Evolution of the solid-to-gas surface density ratio across the protoplanetary disc for the nominal model (top panel) and the midplane model (bottom panel). The solid component includes dust and pebbles, whereas planetesimals are excluded. The two models produce very similar amounts of planetesimals around the outermost planetary gap-edge, resulting in very similar solid-to-gas surface density ratios in this part of the disc. Around the second planetary gap-edge, the midplane model is not quite as efficient at forming planetesimals as the nominal model, and it leaves a ring of dust and pebbles behind that does not exist in the nominal model. At the innermost planetary gap-edge, there is no planetesimal formation at all in the midplane model, and thus the solid-to-gas surface density ratios in the two models are very different.