1) The verb meant to cover (a roof) with wooden boards (OED).
1458 pro sarracione … de les sarkyng bordes, York
1538 the sarking underneath if it might be seen is sore chauffyd and will have much new timber, Knaresborough
1582 In laying the gutters and in sarkinge about the chime, York
1596 one hundred of sixpennye nayles for naylinge down lead & sarking bords on the roufe of the churche, Howden. Note: 1464-5 Pro Ml sarkyngnale, 5s, Durham, presumably the nails used to secure sarking boards on a roof.
2) A garment such as a coat, or one worn next to the skin, a shirt or chemise.
1472 for ij sarkkes mendeng ijd, York
1534 a white cotte and an new sarke, Liversedge
1552 two pare of hoose and one sark, Huddersfield. Occasionally it was a coat of mail: 1515 my sark of mayll and a battell axe, Hunmanby. It is found as an element in early by-names: 1301 Henry Whitserk, Carleton juxta Snaith
1332 William Whiteserk, Thornes near Wakefield.