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Tapeatscreek posted on r/handyman18h
A fan box is made that straddles the joist. https://www.amazon.com/Hubbell-Raco-7120-Ceiling-Saddle-Depth/dp/B07DMS2LY1/ref=asc_df_B07DMS2LY1?tag=bngsmtphsnus-20&linkCode=df0&hvadid=80676876082211&hvnetw=s&hvqmt=e&hvbmt=be&hvdev=m&hvlocint=&hvlocphy=97140&hvtargid=pla-4584276357150727&psc=1&msclkid=03c8637736df1b1cb49509348c973ec5