Group Executive Board

Jan Karel Mak (1957)
CEO Deerns Group  

After completing his graduate studies in Environmental Sciences at Wageningen University and the Harvard School of Public Health, Jan Karel Mak worked for seven years as a consulting engineer, project manager and business leader at DHV consulting engineers. He then spent four years as a management consultant at McKinsey & Company, followed by four years as managing director of Arcadis’ Dutch consulting engineering arm (‘Heidemij Advies’ at the time).

He joined Deerns Group as a managing director in 1999, shortly thereafter became a shareholder, and has been serving as Chief Executive Officer since 2000.

Jan Karel has served as a president and member of numerous Boards of professional, educational and sports organisations and advisory committees such as the Dutch Association of Environmental Professionals (VVM), the European Network of Environmental Professionals (ENEP, Brussels, which he cofounded and led for 10 years), the Open University in the Netherlands, the University of Antwerp (Belgium) and the Royal Dutch Rowing Federation. He is currently the president of the Wageningen University Fund and an advisory Board member of several firms.


Roel Rooskens
CFO Deerns Group

Roel Rooskens is CFO of the Deerns Group and member of the Group Executive Board since January 2012. Prior to joining Deerns in 2011, Roel has worked in various financial roles within the Telecom and IT industry and lastly as a Group Controller for the Insulation & Rooflights division of a large building materials company, CRH.

He holds a degree in Economics from Tilburg University, The Netherlands. In 2004 Roel completed the RC Post Master Controllers Programme at the Nyenrode University, The Netherlands.



Sjoerd Hora Siccama (1954)
Member Group Executive Board
Deerns Group

After completing his graduate studies in Electrical Engineering at the Delft University of Technology, Sjoerd Hora Siccama started working for Deerns in 1983 as a design engineer. Within several years Sjoerd took on the roles of a consulting engineer, project manager and later business leader of the electro technical design department. He joined the Deerns Board of Directors in 1992 and became shareholder and Managing Director of Deerns the Netherlands in 1995. From 2000 onwards Sjoerd worked closely together with Jan Karel Mak to develop Deerns’ international strategy.

Sjoerd has served as a president and member of numerous Boards of professional and educational organisations. Sjoerd was chairman of the Dutch Consulting Engineers Organization (today NL Ingenieurs) for the domain of the building installations and IT and was chairman of various school boards. Currently, he is member of the Board of the Koninklijke Haagse Woningvereniging van 1854, a developer of social housing in The Hague, and he is chairman of the PitZtop Foundation, an initiative with the aim to integrate refugees into the Dutch society. Following from Sjoerd’s membership of the Arbitrators Board of the Royal Netherlands Society of Engineers (KIVI), today he is arbitrator for the Stichting Raad van Arbitrage voor Metaalnijverheid en –Handel, an independent body for the settlement of disputes in the metal and electro technical industry.  



Tjerk van der Meer (1975)
Member Group Executive Board
Deerns Group

Tjerk van der Meer is member of the Group Executive Board Deerns Group since June 2019.

Before joining Deerns, Tjerk was globally responsible for commerce and strategy in the Manufacturing & Technology sectors at Arcadis. Prior to that he occupied various international management positions at, among others, design and engineering firm Royal HaskoningDHV.