Paul gives some good advice in his reply. Basically, unless this level (or stratum) is shown on the strata plan, then it is common property. By default any area within the boundaries of your strata plan not shown as part of a lot is common property.
At some time in the past exclusive use of part of the common property could have been granted, but to take effect this would have required a special resolution at a general meeting, followed by registration of a bylaw. If a bylaw for the roof space had been registered in the past you can readily request a search of your strata records at the Land and Property Information office.
Apart from the Strata issues mentioned by Paul, you could also run into trouble from the council if you modify the roof space for habitation without obtaining development consent.
DaveB