Shaun Jacob, 18, was removing the ridge from a metal sheet roof when a sheet he was standing on buckled, causing him to fall to the ground below.
Mr Jacobs’ employer, Randell and Janes Roofing Specialists Ltd, of Maes yr Hendre, Cwmann, near Lampeter, was prosecuted after the incident by the Health and Safety Executive.
The firm pleaded guilty to a breach of Regulation 4 of the Work at Height Regulations 2005 when it appeared before magistrates in Ammanford.
The company was hired to replace a roof at a single-storey cottage at Derwydd Mansion, Ffairfach, Carmarthenshire, where the incident occurred on 7 October 2009.
HSE’s investigation revealed that there had been no specific risk assessment carried out for the job, and there was nothing to break a fall placed beneath the roof as it was being removed.
Mr Jacob, from Llanybydder also had no training in using roof ladders, and his supervisor also had no health and safety training.
Randell and Janes Roofing Specialists Ltd was fined £2,000 and ordered to pay £1,500 costs.
HSE Inspector Phil Nicolle said: “It is disappointing that a company whose primary business is roof work displayed such significant failings.
“Employers have a legal requirement to properly plan work at height and ensure it is appropriately supervised, but this did not happen in this case.
“There are several ways to reduce the risk of injury from falls and though the company owned a large number of bean bags which would have served this purpose, they were not used.”