Numerical modelling was required to assess thirteen fume cupboard exhausts during roof refurbishment activities. Thus, different wind conditions and exhaust gases were assessed through a series of CFD simulations. The MEP team were able to improve the design by increasing the stack heights and exhaust gases dilution, reducing any adverse effects at the pedestrian areas. For the assessment it was used a dilution criteria of 3000:1 for flue flow of 1000 cfm, as recommended in “A dilution criterion for laboratory fume hood exhaust stacks”, TECHNOTES, RWDI.