Can I deduct the cost of a new roof on my detached garage?

If you are referring to your personal residence, home improvements such as your new roof would not be deductible unless they are qualified energy efficient

If you make improvements which increase your home’s market value and/or extend its useful life, you can add to the cost basis of your home, which may reduce the capital gain when you sell your property.

In addition, you may be able to deduct the sales tax paid for building materials for a major improvement to the home if you itemize deductions and choose the sales tax deduction over state income taxes paid.

See Home Improvements And Your Taxes