A repair restores the property to as-is or as-was condition. An improvement adds value to the property or extends the useful life of the property or system. Consider if a tree limb falls and puts a hole in your roof. If you patch the hole, it's a repair. If you take the opportunity to replace the whole roof (extending the functional lifespan of the roofing system) then it is an improvement, despite the fact that the original need might have been a repair. The same logic would apply to a fence, hot water heater, or other property system.
Do you have photos? Can you show that the scope of the work is different for different units? It seems unlikely that exactly 1/3 of the the cost would be a repair, if "repair" is a fair assessment of the work. Without a more thorough description, it sounds to me like the entire job was an improvement, and you are really just looking for an excuse to call it a repair. What matters is what you can prove to the IRS. No one here will stand behind you if you are audited. If you want a formal opinion, pay an accountant who will defend you if you are audited.
