And I also found this (note the final paragraph):
Identifying the unit of property is very important. For items of personal property, the unit of property is determined based on the functional interdependence of the component parts. For real property, the 2008 proposed regulations considered a building to be a unit of property. This interpretation made it possible to treat the replacement of a roof as a deductible repair under certain conditions. The reason it was possible to expense a roof replacement was that it was not considered a major component or substantial part of the unit of property (the building).
In December, 2011 the Treasury issued new temporary regulations which replace the 2008 proposed regulations. These regulations are effective for taxable years beginning on or after January 1, 2012. Although the tests for betterment, restoration and adaptation are still in the regulations, the rules on buildings have been changed. Under the new rules, building systems are considered separate from the building structure. Building systems include the following: HVAC, plumbing, electrical, elevators, escalators, fire protection, security systems and other items to be determined by regulations at a later time.
The new regulations provide several examples dealing with roof repairs and replacements. They conclude that a replacement of a complete roof or a substantial part of the roof must be capitalized as a restoration of the unit of property. One example concludes that the replacement of a roof membrane but not the decking or insulation can be deducted as a repair because the roof membrane is not considered a major component.
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