C-35 Lathing and Plastering exam questions, with answers
10 practice questions for the CSLB Lathing and Plastering trade exam, each with the answer, a plain explanation and the source it rests on.
These are not real exam questions. CSLB does not release its test questions or answer keys, and nobody else has them. We write our own questions from the C-35 outline and the codes, standards and references CSLB lists for the exam. Each answer names its source so you can check it.
The C-35 exam has 5 sections. The questions below are grouped by section, with the share of the exam each one carries. Read the question, pick an answer, then open “Show the answer”. The other questions of the free test, scored section by section, are in the free 25-question C-35 test.
Planning and Estimating · 15% of the exam
Interpreting plans and specifications; estimating material, equipment and labor; inspecting job sites.
1. The specifications for exterior portland cement plaster on a commercial building say to follow the ASTM standards that the 2025 California Building Code names for cement plaster and for its lath. Which pair is correct?
- Plaster per ASTM C926; lath and furring per ASTM C841
- Plaster per ASTM C842; lath and furring per ASTM C1063
- Plaster per ASTM C843; lath and furring per ASTM C841
- Plaster per ASTM C926; lath and furring per ASTM C1063
Show the answer
Answer: D. 2025 CBC Table 2511.1.1 (and 2510.3 for exterior work) pairs cement plaster with ASTM C926 and the lathing and furring for cement plaster with ASTM C1063. ASTM C842 and C843 cover gypsum plaster and gypsum veneer plaster, and ASTM C841 covers lathing for gypsum plaster, so those pairs belong to interior gypsum work.
2. You are pricing three-coat stucco on the front wall of a house. The wall is 38 ft long and 9 ft high, with one 16 ft x 7 ft garage door and two windows of 5 ft x 4 ft each. Deduct the openings in full. What is the net plaster area?
- 21.11 sq yd
- 25.56 sq yd
- 33.56 sq yd
- 38.00 sq yd
Show the answer
Answer: A. The gross wall is 38 x 9 = 342 sq ft. Deduct the garage door (112 sq ft) and both windows (2 x 20 = 40 sq ft) to get 190 sq ft, then divide by 9 sq ft per square yard: 21.11 sq yd. Forgetting the windows gives 25.56, forgetting the garage door gives 33.56, and 38.00 is the gross wall with no deductions.
Lath Preparation and Installation · 20% of the exam
Coordinating lath and plaster installation; preparing substrates; installing metal framing and gypsum systems; installing lathing materials; sealing penetrations.
3. The OSB sheathing on a framed stucco building got soaked in a storm before the water-resistive barrier (WRB) went on. Under TSIB Technical Bulletin 60.140 on project sequencing, what is the highest moisture content the sheathing may have when the WRB is installed and again just before plastering?
- 10%
- 12%
- 15%
- 19%
Show the answer
Answer: D. TSIB 60.140 says wood-based sheathing may be at no more than 19% moisture content when the WRB is installed and immediately before plastering, so the GC must let soaked sheathing dry before your crew papers and lathes it. Wet sheathing that dries and shrinks under the stucco can crack it. 10%, 12% and 15% are not the limit the bulletin sets.
4. A house in a dry (non-marine) California climate zone gets three-coat stucco over wood sheathing, with no foam board or drainage layer between the barrier and the stucco. Under 2025 CRC R703.7.3.1, which water-resistive barrier meets the code?
- One layer of ASTM E2556 Type I, lapped 6 in
- One layer of 10-minute Grade D paper, lapped 6 in
- Two independent layers of 10-minute Grade D paper
- One layer of 60-minute Grade D paper against the stucco
Show the answer
Answer: C. R703.7.3.1 allows two independent layers of 10-minute Grade D paper (or the equal of two layers of ASTM E2556 Type I), or one layer of 60-minute Grade D paper only when foam sheathing, another non-absorbing layer or a drainage space separates it from the stucco. With nothing between the barrier and the stucco, a single layer of any of these papers does not comply, however it is lapped.
Source: 2025 CRC R703.7.3.1, 2025 California Residential Code, Chapter 7: Wall Covering
Plaster/Stucco Preparation and Installation · 28% of the exam
Mixing and applying scratch and brown coats; installing plaster/stucco systems; specialty plaster/stucco installations; cleaning up job sites.
5. Interior walls of a commercial building get gypsum plaster over gypsum lath (not metal or wire lath). The product is not a veneer or one-coat plaster. Under 2025 CBC 2511.1, what is the fewest number of coats allowed?
- One coat, since gypsum lath gives a flat, solid base
- Two coats, since the base is not metal or wire lath
- Three coats, the same as over metal or wire lath
- Three coats, since the base is a gypsum product
Show the answer
Answer: B. CBC 2511.1 requires at least three coats of interior plaster over metal or wire lath and at least two coats over other bases, such as gypsum lath. One coat is allowed only for approved veneer or one-coat products. Gypsum lath is one of the other bases, so two coats is the minimum.
Source: 2025 CBC 2511.1, 2025 California Building Code, Chapter 25: Gypsum Panel Products and Plaster
6. A California house has a concrete slab-on-grade floor and wood-framed exterior walls. The lath and building paper end on the weep screed at the base of the wall. Under 2025 CRC R703.7.2, how must the three-coat stucco end at the bottom of the wall?
- Covering the lath, paper and screed, but not extending below them
- Extending below the screed onto the foundation to seal the base
- Stopping just above the screed so a sealant joint can be run there
- Covering the lath, paper and screed, and wrapping onto the slab edge
Show the answer
Answer: A. 2025 CRC R703.7.2 says that on wood-frame construction with a slab-on-grade floor, the exterior plaster covers the lath, paper and screed but does not extend below them. Carrying the plaster onto the slab edge or foundation bridges the weep screed, and stopping short of the screed leaves the lath and screed flange uncovered.
Source: 2025 CRC R703.7.2, 2025 California Residential Code (Title 24, Part 2.5), Chapter 7 Wall Covering
Repairs · 17% of the exam
Repairing plaster/stucco installations.
7. You are deciding how to repair several cracks in a stucco wall. According to PCA IS526, which crack should be routed out and filled with an elastomeric sealant instead of a rigid material?
- A crack a monitor shows moving more than thermal cycling explains
- A hairline crack in a fresh patch caused by drying out too fast
- A fine shrinkage crack that has kept the same width for a year
- A crack whose movement matches normal daily and seasonal heating
Show the answer
Answer: A. IS526 explains that a moving crack gives the plaster needed stress relief and acts as a control joint, so it must be allowed to open and close; a rigid filler leads to recurring cracking. A crack movement monitor tells whether movement exceeds normal thermal cycling. Such cracks are routed (widened with a saw or grinder) and filled with elastomeric sealant; cracks that do not move beyond thermal cycling can take a rigid filler.
8. You are pricing crack repairs on a stucco house. According to TSIB Technical Bulletin 60.102, which crack and repair are correctly matched?
- A 0.020-in hairline crack: fill it with finish coat slurry and sand
- A crack through the whole membrane: tool it out, fill with bonded slurry
- A 0.050-in crack in the finish: brush paint into it, leave the texture
- A crack through the whole membrane: brush paint into it, then refinish the panel
Show the answer
Answer: B. TSIB 60.102 does not recommend aggregate patching for cracks 0.030 in wide or less (brush paint into them if the look is a problem). Wider cracks in the finish get a slurry of finish material with a bonding agent. A crack that goes through the plaster membrane is tooled out and filled with a slurry of portland cement, plasticizer, #60 sand and bonding agent, then the panel is refinished.
Safety · 20% of the exam
Personnel safety; job site safety; Cal/OSHA laws and regulations; personal protective equipment.
9. A plasterer says he rubs a barrier cream on his hands each morning so he can work wet cement plaster without gloves. What does federal OSHA's booklet OSHA 3351 on portland cement say about this?
- Barrier creams do not protect skin from cement; he must wear gloves
- Barrier creams work if he washes with a waterless cleaner at lunch
- Barrier creams protect him if he reapplies them after every break
- Barrier creams are enough for base coats but not for finish coats
Show the answer
Answer: A. OSHA 3351 says not to use barrier creams or 'invisible gloves' because they are not effective against portland cement hazards, and it notes that butyl or nitrile gloves are frequently recommended for caustic materials such as cement. Reapplying the cream or pairing it with a waterless cleaner does not make it protective.
10. Your lath and plaster company has adopted its written Code of Safe Practices. Under 8 CCR 1509, how must the Code be made available on each job?
- Kept at the main office and sent to Cal/OSHA once each calendar year
- Posted at the job site office, but only after an accident on that job
- Handed to each new worker, who signs it and returns it the same day
- Posted visibly at each job site office, or given to each supervisor
Show the answer
Answer: D. 8 CCR 1509 requires every construction employer to have an Injury and Illness Prevention Program and a written Code of Safe Practices for its operations. Under 1509(c) the Code is posted at a conspicuous location at each job site office or given to each supervisory employee to keep readily available. Toolbox (tailgate) meetings at least every 10 working days are a separate part of the same section.
Where to go from here
How the license works, from experience to fees: how to get a C-35 license. Everything about the course: the C-35 exam page.
Questions from our free C-35 test, written from CSLB License Examination Study Guide, Lathing and Plastering (C-35), form 13E-35 (03-2026), for examinations scheduled on or after October 1, 2025. CSLB notes that the percentages are approximate. Reviewed 14 October 2026. Study guidance only; we are not affiliated with CSLB or PSI.