You can hear it when Blind Boy Paxton’s thumb bangs his guitar’s bass strings in “Railroad Bill.” And it’s there in the ghostly warble of Michael Hurley’s voice on “Light Green Fellow.” Crooked rhythms and eerie inflections often mark the most memorable stuff when it comes to roots music, and the Brooklyn Folk Festival is expert at brokering such sounds. The action gets more diverse with each edition; this weekend’s bash is built on variety. Blues, rags and hollers; gypsy, jug band and flamenco; mountain music from both North Carolina and Peru. Plus legends John Cohen and Ed Sanders mixed in with keepers-of-the-flame Clifton Hicks and Rayna Gellert. And even if you don’t know the difference between frailing and clawhammering, you’ll need to check out the annual banjo toss: the high lonesome splash of the Gowanus Canal should probably be considered some kind of roots music, too.