Kidderminster magistrates heard that the trainee, who has asked not to be named, was working for Harris Scaffolding Limited on a construction site in Stourport-on-Severn, Worcestershire, on November 16 2011.
The firm had been called back to the site to make modifications to the scaffold it had put up two months earlier.
The changes were needed to provide roofers with working platforms at each corner of the scaffold so they could install rainwater downpipes.
The teenage worker was carrying out the necessary alterations when he fell nearly three-and-a half metres to the ground below. He fractured two vertebrae and was off work and in a back brace for three months.
An HSE investigation found the apprentice was allowed to work unsupervised in areas of scaffold with no boards or guardrails and without a harness.
At times he stood on single-width scaffold boards or directly on tubing and gained access to work areas from an unsuitable ladder and by climbing up the outside of the scaffold.
HSE said the work had not been adequately planned, supervised or carried out in a safe manner.
A more experienced colleague had been sent to work with him but he had not carried out any scaffold construction work for 15 years and had not had any refresher training in that time.
The investigation also found that Harris Scaffolding had not followed its own risk assessment when it first built the scaffold by failing to work in accordance with industry-recognised best practice guidance.
No guardrail or supports were in place for use during modifications of the scaffold.
Neither the apprentice nor the older colleague were given any specific instructions or drawings before going on to site and had not seen a risk assessment or method statement.
Harris Scaffolding of Halesowen, pleaded guilty to safety breaches and was fined a total of £10,000 and ordered to pay costs of £6,156.
Speaking after the hearing, HSE inspector Luke Messenger said: “This was an avoidable incident and a young man was fortunate not to suffer more serious life-changing or even fatal injuries.
“There is a wealth of guidance available, from HSE and the industry, and there is really no excuse for not following basic precautions such as working from a safe area or using a harness.
“In this case the company fell well below accepted standards and a trainee scaffolder was badly injured as a result. It was lucky his career wasn’t ended before it had properly begun.”