Should I enter the full cost of the roof on my 2018 taxes or only the portion I paid for in 2018?
When Does Depreciation Begin and End?
You begin to depreciate your property when you place it in service for use in your trade or business or for the production of income. You stop depreciating property either when you have fully recovered your cost or other basis or when you retire it from service, whichever happens first.
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.
Please note: AMT does not allow for accelerated depreciation so be careful. I always match my assets to AMT rules so I don't have timing issues... just in case you are subject to AMT.
So in your case depreciation would probably be 2019 as that's when the asset was completed and in service.
Hope this was helpful.