Pliny's Natural History (Latin: Naturalis Historia) is a book about the whole of the natural world in Latin by Pliny the Elder, a Roman author and naval commander who died in 79 AD. It is one of the largest single works to have survived from the Roman Empire to the modern day and purports to cover all ancient knowledge.
The book gives us insights into the methods of kindling fire during the Roman Empire.
The below is taken from this translation of Pliny's work.
Pyrodes, the Son of Cilix, first obtained Fire from the Flint;
and Prometheus, the Means to preserve it in Ferula (or Fennel)
(On searching more on Pyrodes I've only found mention by Pliny so unsure which myth this comes from!)
From Genus, the son of Protogonus and (Eon, other mortal issue
were begotten, whose names were Light, Fire, and Flame. These found
out the way of generating fire by the rubbing of pieces of wood against
each other, and taught men the use thereof.
(after a bit of googling this seems to be from the THEOLOGY OF THE PHŒNICIANS)
The below extract is taken from this translationof Pliny's work:
The softest of all woods is lime, and it is also apparently the hottest as well: it is adduced in proof of this that it turns the edge of adzes quicker than any other wood. Other hot woods are mulberry, laurel, ivy and all those used for making matches.
LXXVII. This has been discovered by experience in the camps of military scouting parties and of shepherds, because there is not always a stone at hand to strike fire with; consequently two pieces of wood are rubbed together and catch fire owing to the friction, and the fire is caught in a lump of dry tinder, fungus or dead leaves catching most readily. But there is nothing better than ivy wood for rubbing against and laurel wood for rubbing with; one of the wild vines (not the claret-vine), which climbs up a tree like ivy, is also spoken well of. The trees that have the coldest wood of all are all that grow in water; but the most flexible, and consequently the most suitable for making shields, are those in which an incision draws together at once and closes up its own wound, and which consequently is more obstinate in allowing steel to penetrate; this class contains the vine, agnus castus, willow, lime, birch, elder, and both kinds of poplar.
My conclusions are that both friction fire and fire by percussion (striking of flint) were used during the Roman Empire. Again it is interesting to read on the types of wood that Pliny mentions and is similar to what the Ancient Greek Botanist Theoprsatrus wrote hundreds of years earlier. Metal fire strikers were common and there is archeological evidence of these from the Roman Empire.