The OFT found that the six Scottish firms agreed to fix the prices of flat roofing services between 2000 and 2002 through collusive tendering. They were fined almost £260,000, reduced to about £138,000 for leniency to the whistleblowers.
Advanced Roofing was fined £1,963, Geo. Brolly & Co was hit with a £22,239 fine, Bonnington Contracts £45,187 and McKay Roofing £27,219.
The contracts affected were for felt and single ply flat roofing services.
OFT chairman Sir John Vickers said: 'Collusive tendering deprives customers of the benefits of competition. In this case a hospital, school and council taxpayers were disadvantaged by these anti-competitive agreements.'