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bay

In Derbyshire, England, a unit of area (or of work performed), = 500 square feet¹ of slate installed on a roof. Other sources, not specifying a locality, say 405 or 450 square feet (see source 1).

1. James Britten.
Old Country and Farming Words.
English Dialect Society, number 30.
London: Trübner and Co., 1880.

Page 167. Britten's source was Morton's Cyclopedia of Agriculture (1863), which in turn drew from the Second Report of the Commissioners (1820, page 8).

sources

Square Measure… 45 square yards, 1 bay of slating, but, when counter measure is allowed, which is commonly half a yard at the eaves, then 50 yards is 1 bay.

John Wilkes, compiler.
Encyclopedia Londinensis etc., vol. 2.
London: printed for the proprietor by J. Allard, etc. 1810.
Page 166.