BSS will pay an additional amount of up to £3.7 million over the next three years.
Birchwood supplies safety workwear, lighting, heating and cooling products to over 6,000 distributors. BSS said the purchase will boost its growing specialist division.
Birchwood's senior management team including Anthony Smith, Paul Swanwick and Paul Simpson will remain with the business.
Turnover at the Nottingham-based firm reached £22.3 million in the year ended 31 October 2007 with an operating profit of £1.8 million.
BSS chief executive Gavin Slark said: “Birchwood is a high quality business that significantly enhances our product offering within the group's specialist division.
“This business has substantial growth prospects and we see good opportunities for synergy benefits across the Group. The senior management of Birchwood have committed to BSS to enable us to realise that potential.”