They did not know the horizontal loom as we do know it. That is an invention of the industrial era, or better to say that its invention was the start of the industrial era.
Mostly the romans used the vertical loom as seen on several reliefs like the ones from the forum transitorium (forum of nerva) in Rome where the competition between Arachne and Athene is shown (See pictures in Blanckenhagen, Flavische Architektur und ihre Dekaration untersucht am Nervaforum, Berlin 1940, Tafel 41). Additionally there are the thousands of loomwheigts that where found all over the roman empire, they do belong to that vertical looms.
But there are speculations about horizontal looms used in northern africa to produce tunicas in one piece. We do not have finds of those looms (they must have been around 2,50m wide) and the archeological evidence is not given by any finds, but Annette Paetz gen. Schiek writes, that in egypt the cloth and tunicas were woven in big frames with two warp beams and that those weaving frames could be arranged vertical and horizontal (horizontal means laying on the floor). Those frames were known since the 15th century a.d. and still prooved for the roman times by letters naming them. The difference between those weaving frames and vertical looms are following:
You can fix the threads more thight and you do strike the filling thread downwards (while the filling thread is stroken upwards to the werp beam). That's all I know of horizontal looms in antiquity and can be read at A. Paetz gen. Schiek, aus Gräbern geborgen. Krefeld 2003, p. 12f) Maybe you can find more in D.L. Carroll, Looms and Textiles of the Copts, Seattle 1988. Unfortunately we don't have this book in our library.
I hope that will anser your question
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