210ltr horizontal cylinder in loft

[finchy01]

Evening all, just after a bit of advice. I'm due to carry out an install in a couple of weeks. Part of this is putting a 210ltr unvented cylinder in the loft. I've advised we need some some extra support. Beams are 600mm apart and the customer has asked if 18mm board is good enough. First thoughts are yes but what are your thoughts? Or is there a direct answer? Thanks.

 

[ShaunCorbs]

Needs to crossspan with 4x2 a min of 8 joists

Then ply over the top as a base