The construction technique of trulli is an evolution of that still used in Apulia to build stone walls. The compact Apulian limestone, which can also be heated up to produce quicklime, is roughly cut into shape with a hammer and then cast in several layers of interlocking dry masonry (opus incertum). Two parallel courses of dry masonry constitute the two faces of the wall, and the space between them is filled with smaller stones. Walls are tapered, becoming thinner as they grow, for an optimal discharge of the static forces.
A mastro paretaro building a section of dry masonry wall
To build a trullo, the mastro trullaro would first identify the right place: a slightly lifted area with a rocky bottom. Trulli had no foundations: the walls and pavement would rest directly on the bedrock, which in many places spontaneusly surfaces amidst the clay-rich, red soil of the Apulian countryside. Then he typically carved a cistern in the bedrock, which would later be used to collect rainwater. With the stones extracted while carving the cistern, together with other stone material gathered in the immediate surroundings, he would then proceed to rise the walls. The earliest trulli only had one circular room, while more modern examples are true beehive architectures, with several square rooms connected by semicircular arches.
The roof was built by gently rounding up the square walls, and then laying smaller and smaller concentric circles of stones to create a cone, sealed on its top by a large slab. Next, an outer cone was built on top of it with overlapping rings of of thin, flat stone slabs (chiancarelle), always slightly bent outwards, to guarantee an effective drainage of rainwater.
Each cone was finally topped off with a plastered hat, typically culminating with a pinnacle also made of limestone, or of sandstone. The symbolism of the pinnacles is partly lost, but some shapes have been traced back to pre-classical cults.
An interior view of a traditional Apulian trullo
Trulli were finished off with a lime-based plaster, which unlike concrete-based plasters allowed the interior to "breathe", contribuited to thermal stability, and created a favorable hygroscopic equilibrium.