AtD translation: rough-ins overlapping faster and faster

[Mike Jing]

P184.16-24   And there was the attraction known as the Dynamite Lazarus,
where an ordinary-looking workhand in cap and overalls climbed inside a
pine casket painted black, which a crew then solemnly proceeded to stuff
with a shedful of dynamite and attach a piece of vivid orange fuse to that
didn’t look nearly long enough. After they’d nailed down the lid, their
foreman flourished a strike-anywhere match, ignited it dramatically on the
seat of his pants, and lit the fuse, whereupon everybody ran like hell.
Somewhere a drummer began a drumroll that grew louder, rough-ins
overlapping faster and faster as the fuse burned ever shorter—

What are "rough-ins"?