As I've been looking into roman baths, particularly the large public ones, I've been puzzled by the floor construction. On top of the masonry supports are the base tiles, which seem to be a couple inches thick generally. Above this a slab of concrete several inches thick is poured, and finally the finishing layer is added.
The end result is a fairly massive piece of masonry construction, and it seems unnecessary. A thinner floor would be sufficiently strong and heat the baths more efficiently. So, why did the Romans build the floors as they did?
From an architectural point of view, acting as a buffer through heat accumulation properties of masonry and fuel logistics.
In architecture, there's generally two types of heating insulation 'concepts' used.
One relies on making the external shell of the heated area before the insulation material (like expanded polysteren or mineral wool) as thin as possible. This is commonly used in buildings like lecture halls, cinemas, sports halls which don't have a constant regime of use, with use of for example, prefabricated insulation panels of polyurethane foam in aluminium cladding.
This allows the building's heated spaces to be heated quickly, but lack any kind of accumulator material to store heat. To banalize, if you have an event at 16:00, you start heating the building in 15:45, and if the event ends at 17:00, you stop heating and it's cold again by 17:15.
However, for use in buildings that are used throughout the day and night, like habitation buildings, hospitals and other public access buildings, we get better results using a thicker slab of masonry or concrete wall we then mount insulation on. This allows the masonry wall to act as an accumulator of heat; the extra heat provided by passive sources like sun's energy, internal heating systems can be used to discharge that stored heat back into the space when such systems aren't there (for example, during the night, allowing you to not wake up in the morning in an outside-temperature-cold bedroom)
Why use a thicker accumulation ceiling slab in a Roman hypocaust system, however? Because of fuel logistics.
These days, when we have our fuel brought into our buildings via heat pipes, gas pipes, or electricity, we tend to take it for granted.
But back in ancient Rome, a thin hypocaust ceiling with no heat accumulation property would mean someone would have to stay down there all day, literally throwing new fuel into the fire, constantly. Using an thick accumulation slab, you can instead supply more fuel in longer intervals, and let the heat accumulation work on keeping the temperature running at acceptable temperatures without having to constantly and continuously work the oven.
For the same reason, the hypocaust's furnace uses a 'buffer' as a large water tank for heating up the tepidarium and caldarium pools.