ROMAN ROAD CONSTRUCTION AND DIMENSIONS
A standard Roman road consists of a metalled surface (eg. gravel or pebbles) on a solid foundation of earth or stone. There is an enduring myth that all Roman roads were paved but examples of paved roads in Britain (eg. Blackstone Edge, the Dean Road) are highly exceptional and in most cases doubts have been cast on whether they are Roman at all.
The Agger is a well-drained base in the form of a bank of earth or other layered material dug out from lateral ditches or quarry pits. It can be up to 6ft (1.8m) high and 50ft (15m) wide or, at the other extreme, very slight or even non-existent with the road surface laid straight on the ground - this is especially true of minor roads. Local materials are used where possible - a layer of large stones may supplement or replace the agger if available.
The road surface itself consists of layers of finer material with a total thickness of between 2-3in (5-7.5cm) and 1-2ft (30-60cm). Additional layers are added by resurfacings. The width of the road is up to 30ft (9m) but more usually around 25ft (7.5m) with minor roads 15-18ft (4.5-5.5m) down to 10-12ft (3-3.5m).
Apart from the scoop ditches, the road might also be flanked by shallow boundary ditches 2-4ft (60cm-1.2m) wide. These may serve to define an official "road zone" especially in areas where the surrounding terrain (eg. woodland) offers the possibility of ambush. The distance between these ditches would seem to indicate two classes of road - 84ft (25.5m) and 62ft (19m). The road in Sutton Park, Birmingham, featured on our "Take a Walk along a Roman Road" page, fits into the latter class (other measurements - agger up to 3ft (1m) high and 30ft (9m) wide).
Where a road passed over unstable ground it might be supported on a wooden structure - at the marshy north end of Sutton Park this consists of a raft of gorse and brushwood.
Additional metalling is sometimes found in or beyond the side ditches. It may also occur on the "berm" between the principal metalling and side ditches of roads with very wide aggers. This may indicate alternative arrangements for different modes of transport or a slapdash response to wear-and-tear on the main carriageway.