Floor and Ceiling tile
A floor (and by extension ceiling) is a vital component of a house. They can be built via the Make menu, under Buildings and Constructions.
Floors take 2 slender tree trunks and 6 boards to complete, both of which should be on a tile within 1 square of the player. Once built, the square that they are on functions as an interior, allowing a player to sleep inside during rain.
Building a fire on a floor tile without a fireplace will destroy it.