Where do I enter repairs/maintenance for my rental property?

In the rental expenses section, there are boxes clearly labeled Repairs and another for Maintenance expenses.

The new heater, depending on it's type and cost, would probably be an asset that gets depreciated over time.

For the roof, if an entirely new roof was put on that would be an asset that gets depreciated also. Otherwise, if less than 25% of the roof was fixed, that would be a repair.

You enter rental expenses for maintenance and repairs, as well as setting up new assets in the "Rental Properties and Royalties" section.  To navigate there, you can either go to Wages & Income, and then select Rental Properties and Royalties.  Or, you can click on the "Search" icon and type in "Rental Expenses", and hit enter.  Next, click on "Jump to rental expenses".  Of course, if  you haven't already, you will have to set up your rental property prior to entering any expenses.