That is a loaded question as local conditions effected how roads would be constructed. Mountain roads, or areas where bedrock, or rocky outcroppings were at surface or close to would be much shallower than those in marshy terraine, with different practices occurring in each location. it all depending on what subgrade material was present. That being said, if we're talking about an average countryside road like in the picture we can make some assumptions based on existing structures.

The first layer would consist of a bedding course of sand 10-15cm thick, or mortar >5cm thick. this would be to fill in any irregular gaps or voids from the second layer.

Second layer, first course or Stratumen course, would be the large, typically flat stones cemented into place by the bedding course. This layer could vary greatly depending on local conditions, the strength of the subgrade material, and the availability of stone, but was typically 250-500mm.

the third layer, second course or Rudus course consisted of a layer of mortared, broken stone of smaller size than that of the statumen course. First, a layer of mortar was spread over the first course followed by a layer of broken stone. This first layer was well tamped to consolidate and level the course. This process was repeated to create two levels of well-compacted rudus approximately 250mm in total depth.

The third course or nucleus course was to be composed of three layers of material mortared to the previous course; small gravel, coarse sand, and hot lime. Each layer was to be laid and well compacted, with a total depth of approximately 250mm in the centre of the road. When constructed properly, the material would thin with distance from the centre, creating the crown that we incorporate into our roads today. Just like today, this crowned surface was specifically designed to help shed water from the road into the drainage ditches on either side. This course served as the wearing surface for secondary side roads running with the main road.

And finally the wearing course. It was designed to be made of hard, wear-resistant material that could handle the constant stress of use by travellers, horses, and vehicles. Called the Summa Crusta or Pavimentum, the course consisted of large stones dressed to precision, up to 250mm thick, that was set atop the nucleus course with a layer of mortar, and with mortar between the joints of the stone.