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5.4 Drafting Over / Under Roster Limit
An owner is allowed to acquire more draft picks than the roster will allow. The owner is allowed to draft all draft picks in his/her possession, even if it exceeds the roster the limit. Any owner with more than the roster limit (16 players) after the draft must "drop down to the roster limit (16)" seven (7) days after the FF draft is complete or three (3) days before the first Waiver date, whichever is first. If the draft is not complete by this 'computed date' then the "drop down to roster limit date" will be determined by the League Commish.
Those owners that do not fill out their complete roster during the draft are allowed to “ramp up” during the first league-wide waiver (blind bid) and FA (FCFS) processes.
So you have a week to trim your roster after the draft is complete.