signs on the building indicating the presence of fragile rooflights and any work on the roof should have been subject to a thorough risk assessment and supervision.
“Simple, straightforward, commonsense procedures could have saved Mr Stone’s life.”