So, what's the difference between ARDEX K 40 HB and ARDEX K 40 Flow? ARDEX K 40 Flow is idea for levelling and smoothing floors up to 20mm in thickness, but for thicknesses from 10mm up to 40mm, it's more economical to use ARDEX K 40 HB. So, if it's up to 10mm, go with the flow, and for thicknesses up to 40mm, use the fibre reinforced ARDEX K 40 HB.