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 translation of Pliny's work "Pliny the Elder, The Natural History" by John Bostock in Book XVI Chapter 77 METHODS OF OBTAINING FIRE FROM WOOD (http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0137%3Abook%3D16%3Achapter%3D77)

"This is a method1 which has been employed by the outposts of armies, and by shepherds, on occasions when there has not been a stone at hand to strike fire with. Two pieces of wood are rubbed briskly together, and the friction soon sets them on fire; which is caught on dry and inflammable substances, funguses and leaves being found to ignite the most readily. There is nothing superior to the wood of the ivy for rubbing against, or to that of the laurel for rubbing with. A species of wild vine,2 too—not the same as the labrusca—which climbs up other trees like the ivy, is highly approved of. "
1 The savages of North America, and, indeed, of all parts of the globe, seem to have been acquainted with this method of kindling fire from the very earliest times
.2 See B. xxiv. c. 49. The Viticella, belonging to the genus clematis.
Conclusion:
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.