in july 1794, he became the engineer for the montgomeryshire canal, which extended the llanymynech branch of the ellesmere canal from its terminus at llanymynech to newtown. it had been authorised in march 1794, and john was to be assisted by his brother thomas. the route involved crossing the river vyrnwy, where a five-arched aqueduct was built, each with a span of , and crossing the river rhiw at berriew, where two arches spanned the river, and there were two more arches on the land. one of the arches of the vyrnwy aqueduct collapsed, and there were troubles with the rhiw aqueduct. he resigned in july 1796 and emigrated to america. on his brother's suggestion, he was replaced by his father. the committee were critical of the dadfords, but when they called in william jessop to advise them, he suggested that such issues were merely teething problems, and were quite normal on such a project.