On Sat, 2 Sep 2017 09:40:25 +0100, Andy Burns
wrote:

When I moved into this house in 1990, I found in the loft two
interlocking concrete tiles, and thought what a good idea for the
builders to leave a couple of spares.


At my last house, left 19 years ago, there were spare tiles, and ridge
tiles, in the garage. The window cleaner, ex builder, who installed
them to repair storm damage just before I left said that on some
contracts they would be told to leave spares.