1) It has more than one meaning and in some late examples was a 'panel', a square, timber framework filled in with bricks or plaster (OED). More usually it was the horizontal timber along the top of a wall which received the ends of the rafters, although the evidence is not always explicit.
1284 ‘one pannepece of oak, 40ft long’, Scarborough
1420 in hys tenement in Coppergate ... walles ... fra the grunde uppe to the panne, York
1501 the sparrez & tymbre … is shot & hyngeth over the ground … by viij ynchez and more anenst the pan of his house, York
1519 stayd with proppes haldyng up the pannes, Bishop Burton
1576 for a dogg of yron nayled on the joynynge of two pannes in the new house, York
1682 well wrought roof … with pans or wall plates, Scriven
1739 2 pann pieces 22 yards in length, Lofthouse. The term ‘post and pan’ has a long history: 1341 et les pannes et les postes au tizon seront de leaese et espesse solonc le scanteloune fait entre les parties, Brandsby
1617 stronge poasts and pannes, Brandsby
1619-21 an auntient howse buylte upon timber or postes and pan, Pickering.