But the contractor remained in the black and saw order books hit record levels.
Latest results for the year to December 31 2020 show pre-tax profits fell to £8.9m from £44.2m last time on revenue down to £2,043m from £2,5987m.
Paul Cossell, CEO of ISG, said: “In one of the most challenging years for our industry, clients and society in general, our resilient 2020 financial performance is testament to the outstanding contribution and commitment of our people and supply chain partners to drive projects forward safely.
“Four years of consistent profitability and growth provided the financial and structural resilience to weather an unprecedented period of market uncertainty and emerge with a record forward order book, which resets us on our previous growth trajectory.
“I’m passionate that ISG continues to be at the forefront of creating many of the world’s most innovative and advanced buildings that have a profound and positive impact on our environment, communities and helping to transform lives.”