Colloseum Arch of Titus theator of Marcellus emperors palace
Depends on which buildings you're referring to.
too build alot of buildings.
Cement.
The Romans used the arch to build large buildings and bridges.
The Romans built the piping for the water systems that was waterproof and leakproof; they also built the roofs of their buildings of waterproof tiles.
Aqueducts, roads and some buildings were all things built by the Romans that are still around today. Some aqueducts can and are still used, while many European highways follow the ancient Roman roads. The Colosseum, Pantheon and may of the tombs along the Via Appia are buildings still standing.
The Romans made extensive use of the arch to build large buildings, bridges, and water aqueducts.
To build underwater foundations for the docks of some ports the Romans used concrete.
Usually concrete. Romans invented concrete.
We do not always build differently than the Romans. There are places on the planet that still build houses and small shops with four walls a tile roof and a floor. The identical way the Romans built their small structures. In our domestic buildings we differ from the Romans in our choices of materials and the safety of those materials concerning fireproofing, allergens and personal safety. In our public or industrial buildings we use steel beams, basements and subbasements and reinforced concrete. Power machinery is used in all of our construction work while the Romans used manpower.
The Romans. They built them to carry water to the cities from the mountains, plus they had a complete water system to provide water to fountains and in some cases homes. The Romans also invented the arch and concrete which allowed them to build the aqueducts, buildings, bridges, and the colosseum.
Builders build buildings. Carpenters build things.