new roof

Yes.

The only consequence is the possible additional depreciation for the first and last year of the life of the asset. 

It determines what month the deprecation starts and then stops. 

"The mid-month convention only applies to residential rental property, nonresidential real property, and railroad grading or tunnel bore. It simply means that you get a half month's worth of depreciation no matter when that asset was placed into (or taken from) service during that month, whether that was at the beginning, middle, or end of the month.

The half-year convention works the same way, but instead of the month it goes by the year. In other words, you'll get 6 months' depreciation if the asset was placed into service or disposed of during the year, no matter if it was in January or December.

Finally, there's the mid-quarter convention, which can be used for assets that the mid-month convention doesn't apply to, if the total depreciable basis of assets subject to MACRS placed into service during October, November, and December amounts to over 40% of the depreciable basis of all assets (other than residential rental property, nonresidential real property, railroad grading, tunnel bore, and assets that were both placed into service and disposed of during the same year) subject to MACRS that were placed into service during that year. This convention gives you a half-quarter (1.5 months' worth of depreciation) for the quarter in which the asset was either placed into service or disposed of."

More information 

I assume this new roof is on rental property or some other type of business property you are already depreciating. You really don't have a choice. A new roof is a property improvement and you are required to depreciate it. You can not expense it. If you don't depreciate it, then in the tax year you sell or otherwise dispose of the property you are still required to recapture the depreciation you should have taken, and pay taxes on it.