If you break them up They act as drainage.
Also good for security as walking on them can be heard thus alerting you of visitors around your house.
If they are still intact and in good condition you could give them to a builder who might use them or keep them yourself in case some of yours fall off.
Use them as edgings along paths, also to separate herbs in a garden bed. Slate is impervious to most things.
Drill two holes near the corner on each edge of one and on three edges of four more. (Make sure you drill at a low speed else they’re likely to crack).
Wire the five pieces together (the one drilled on 4 sides on the botom) to make planters.
The rust from the wire he’d used looked quite pretty, rather surprisingly, and the gaps around the edges of the tiles made for excellent drainage.
If you’re adept with a tile cutter or any other way of cutting slate you could make any shape planter you liked.
You could cut them up into small rectangles and use them as miniture roof tiles on a reproduction Victorian dolls house and give it to a small girl as a unique gift.
In New Orleans, old roof slates have New Orleans scenes hand painted on them. Think you could probably use a stencil on them, drill a hole for cord to hang.
I have seen them used as a border to flower beds they looked pretty cool.
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One of my fave gifts was receiving a slate tile with rawhide lacing for hanging. Mine has snowman and “merry Christmas” painted on it - you could paint them for any occasion, or decoupage ( and use ribbon instead of rawhide to make it less rustic). They also might make cute mini- blackboard, or cheese board.