I do not know if the I-Codes are applicable, as only a few California
jurisdictions are adopting a portion of the codes. So for what it may
be worth, here is the IBC reference that may help.
2512.1 General. Plastering with cement plaster shall not be
less than three coats where applied over metal lath or wire fabric
lath and not less than two coats where applied over masonry,
concrete or gypsum board backing as specified in Section
2510.5. If the plaster surface is to be completely covered by veneer
or other facing material, or is completely concealed by anotherwall,
plaster application need be only two coats, provided
the total thickness is as set forth in ASTM C 926.
2510.5 Backing. Backing or a lath shall provide sufficient rigidity
to permit plaster applications.
2510.5.1 Support of lath. Where lath on vertical surfaces
extends between rafters or other similar projecting members,
solid backing shall be installed to provide support for
lath and attachments.
2510.5.2 Use of gypsum backing board.
2510.5.2.1 Use of gypsum board as a backing board.
Gypsum lath or gypsum wallboard shall not be used as a
backing for cement plaster.
Exception: Gypsum lath or gypsumwallboard is permitted,
with a weather-resistant barrier, as a backing
for self-furred metal lath or self-furred wire fabric lath
and cement plaster where either of the following conditions
1. On horizontal supports of ceilings or roof soffits.
2. On interior walls.
2510.5.2.2 Use of gypsum sheathing backing. Gypsum
sheathing is permitted as a backing for metal or wire fabric
lath and cement plaster on walls. A weather-resistant
barrier shall be provided in accordance with Section
Combination Building Inspector