Section 11(3)(iii) in The Merchant Shipping (Equivalent Arrangements For Carriage Of Grain) Order, 1977
(iii)Within filled hatchways and in addition to any open void within the hatch cover there is a void of average depth of 150 mm measured down to the grain surface from the lowest part of the hatch cover or the top of the hatchside coaming, whichever is the lower.