Why does Roderick place Madeline in the vault?

Roderick temporarily places Madeline in the vault because she is dead.

At the request of Usher, I personally aided him in the arrangements for the temporary entombment. The body having been encoffined, we two alone bore it to its rest. The vault in which we placed it (and which had been so long unopened that our torches, half smothered in its oppressive atmosphere, gave us little opportunity for investigation) was small, damp, and entirely without means of admission for light; lying, at great depth, immediately beneath that portion of the building in which was my own sleeping apartment.