Whenever it shall appear to the commissioners that any public roads or driftways set out by the valuer in the matter of any inclosure are of such a nature or so situate that it is not necessary, for public convenience, that the same should be fenced, or made of hard materials, and certified by two justices, the commissioners may, by any order under their hands and seal, certify that the said roads or driftways are sufficiently formed, having reference to the nature or situation thereof; and such roads shall thenceforth be kept in repair by such persons and in such manner as like public roads within the said parish are or ought by law to be kept in repair; but such last-mentioned persons shall not by such obligation to repair be liable to convert the said roads so set out into fenced roads, or into roads made of hard materials.