The Stirling-based services provider, which has been celebrating its 50th year in business, saw turnover leap to £180.8 million for the year ended August 31, up from £163.2m the year before.
But it was not enough to prevent a slide in pre-tax profits to £5.41m from £7.53m, accounts newly filed at Companies House reveal.
Chairman Duncan Fletcher said: "For a fourth year in succession we have achieved an increase in turnover, to £180m, with overall profitability remaining at acceptable levels. We achieved this while maintaining our commitment to invest in training and development of our people and substantial investment in the areas of quality and health and safety."
Mr Fletcher said the diversification of Forth's activities has helped it generate a "strong pipeline" of business, reporting that its accepted order book had increased by £250m to total more than £1.5 billion. And he emphasised its commitment to staff development had not been hindered by difficulties in the construction sector.
Highlighting the firm's investment in a purpose-built training academy, Mr Fletcher said: "Our steady intake of young people, being equipped with appropriate industry skills, will provide us with an invaluable future asset."
The firm employed an average of 1896 last year, up from 1798 in 2012.