Make a roof for the ark, and finish it to a cubit above...
Genesis 6:16 ESV

A window or a roof? BDB says the word literally means “noon”, but might refer to a roof in the sense of something straight up or sunward. Strong’s says it means a light rather than noon, and so refers to a window.

What is a cubit finished above? Nobody knows what this means either. It could be a covering over the peak to allow openings for ventilation without allowing rain to enter. It could also be a soffit extending a cubit beyond the sides or the height of the rafters or the height of the roof peak.