Members of the GMB union at the contractor Emcor will walk out on September 7 after rejecting a pay offer below the rate of inflation.
A mass meeting of workers fixed the strike date, although talks were to be held later today to try to avert the action.
A GMB official, Rob Kelsall, said: "Emcor is a profitable company and it can afford to pay a fair increase to our members who live in the expensive south-east.
"We hope the pay talks today move us in the right direction as the company are now clear where they stand with the determination of their workforce on this matter."
