Info on Stucco and Plastering-December, 2019-fifty-ninth issue
Lath and stucco with no sheathing
(That is, no plywood, OSB, etc.)
Hello Reggie: It has been a few years since we communicated. I hope you are doing well.
I have been in a lengthy technical debate with an Architect about the requirement and ability to properly waterproof plaster applied to metal plaster base.
My firm was hired by a General Contractor to prepare waterproofing details related to architectural details (banding, eyebrows etc.) to a high-rise building --> Most important to this discussion, all of the details are outside of the building envelope but structurally attached to the walls.
Our recommended details followed all provisions outlined in ASTM C926 and C1063 (went with the 3.4 lb./s.y. --- but we did not include sheathing (i.e. solid base).
A 3rd party consultant hired by the Architect disagreed with our details and first tried to assert "sheathing was required per code and my client was required to build to code" (hence... it should have been included in our construction cost). --> We won that first battle because the consultant didn't understand C926.
The most current assertion is that sheathing/solid base is necessary to properly waterproof the detail. We disagree and believe properly-applied plaster installation coated with a hybrid waterproofing coating system (not elastomeric) will be sufficient to keep water from penetrating and ultimately deteriorating the structural bond of the framing (a stated prediction by one of the opposing parties).
Wanted to get your opinion.
Using guide wires is a good method for lath with no sheathing.
I haven't seen this method in almost 30 years.
I am sure guide wires are still used in California.
Nails are driven into the studs, leaving the nail head sticking out about a quarter inch. In other words, the nails aren't driven tight.
Tie wire is then wrapped around the head of the nail every foot across the wall. 2 foot spacing works out well for expanded metal lath, which
varies from 25 inches to 29 inches in width. When woven wire lath or other kinds of lath are used the idea is the wire supports the lath where the sheets or rolls are overlapped, or the "laps".
The idea of the guide wires is twofold: The wire suppots the lath to prevent the laps from being knocked in by the scratch coat.
Also, the wires in this case hold the lath out away from the studs, allowing mortar to form a full key. The lath can be tied to the wires or nailed to the stud. It is a good idea not to drive the nail tight to the stud, so mortar keys behind the lath.
The guide wires work well to support the tarpaper, such as for paper back stucco mesh.
Lath can be stretched over the studs, provided the laps are tied. Here is an interior plaster application where no tarpaper is needed. The rule is to tie the
lath with tie wire in at least two places between every stud.
Tying the laps prevents the lath from separating until the scratch coat sets up.
Speed bracing, or diagonal metal bracing can be used instead of sheathing boards.
You made need to check the codes in your area. I have seen metal diagonal used out west for stucco applications. Diagonal bracing didn't catch on here in Virginia, because siding doesn't lay flat without notching the studs. Diagonal bracing works well for stucco because the strap is buried in the mortar. Stucco doesn;t have to follow the contour of the framing.
I found this diagonal bracing interesting so I took a picture. The picture shows the inside of a garage built in the 1930's in Washington, DC. The studs
were marked and notched to allow room for a one by four diagonal brace. The stucco was "backed up", that is the back side of the stucco was filled and evened out.