George Nicholls, 25, used the ladder to paint a shop frontage on St Marys Road in Southampton on 14 March 2013.
He was caught on camera by a council environmental health officer following a tip-off from a concerned member of the public.
Southampton Magistrates’ Court heard Nicholls, trading as Laser Roofing London and South East Roofing Limited, had been sub-contracted by Norfolk-based Maintenance 24-7 Ltd for the paint job because the company did not possess the correct equipment or expertise.
Ladders were specified as the chosen method of work, but after the finding the façade was higher than the ladder he had with him, the roofer opted to improvise.
He placed it on the roof of his van and worked from it fully-extended eight metres above the ground with a labourer providing the footing.
HSE also found that the van in question was also parked over a bus stop on a busy road with double yellow lines.
Maintenance 24-7 Ltd, of King’s Lynn was fined £10,000 with £784 in costs after admitting a safety breach.
Nicholls, of Newdigate, Surrey, was fined a total of £4,000 and ordered to pay £666 in costs.
After the hearing, HSE Inspector Frank Flannery said: “Anyone can see the system of work is plain wrong, so why a supposedly competent roofer chose to work in this way is anyone’s guess.
“George Nicholls blatantly and recklessly risked harming himself and others, and he did so on behalf of Maintenance 24-7 Ltd, who had clear duties of their own to ensure the work at height was properly planned, managed and executed in a safe manner.
“The standards of both parties fell far below those required, and I would like to thank the concerned member of the public who initially brought the matter to the council’s attention.”