But the move will also mean the end of Unimer’s 78-year history, the closure of its offices and the loss of up to 40 jobs at the company.
The deal, expected to come into effect on February 1, has been under discussion for some time and is partly due to Grafton’s decision to withdraw from Unimer – a move which is significantly reducing the company’s turnover.
The existing Unimer operation will be wound down, with the group’s managing director Howard Grant deciding to retire from full-time business. Unimer independent merchant members will be offered membership of NMBS by way of immediate or deferred terms.
The merger will create a buying group with a turnover in excess of £1bn and comprise more than 1,000 members and 600 suppliers.
NMBS chairman Tim Allen said the agreement represented a "quantum leap" forward for the independent builders merchant sector.
"Undoubtedly it will deliver hitherto unseen scale to all stakeholders and it will provide our members with the advantage they need to compete more effectively with the national rivals on a daily basis," he said.
Howard Grant said Unimer’s membership had in recent years been about 1,000 merchants trading from over 3,000 outlets and with an annual turnover of around £900m, while NMBS’s turnover was in excess of £600m. But he said Unimer’s business had been reducing because of Grafton’s decision to withdraw and now both organisations had a similar turnover.
Grafton made its Unimer exit decision a year ago and the withdrawal process has been gradual over the last 12 months.
"It’s [Grafton’s decision] an element in formulating this decision," said Mr Grant. "But NMBS and Unimer have had a number of discussions over recent years as to whether it would make sense to merge the businesses."
He said the merged company would create a single interface for merchants and suppliers, as well as a single and major voice for the independent merchants.
Mr Grant confirmed the deal was "momentous" for Unimer and would write the final chapter in its long history – it was established in 1935. The Rustington office in West Sussex will be wound down and closed during 2014.
The 40 staff, he added, are facing redundancy. Some positions are to be offered at NMBS’s Leicester offices, but the distance away is not expected to make this a realistic option for most.
"The team here are very disappointed and NMBS’s Leicester team are very delighted," said Mr Grant.
"There is a people cost to this, but the board believes creating a single entity is the right thing for shareholders."
