The word plumber dates from the Roman Empire. In Roman times, some roofs were made of lead, or plumbum in Latin (hence the periodic table of the elements symbol of 'Pb' for lead). Lead roofs were waterproof, and the workers on such roofs were what are now called "plumbers".
Roman baths later used lead for piping and for the main baths. Thus, a person with expertise in working with lead was known as a Plumbarius, eventually shortened to plumber.
From: 'Wikipedia: Plumber'
The ordinary of this society, anciently consisting of Goldsmiths, Plumbers, Glaziers, Pewterers, and Painters, and dated September 1, 1536, enjoined them to go together on the feast of Corpus Christi, and maintain their play of "The Three Kings of Coleyn;" to have four wardens, one Goldsmith, one Plumber, one Glazier, and one Pewterer or Painter; to be sworn on admission not to interfere with each other's occupation; that no Scotsman born should be taken apprentice, or suffered to work in Newcastle, on pain of forfeiting 3s. 4d. one half of which to go to the upholding of Tyne Bridge, and the other to the society. Among other orders in the old books of the society, the following occurs: "September 7, 1730, no brother to lend his diamond, except to a free brother of this company, on pain of forfeiting 6s. 8d."
This society hold their meetings in Morden Tower. A gilded ball, suspended from the centre of this meeting-room, probably had been shot from the cannon of the Scottish army during the great siege of the town in 1644 and, having lodged in the wall, was discovered on the alteration of the tower. The outside of the adjoining wall bears marks of this memorable siege.
G. M. Jobling
5 Higham Dykes
Newcastle upon Tyne