1) A long beam, one of the roof timbers.
1527 ij spare and a wyver xijd, York
1682 one well wrought roof … with wind-bands and wyvers, Scriven. The builder’s contract in this case required him to place his balkes and principle spars soe that the wyvers [should] not beare above ten foot. The said roof to be doubly wyverd, both ends and sides. It is on record much earlier as a verb: 1471-2 ‘For a tree and stowres bought for wivering’, Hull. It is said to be a form of ‘waver’ but that spelling is late.