The construction materials company will pay a combination of cash and shares at a price of 50p per share for Stone.
The deal is made up of a stage consideration of up to €1.3mln and €0.9mln of assumed debt.
Stone, which operates two quarries, a wharf and a contracting business in Belgium with a focus on armour rock for river and sea defence work, generated an average revenue of €3.8mln and earnings (EBITDA) of €500,000 for the financial years between 2016 and 2018. That means the value of the acquisition represents a multiple of 4.4 times average earnings (EBITDA).
Last year Stone reported profit before tax of about €355,000.
"The acquisition of Stone will offer SigmaRoc an operating hub in the Benelux region, from which the group can expand," the UK company said.
Selling shareholders Christophe de Limburg Stirum and Jacques Emsens own 40% and 60% of the business respectively, with Emsens joining the enlarged group as a non-executive director following completion.
SigmaRoc will post a circular to shareholders about a general meeting to vote on a resolution to increase the board’s general authority to issue new ordinary shares to fund the deal and other acquisition opportunities.
This will include exercising an option to acquire the remaining 60% of Wales-based quarrying group, GDH, that it does not already own in the fourth quarter of this year.
Since buying a 40% stake in GDH, the business has achieved nearly £1.8mln EBITDA for the first six months of 2019, which is well ahead of its performance for the same period of last year, SigmaRoc said.
GDH is seen as becoming the "cornerstone" of the group's new construction materials hub in South Wales.
“In addition to the board's current deal pipeline, following the appointment of Jacques Emsens, who will bring a wealth of contacts and expertise, SigmaRoc expects to have access to additional acquisition targets and will be well placed to continue its buy-and-build strategy in Northern Europe.”