The wall has been plastered as one uninterrupted field 12 feet high by 30 feet long, with no control joints anywhere in it and none shown at changes in the framing behind. What does the code require of this plaster field?
- Control joints dividing the plaster into panels of not more than 144 square feet, with a length-to-width ratio no greater than 2 1/2 to 1
- BControl joints only where the plaster crosses from one substrate material to a different one, a condition that does not occur anywhere on this wall
Substrate changes are one trigger for a joint, but not the only one. The area and proportion limits apply on their own, and a wall of one continuous substrate still has to be divided.
- CNo control joints at all, provided the plaster is reinforced with metal lath run continuous across the full length of the wall
Lath reinforces the plaster and distributes shrinkage into many fine cracks, but it does not remove the panel size limits. Continuous lath across a 360-square-foot field is exactly the condition the limits address.
- DControl joints at not more than 20 feet on center horizontally only, since vertical shrinkage is taken up at the screeds
There is no horizontal-only rule here, and vertical joints matter as much as horizontal ones on a long wall. Screeds terminate the plaster at edges; they do not act as intermediate control joints.
Why A is correct
The referenced plaster standard limits a panel to 144 square feet with a length-to-width ratio no greater than 2 1/2 to 1, and calls for joints where the substrate or the framing direction changes. This 360-square-foot field exceeds the area limit and must be divided by control joints in both directions.
What this question is testing
Whether the candidate knows exterior plaster carries panel size and proportion limits from the referenced standard, and does not accept reinforcement or edge accessories as a substitute for dividing the field.
On the job
Stucco crack complaints are among the most common in residential warranty work, and the cause is almost always a field that was never divided. The inspection happens before the finish coat, while the accessories are visible: an inspector looks for joint accessory in both directions, checks that lath is cut at the joints rather than run through, and confirms the joints line up with framing changes and with the corners of openings.
Memory technique
Big blank stucco walls crack; divide them before they divide themselves.
Exam tip
Stucco panels: 144 square feet and no longer than 2 1/2 times their width.
Where to look it up
IRC Chapter 7, exterior covering, exterior plaster and the referenced application standard covering control joints.