Then he saw he was in the wonderful, mysterious place between the ceiling and the roof of the house.

The roof is beams and tiles.

Slits of light show through the tiles here and there.

The ceiling, on its other and top side, is made of rough plaster and beams.
If you walk on the beams it is all right--if you walk on the plaster you go through with your feet.

Oswald found this out later, but some fine instinct now taught the young explorer where he ought to tread and where not.