Many wall and ceiling contractors, particularly those doing lathing in schools, health care facilities and other projects funded by the state of California are being surprised, to their dismay by code requirements of which they should have been aware.
For example, Table 25-B of the Uniform Building Code (UBC) is entitled Types of Lath – maximum Spacing of Supports. Supports are considered to be studs and joists, and the fasteners described in this Table and their required spacing is based on their holding power into the supports. Staple legs, you will notice, may be required to minimum 7/8 inch long but the minimum nail size is 1 inch and requires a heavier gauge than the staples.
Since the code does not consider sheathing as a support for attaching lath, all fasteners must be long enough to pass through any sheathing and penetrate the supports. Randomly nailing or stapling self-furred lath only to the sheathing is unacceptable.
This means that 3/8 inch thick plywood sheathing or OSB over the framing will necessitate that the lather uses 1-1/4" long staples in order to get 7/8 inch penetration into the studs. 1-1/2 inch long staples are necessary for attaching lath to supports through 1/2 inch thick sheathing.
To compound this issue, the California Building Code, which is applicable on state funded work, is more restrictive than the Uniform Building Code and does not recognize staple attached, self-furred lath for application over plywood sheathing or similar rigid backing.
Consequently the only way lath might be installed over sheathing on this work is with furring devices other than deformations in the lath. Furring nails are the most obvious example which comes to mind and Table 25-C of the building code describes furring nails as being 1-1/4 inch long No. 12 gauge with 3/8" diameter heads.
This attachment is usable with wire fabric lath but the furring wad is too large to be used with expanded metal lath. Additionally, nails are not suitable for attaching lath to light gauge steel supports. At this writing the author is not aware of any screws which might provide furring of the lath away from rigid backing.
Finally, it must be repeated that vapor retardant weather resistive barriers are not acceptable for application over wood based sheathing. Section 2506.4 of the code states that weather resistive barriers shall be installed as required in Section 1402.1 and when applied over wood based sheathing shall include two layers of Grade D paper.
Author's comment: Grade D kraft building paper is vapor permeable and while it is effective in preventing liquid moisture from the exterior penetrating of the walls, it will still enable vaporous moisture generated on the structure from cooking, bathing, breathing, etc. to pass through the sheathing and weather barrier to escape to the outside air.
