Leeds Crown Court heard how, on 3 October 2014, the two subcontracted joiners were working on the refurbishment and construction of new homes at the former Wharfedale Hospital in Otley, West Yorkshire.
They were working in the bell tower at the site on a temporary access platform when it collapsed.
The men fell approximately 3.4 metres onto a lower platform which also collapsed. They then fell a further 3.4m onto the ground floor of the building.
The platforms were installed by PJ Livesey Living Space (North) Ltd to allow other contractors to access the roof structure.
One of the workers injured their ankle while the other has suffered permanent nerve damage to the face.
An HSE investigation found the work platforms had been designed by the site manager, an employee of PJ Livesey.
The site manager was not competent to carry out such a design and the design had not been reviewed by another competent person. Consequently, the platform was unsafe and collapsed.
PJ Livesey Living Space (North) Ltd of Trafford Park, Manchester pleaded guilty to safety breacehs and was fined £56,000 and ordered to pay £7262.16 costs.
After the hearing, HSE inspector Chris Tilley said: “The risks associated with working at height are well known.
“Temporary works must always be designed to meet British Standards by a competent person and in the case of more complex works such as this, be subject to a secondary overview.”