Subcontractor Malcolm Dunn was also fined £2,000 at Southwark Crown Court following the death in July 2007 of Paul Morrissey, 57, of Penarth, Cardiff.
Morrissey had been on the roof of Unit 37 Fairview Industrial Estate, Hayes, cutting the bolts which were securing an old rooflight, intending to replace it with a new one.
It is not known what caused him to fall nearly seven metres through the rooflight onto the concrete floor below.
The removal of the fixings securing the rooflight meant that only slight pressure would have made it give way beneath him. There was no one else on the roof at the time and critically, there was nothing to break his fall. He died at the scene.
The investigation by HSE found that the contractor appointed to carry out the work had little knowledge or experience of this type of roofwork. There was also evidence that there was a basic failure to plan the work adequately.
HSE Inspector, Giles Meredith, said: “The dangers of working on fragile roofs are well known and yet this senseless waste of life continues. This incident was all too familiar.
“It is vital that anybody planning or carrying out roofwork of this nature has the right experience and manages the risks involved. Whoever is responsible for selecting contractors needs to ensure that the people they get in know what they are doing.
“In this case, the provision of safety nets would have kept Paul Morrissey alive.”
Open Contracts was ordered to pay costs of £29,757. Subcontractor Dunn, of Caerau Park Road, Cardiff, who was trading as 3D Coatings, was ordered to pay his fine within two years or face a custodial sentence in the event of default of payment.