Ucatt said the 85% in favour was a “landmark” decision, which would strengthen campaigns on behalf of construction workers.
Both unions put “bad bosses” on notice, saying their combined resources would zero in on ending bogus self-employment and stopping firms undercutting pay, terms and conditions.
With major infrastructure projects coming on stream or in the pipeline, the unions said the move would give construction workers a powerful voice in pressing for a joined up industrial strategy, securing good quality jobs and boosting the number of proper apprenticeships.
Unite general secretary Len McCluskey said: “This is a historic moment for the construction industry and the men and women who work in it.
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“From January 2017, construction workers will for the first time have one united powerful union on their side at work.
“Bad bosses and construction firms employing sharp practices should beware.
“Our new combined industrial voice will be used to defend the rights of construction workers across the UK and to campaign for secure decent jobs which pay the rate for the job.”
Ucatt acting general secretary Brian Rye said: “By joining with Unite, the industrial muscle of all construction workers will double overnight, which is good news for all members.”