Advice needed for fibreglass flat roof please 😊

[Samg235]

Hi, my husband has just put a fibreglass flat roof down and noticed that the OSB boards are softer than the ones he usually uses, merchants changed suppliers. It's all finished with top coat etc but when he went back to do some more work he noticed that the roof had dipped a bit. He was wondering if he put another layer of fibreglass on top would it help to strengthen the roof? Grateful for any advice please. Thank you 😊 

[Mr Punter]

18mm OSB should not sag if correctly installed on 400 centre joists / firrings.  If the boards are defective, they need replacing.  The grp is there as a waterproofing layer, not to make up for dodgy OSB.