nothing can be deducted until the whole roof is placed into service. .   however if separate structures are involved - say a the building and a detached garage, if one was finished before the end of 2019, then the cost allocable to that is depreciable.   

from IRS pub 946 on depreciation

Placed in Service
You place property in service when it is ready and available for a specific use, whether in a business activity, an income-producing activity, a tax-exempt activity, or a personal activity. Even if you are not using the property, it is
in service when it is ready and available for its specific
use.
Example 1. Donald Steep bought a machine for his
business. The machine was delivered last year. However,
it was not installed and operational until this year. It is considered placed in service this year. If the machine had
been ready and available for use when it was delivered, it
would be considered placed in service last year even if it
was not actually used until this year.
Example 2. On April 6, Sue Thorn bought a house to
use as residential rental property. She made several repairs and had it ready for rent on July 5. At that time, she
began to advertise it for rent in the local newspaper. The
house is considered placed in service in July when it was
ready and available for rent. She can begin to depreciate
it in July.
Example 3. James Elm is a building contractor who
specializes in constructing office buildings. He bought a
truck last year that had to be modified to lift materials to
second-story levels. The installation of the lifting equipment was completed and James accepted delivery of the
modified truck on January 10 of this year. The truck was
placed in service on January 10, the date it was ready and
available to perform the function for which it was bought.