Rashi on Numbers 19:16/רש"י על במדבר י״ט:ט״ז

1 על פני השדה [AND EVERYTHING WHICH TOUCHES] ON THE OPEN FIELD — Our Rabbis explained that this is intended to include the upper board and the side board of a coffin (which were on the surface of the field — that these, too, render a person unclean) (Chullin 72a). The literal meaning of the words על פני השדה is, however, that even where there is no אהל, covering, there too, the dead body may cause uncleanness, but only by contact with it.