The new floor and any structure supporting it must have a fire resistance of 30 minutes from the underside. This should extend to the inside face of the external walls. Separating walls between dwellings should have at least 60 minutes fire resistance. Some older properties do not have separating walls within the roof space, or may have holes or gaps in them. These will need to be sealed up to the underside of the roof coverings to maintain the integrity of the separation.