The IRS considers this as an "Improvement" and has a 27.5 year MACRS depreciation period. Be sure to keep this in mind if/when you sell the property. Part of the sale proceeds can be applied to the remaining value of this asset. 

According to the IRS:

"Residential rental property (buildings or structures) and

structural components such as furnaces, waterpipes, venting,

etc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27.5 years "

https://www.irs.gov/pub/irs-pdf/p527.pdf