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it must be open to interpretation to give the AO lee way on the insulation. as long as it is deemed to be stout application. if it heavy enough angle iron it will spreed the load across the floor...

when i put my E cat safe in back in about 1997 the Tauranga AO was adamant that 6mm was used on the base plate being 6mm mild steel and of coarse larger then the area of the safe. interesting i see...

You maybe able to coach screw directly into floor joist, but the only person qualified to answer your question is your local arms officer.