Pilae Stacks are stacks of pilae tiles, square thin tiles, that were used in Roman times as an element of the underfloor heating

Underfloor heating

Underfloor heating and cooling is a form of central heating and cooling which achieves indoor climate control for thermal comfort using conduction, radiation and convection...

 system, common in Roman bathhouses

Thermae

In ancient Rome, thermae and balnea were facilities for bathing...

, called the hypocaust

Hypocaust

A hypocaust was an ancient Roman system of underfloor heating, used to heat houses with hot air. The word derives from the Ancient Greek hypo meaning "under" and caust-, meaning "burnt"...

. The concept of the Pilae stacks is that the floor is constructed at an elevated position, allowing air to freely circulate underneath and up, through the hollow bricks, into the structure walls. Examples of such baths are found not only in Rome

Rome

Rome is the capital of Italy and the country's largest and most populated city and comune, with over 2.7 million residents in . The city is located in the central-western portion of the Italian Peninsula, on the Tiber River within the Lazio region of Italy.Rome's history spans two and a half...

, but also in Roman Britain

Roman Britain

Roman Britain was the part of the island of Great Britain controlled by the Roman Empire from AD 43 until ca. AD 410.The Romans referred to the imperial province as Britannia, which eventually comprised all of the island of Great Britain south of the fluid frontier with Caledonia...

 and distant parts of the Roman Empire such as the baths at Chellah

Chellah

Chellah, or Sala Colonia is a necropolis and complex of ancient Roman Mauretania Tingitana and medieval ruins at Rabat, Morocco. It is the most ancient human settlement on the mouth of the Bou Regreg River.-History:...

, in modern day Morocco

Morocco

Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...

.

This architectural technique was the first form of underfloor heating and the same principle is still used today.