Can I ask  one question - why it is required, I mean how this question came up.

Depending up on the construction and purpose of the ship it can be greater or lesser.

Ex : Helicoper landing area on main deck is constucted with additional strengthening materials below the deck as compared to other deck area. So that landing area have more load density.

similarly If any ship is desined to carry cargo on main deck, those ships  have more load density on main deck than forecastle deck.