Sunderland building firm Murray Construction and Development was fined £10,000 at South Tyneside Magistrates’ Court after an employee was injured when he fell 2.8m onto a concrete floor.
The firm pleaded guilty to health and safety breaches following the incident at Andes House, South Tyneside in May 2007.
Three employees were working on the construction of a new house and moving one of the rafters to accommodate a roof window.
One man standing on a joiner's stool lost his balance and fell through a gap onto the concrete floor 2.8m below him. He suffered serious injuries and has been unable to return to work since the incident.
HSE Inspector Michael Brown, said: "Companies must assess the risks from work that they are undertaking at height and make sure that suitable measures to prevent a fall are in place.
"They should take steps to avoid, prevent or reduce risks, choose the right work equipment and take collective measures to prevent falls, such as guardrails and working platforms, before other measures which may only mitigate the distance and consequences of a fall, such as nets or airbags, or which may only provide personal protection from a fall. Had this happened this worker may not have been injured."
Murray Construction and Development was ordered to £5,000 compensation to the injured worker, £1,414 costs and a £15 victim surcharge.