Adrian Goldsworthy provides a good description of a Roman shield, called a scutum, in his book The Complete Roman Army on page 129. A well-preserved shield was found at Dura Europus that dates from the 3rd century.
The shield is 3’ 3” tall by 2’ 8” wide in a curved shape.
It is two inches thick, consisting of three layers of wood glued together.
Update: More images added on 4/7/19.
The center layer is vertical and the two outside layers are perpendicular. On the inside there are strips of wood attached for extra strength. There is a thin layer of leather on the inside and outside. The edge has a wide leather binding and there are leather reinforcements on the corners.
There is an opening in the center of the shield for a boss (a hand grip), but one was apparently not attached. Instead there is a horizontal handgrip. I’m guessing that means there was a bar across the shield.
A reconstruction of this shield with a boss weighed in at 12 pounds.
The leather covering and wood core would have provided great protection against arrows, including flaming arrows.
Seems like the 2 inches of laminated wood have done a good job protecting from spears. Of course, if the spear penetrated you would likely have to drop the shield. If the spear stuck you would have to take precious seconds knocking it loose.
Update 4/9/19: Wikipedia reports Roman soldiers carried an oval shield, called a clipeus, until sometime in the 4th century BC. The clipeus was similar to what the Greek hoplites carried. By the start of the 4th century an oval shield was in use.
Article says a scutum weighed in at 22 pounds.
Wikipedia quotes Polybius as saying the scutum measured 2 1/2 feet wide and 4 feet tall, with iron reinforcement on the top and bottom edges.
Update 4/7/19: The Roman soldiers each carried a pilum, or spear, which was thrown en-mass immediately before advancing on foot. If the pilum got a good hit on a shield, the bearer would have to drop the shield, losing most of his protection moments before the Romans arrived.
On a day-to-day basis, a scutum would be carried in a leather bag to provide protection to the shield.
Other posts on weaponry of a Roman Legionnaire:
- Armor – Breastplate
- Pilum – javelin