The tiles of SU5801 clearly were placed here for purposes of drainage. An opus signinum lining extends up the sides of the channel to allow for water-proofing. To the south the drain extends beneath the floors of room 22, where it is covered in places by tufo slabs. The channel appears to originate somewhere to the north, but this remains uncertain, as the backfill and accumulation in the north remains unexcavated beneath stone features jutting out from walls 5615 and 5789. Another possibility is an association with the E-W channel along the north wall of room 21.