The shindenzukuri style of house consisted of moya and hisahi rooms. The moya was where people slept and the hisashi was where waking hours were spent. This was a wide room. Among the aristocracy, important formal functions took place in the hisashi on the southern side of the house and the hisashi room on the northern side was used for everyday life. These became gradually bigger.
Finally the interior of a room was divided by walls and fusuma (sliding paper-covered doors). Instead of shitomido, a latticework door with a board attached to the back which protected the room from sunlight and rain, hikido (sliding doors) and akarishoji (sliding doors covered in thin paper) were used. Now light could be let into the room at any time.
In ancient times floors were made of wood and tatami mats were placed on top of them to sit on. Then there was a demand for tatami to fill the whole floor surface in the room instead of having a cold, hard wooden surface. So the size of tatami was fixed and they began to be mass produced. However big a room was, the whole floor surface was now filled with tatami. This kind of room was called a zashiki. This happened at the time of the Ashikaga shogunate.