I am having a new extension built with a flat roof measuring 3m x 5m including a sky lantern 1.5m x 3m, so very little roof surrounding the lantern. It is a cold flat roof, therefore the void which is 60mm deep needs ventilation. We have been recommended to use 4 x Oft1f 166 mushrooms by the company advisor. Not only are these enormous and will be obstructive to access if needed, I think it is somewhat overkill. What alternatives do I have to convince Building Control, will be grateful for any suggestions.