Shields are not easy to penetrate, of course not, that is why they were used (and still are by the police). I hope I didn't give the impression that they were. Shields worked! Period.
But that doesn't mean that they were able to withstand everything or even a whole lot. Shields were broken, penetrated and generally lost in battles often enough, and they were then replace afterwards. If they protected a man for a single battle, then it did it's job as it should. Afterwards the man could get a new one.
The shields used against the Parthians seems to have been the lighter imperial version, or a step in that direction. They were a bit smaller (cut-off ends) and two layers of wood rather than three.
If subjected to continual archery it seems to me that at some point an arrow would hit a junction of the strips (the shields were made of little strips glued together) on both layers. That would make a penetrative hit possible if not likely. And of course there were always weakspots and little flaws and weaknesses in the wood ect ect... I'm not surprised that the Romans would experience arrows going through their shields in cases where arrows rained on them for hours. But interestingly it isn't a feature the Romans seem to fear much later on, and I haev to personally assume it wasn't anything that happened regularly, or when it did, it wasn't dangerous to the man behind the shield.
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