The 20-year-old arrives at Victoria Road as Daggers midfielder Billy Bingham awaits the results of a scan to a knee injury which forced him off in the first half of Tuesday's 1-1 draw at AFC Wimbledon.
D'Ath has graduated through the Academy ranks with the Royals, making one substitute first-team appearance, while he has enjoyed loan spells with the likes of Yeovil, Cheltenham and Exeter in recent seasons.
He will remain with Dagenham until December 29 but faces a late fitness test ahead of Saturday's home clash against Wycombe due to an ankle knock.
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