Peterborough Crown Court heard that Nigel Sewell, 57, from St Neots, died as a result of his crush injuries following the incident at Universal Builders Supply Ltd in Kneesworth on 19 September 2011.
The week-long trial hard that Sewell was part of a four man team assembling a tri-mast in the rear yard of the premises.
It consisted of three standard mast sections, in a triangular formation and each section was being lifted and then lowered vertically into a purpose-built jig using a telehandler and a four-leg chain sling.
Two sections had been lowered into the jig and bolted together, but the second section did not sit correctly so Sewell and a colleague attempted to force it into the jig using a sledge hammer and crowbar.
The court heard that their attempts failed, so the driver of the telehandler was instructed to use the machine to push the mast section into the jig, which he did.
He then positioned the telehandler ready to move the final section into place, but as he did so the two three-metres tall mast sections already in place, each weighing around one tonne, toppled onto Sewell.
The HSE found that there was inadequate planning and supervision of the work and there had been no separation of vehicles from the assembly process.
This meant that the telehandler, used to lift and lower the sections, inadvertently pushed over the two sections of the tri-mast.
Universal Builders Supply Limited, of Rifle Street, London, was fined a total of £125,000 and ordered to pay £40,000 costs after being found guilty of safety breaches.
Speaking after the hearing HSE Inspector Graham Tompkins, said: “The loss of Mr Sewell was devastating for his family and friends and is made even more incomprehensible by the fact that it was completely avoidable.
“This tragic death could have been prevented had simple safety measures been thought through and put in place.
“Universal Builders Supply Ltd failed to plan the work properly, to provide appropriate instruction and to ensure there was competent supervision of the operation.”
