No weight restriction on headsails, but there is on the main. See extract from class rules below:
5.12.1.3 Sailcloth in the #1, #2 and #3 Genoa is not restricted. This change in approved sailcloth goes into effect no later than October 17, 2005. Individual
J30 Class Districts may adopt this change for local events at an earlier date.
5.12.2 Mainsail
5.12.2.1 Minimum cloth weight is 6.25 oz.