If the replacement or repair of the deck would not materially increase the value or substantially prolong the useful life of the property, the cost can be expensed. Otherwise, it must be capitalized and depreciated. 

An "improvement" must be depreciated.  A "repair" can be expensed.  The difference is that a repair maintains the property in as-was condition.  An improvement, also referred to as a "betterment", improves the value of the property and/or extends the useful life of the property or one of its subsystems.  (Hence, replacing a 50 year old furnace is an improvement because it extends the useful life of the HVAC system even though it might not get you a better sales price.)

To me, "replacing" a structurally unsound balcony sounds like an improvement and not a repair.  But you are the person who will have to prove their case to the IRS if audited. 

The Coop Association is requiring specific owners to replace/repair our balconies (built in the 50's) and pay the entire cost.

REPAIR - Those expenses incurred to return the property or it's assets to the same usable condition they were in, prior to the event that caused the property or asset to be unusable.

PROPERTY IMPROVEMENT -

Expenses you incur that add value to the property. Expenses for this are entered in the Assets/Depreciation section and depreciated over time. Property improvements can be done at any time after your initial purchase of the property. It does not matter if it was your residence or a rental at the time of the improvement. It still adds value to the property.

To be classified as a property improvement, two criteria must be met:

1) The improvement must become "a material part of" the property. For example, remodeling the bathroom, new cabinets or appliances in the kitchen. New carpet. Replacing that old Central Air unit.

2) The improvement must add "real" value to the property. In other words, when  the property is appraised by a qualified, certified, licensed property appraiser, he will appraise it at a higher value, than he would have without the improvements.

Now, if there was no "event" that caused the balcony to be unusable, (such as hurricane, tornado, fire, or something of an "immediate damage" cause), then this is not a repair. Considering the balcony is 50 years old, there's no question that replacing the balcony will increase the value of the property as a whole.  The balcony has become unsafe over time, and "time" is not an event that caused something usable yesterday, to become unsafe and therefore unusable today.

This particular property improvement would be classified as residential rental real estate and depreciated over 27.5 years with depreciation starting on the date the project is completed and the balcony can actually be used for its "intended purpose".

@golomb-andrew It is a fairly safe bet to state that the IRS would be expecting the balcony replacement to be capitalized, assuming the replacement falls outside the de minimis safe harbor, considering the virtual definition of an improvement that is found in the treasury regulation at the link below. 

https://www.law.cornell.edu/cfr/text/26/1.263(a)-3#d