What is the typical thickness of insulation of fibre glass in flat roof?
The thickness of fibreglass insulation that can be put in a flat roof is governed by the roof timber size so if they are 6″x2″ then the max that could have been put in is obviously 6″. But maybe perhaps only 4″ was put in. Nowadays a gap would have to be left above the insulation to allow for ventilation so in the case of 6×2’s if using fibreglass a max. of 4″ would apply. Because that would not give, by todays standards, adequate insulation, fibreglass wouldn’t likely be used. Modern insulation boards that may be only 2-3″ thick give the equivalent maybe of 6-10″ of fibreglass, and still leave the required gap for ventilation.