At what point does a Repair become an Improvement?

[Chris T.]

After just going through my 2013 taxes and now looking ahead to 2014 I have a question about repair vs improvement in the eyes of the IRS. At what point does a repair become an improvement?

I know that if you repair a roof, that is a repair and can be deducted in a single year. If you replace the entire roof that is an improvement and must be depreciated. Is there any rule of thumb as to where that break is? If over half the roof is replaced in a "repair", does that still count as a repair? What about 25%, 75%? Is there a dollar amount for a repair that might start raising a red flag?

My specific situation is with the wood siding on one of my properties. It was neglected for many years and will be in need of major repair. There was a newer addition to the house that is Hardy board and does not need to be replaced. If an entire side of a house is replaced is that still considered a repair? What if it is the bottom 50%, 75% of the boards that need to be replace?