Look at the original plans. Thick black lines define the boundaries of common property.
Otherwise, take a decision at EC level not to repair the deck and let the owner prove that its common property rather than you having to do anything.
I recall a case where a unit owner took the Owners Corp to court to prove that an added glass enclosure on their balcony, which had become dangerous, was common property and had to be maintained by the strata scheme.
Once the decision was made in the owners favour, the Owners Corp asked the judge to confirm that the glass enclosure was theirs and they could deal with its problems as they saw best. The judge confirmed this so they simply had it removed.