- Exam Overview: How the 10 Domains Fit Together
- The 10 OK-PLB Domains and Their Item Counts
- The High-Weight Domains: Vents, Drains and Sewers, and Water Supply
- Mid-Weight Domains: Piping, Fixtures, Fuel Gas, and General Knowledge
- Lower-Weight but Still Critical: Traps, Isometrics, and Safety
- Code Version Matters Inside Every Domain
- Mapping a Study Schedule to Domain Weight
- Frequently Asked Questions
- The OK-PLB exam has exactly 85 scored questions split across 10 domains, from 4 to 13 items each.
- Vents is the single largest domain at 13 items - nearly 15% of the scored exam.
- You need 64 correct answers out of 85 to hit the 75% passing standard.
- Isometric Analysis (Domain 8) has only 4 items but tests a skill unlike any other domain.
Exam Overview: How the 10 Domains Fit Together
The Oklahoma Plumbing Journeyman and Natural Gas License Exam is administered by PSI Services LLC on behalf of the Oklahoma Construction Industries Board (CIB), and it is built from ten distinct content domains rather than a loosely organized question bank. Every one of the 85 scored questions on the computer-administered exam belongs to one of these ten areas, and the official bulletin publishes exact item counts for each. That means domain weighting isn't a guess - it's published, and it should drive your entire preparation strategy.
Candidates get 180 minutes to complete the open-book exam, and up to 10 additional unscored experimental questions may be mixed in without extending the clock. Passing requires 75%, which works out to 64 correct answers out of the 85 that count. If you want the full mechanics of scoring, eligibility, and registration before diving into content, the OK-PLB Study Guide 2026: How to Pass on Your First Attempt is a good companion piece to this one.
The 10 OK-PLB Domains and Their Item Counts
Here is the official domain list in order, with the scored item allocation for each, based on the Board-linked PSI candidate bulletin effective December 18, 2025:
| Domain | Content Area | Scored Items | Share of Exam |
|---|---|---|---|
| 1 | General Plumbing Knowledge and Regulations | 10 | ~11.8% |
| 2 | Piping, Valves, and Control Valves | 8 | ~9.4% |
| 3 | Fixtures and Equipment | 8 | ~9.4% |
| 4 | Water Supply | 10 | ~11.8% |
| 5 | Drains and Sewers | 10 | ~11.8% |
| 6 | Vents | 13 | ~15.3% |
| 7 | Traps, Interceptors, Indirect, and Special Waste | 5 | ~5.9% |
| 8 | Isometric Analysis | 7 | ~8.2% |
| 9 | Fuel Gas | 10 | ~11.8% |
| 10 | Safety | 4 | ~4.7% |
Note the totals add to 85, matching the scored question count exactly. If you're still confirming your eligibility path before worrying about domain content, review OK-PLB Requirements 2026: Eligibility, Prerequisites & How to Qualify first - you'll need CIB approval before PSI scheduling opens up.
The High-Weight Domains: Vents, Drains and Sewers, and Water Supply
Three domains carry 10 or more items each, and together with Fuel Gas they represent the backbone of the exam.
Domain 6: Vents (13 items - the largest domain)
Vents is the single biggest content area on the exam, and candidates who underestimate it tend to leave points on the table. Expect questions on vent sizing, distance limitations, wet venting, common venting, island venting, and how vent requirements interact with drainage design under IPC provisions with Oklahoma revisions.
- Know minimum and maximum vent pipe sizes relative to fixture units and trap arm length
- Understand when a fixture requires individual venting versus combined approaches
- Be able to trace vent termination requirements above roof lines
Domain 5: Drains and Sewers (10 items)
This domain covers drainage fixture unit (DFU) calculations, pipe sizing based on slope and diameter, building drain versus building sewer distinctions, and cleanout placement requirements.
- Practice DFU-to-pipe-size lookup tables until they're automatic
- Understand minimum slope requirements for different pipe diameters
- Know cleanout spacing and access requirements
Domain 4: Water Supply (10 items)
Water Supply questions test pipe sizing for supply lines, water hammer arrestor placement, backflow prevention devices, and fixture supply fixture unit calculations.
- Master water supply fixture unit (WSFU) tables and demand curves
- Know cross-connection control and backflow device selection by hazard level
- Understand pipe material compatibility rules under the adopted code
Mid-Weight Domains: Piping, Fixtures, Fuel Gas, and General Knowledge
General Plumbing Knowledge and Regulations (Domain 1, 10 items) covers licensing law, CIB regulatory authority, and general code application - the "rules of the road" a journeyman must know regardless of task. Piping, Valves, and Control Valves (Domain 2, 8 items) tests pipe material identification, joint types, and valve function including check valves, gate valves, and pressure-reducing valves. Fixtures and Equipment (Domain 3, 8 items) covers installation clearances, minimum fixture requirements per occupancy, and equipment like water heaters and grease interceptors.
Domain 9: Fuel Gas (10 items)
Fuel Gas is graded against the International Fuel Gas Code (IFGC 2018) with Oklahoma revisions, and it's the domain most distinct from the plumbing-specific content elsewhere on the exam. This is also the domain most affected by the exam's natural gas scope - remember, this is the full Plumbing Journeyman and Natural Gas exam, not a standalone gas-only test.
- Gas pipe sizing methods based on BTU load and pipe length
- Appliance venting and combustion air requirements
- Shutoff valve placement and gas meter location rules
Because Fuel Gas draws from a different code book than the plumbing-only domains, many candidates find it helpful to study it as its own block rather than blending it into general plumbing review. For a broader sense of where candidates commonly struggle across all domains, see How Hard Is the OK-PLB Exam? Complete Difficulty Guide 2026.
Lower-Weight but Still Critical: Traps, Isometrics, and Safety
Don't mistake a low item count for low difficulty. Three domains sit at the smaller end of the weighting scale, but each tests a skill set that can't be skipped.
Domain 7: Traps, Interceptors, Indirect, and Special Waste (5 items)
Covers trap seal depth, trap design restrictions, grease and oil interceptor sizing, and indirect waste connection requirements for equipment like ice machines and dishwashers.
Domain 8: Isometric Analysis (7 items)
This domain is unique on the exam: instead of recalling a code number, you interpret isometric drawings and identify errors or missing components in a piping layout. It rewards spatial reasoning and practice reading diagrams far more than memorization.
- Practice tracing vent, drain, and supply lines on sample isometric drawings
- Learn to spot missing cleanouts, incorrect vent connections, and improper trap placement visually
Domain 10: Safety (4 items)
The smallest domain, drawing on OSHA construction references alongside general jobsite safety practice. Expect trench safety, ladder use, and personal protective equipment questions.
Key Takeaway
Isometric Analysis is worth studying with actual practice diagrams, not flashcards - it's a visual skill, and no amount of code memorization substitutes for drawing repetition.
Code Version Matters Inside Every Domain
Every domain above is graded against a specific code baseline: IPC 2018 and IFGC 2018 with Oklahoma revisions, plus authorized plumbing-mathematics and OSHA construction references. But 2026 candidates face a wrinkle that touches every domain equally - a code transition window. Candidates testing between October 8, 2026 and January 8, 2027 may choose either the previous code version or the newly adopted 2024-code version for their full Plumbing Journeyman and Natural Gas exam. Starting January 9, 2027, the 2024-code exam becomes mandatory.
This transition date is specific to the full journeyman and gas exam and differs from both the September 14, 2026 construction-code effective date and the transition schedule for the residential-only and standalone natural-gas exams - so don't assume a deadline you read elsewhere applies to your exam version. Whichever code you select, make sure every practice question and code citation you study matches that same version. For the exact dates and how they interact with scheduling, see OK-PLB Exam Dates 2026: Testing Windows, Deadlines & Scheduling.
Mapping a Study Schedule to Domain Weight
Rather than studying the ten domains in numerical order for equal time blocks, build a schedule around the published item counts. A simple, weighting-based approach looks like this:
High-Weight Domains
- Vents (13 items) and Drains and Sewers (10 items) - the two largest drainage-related domains
- Build DFU and vent-sizing tables into memory through repeated practice problems
Supply and Gas
- Water Supply (10 items) and Fuel Gas (10 items) - different code books, so alternate study days between them
- Drill WSFU tables and gas pipe sizing charts separately
Mid-Weight Content
- General Plumbing Knowledge and Regulations (10 items), Piping/Valves (8 items), Fixtures and Equipment (8 items)
Smaller Domains and Diagram Practice
- Isometric Analysis (7 items) - dedicate real time to reading diagrams, not just terminology
- Traps, Interceptors, Indirect and Special Waste (5 items) and Safety (4 items)
Full Review and Timed Practice
- Full-length timed practice runs under the 180-minute limit
- Re-test weaker domains identified during practice, using your selected code version consistently
This is one reasonable pacing model, not a rigid rule - adjust it based on your baseline knowledge and how many years of on-the-job experience you're bringing in. For a deeper look at pass-rate expectations and what a realistic timeline looks like overall, see OK-PLB Pass Rate 2026: What the Data Shows, and for a condensed reference sheet once you've covered all ten domains, bookmark the OK-PLB Cheat Sheet 2026: One-Page Review of Must-Know Facts.
It's also worth remembering what this credential authorizes and what it doesn't. Passing the OK-PLB exam and completing CIB licensing lets you work as a journeyman under a licensed plumbing contractor - it does not authorize independent contracting, since Business and Law content sits outside this exam entirely. If you're mapping out career direction, OK-PLB Jobs and OK-PLB Salary Guide 2026: Complete Earnings Analysis cover what comes after licensure, while Is the OK-PLB Certification Worth It? Complete ROI Analysis 2026 weighs the licensing investment against the career payoff.
Once you're ready to test your domain-by-domain readiness under realistic timed conditions, practicing with full-length OK-PLB practice tests is the most direct way to see which of the ten domains still need work before exam day. Structured practice modeled on the actual 85-item, 180-minute format - available through OK-PLB Exam Prep - lets you simulate the real weighting instead of guessing at it.
Frequently Asked Questions
Vents (Domain 6) is the largest domain with 13 scored items, roughly 15% of the 85-question exam. Drains and Sewers, Water Supply, Fuel Gas, and General Plumbing Knowledge and Regulations each carry 10 items.
There are 10 domains totaling 85 scored questions, per the official item allocation. Up to 10 additional unscored experimental questions may also appear without affecting your score or the exam time limit.
Many candidates find it disproportionately challenging because it tests visual diagram-reading rather than fact recall. Despite its smaller weight, dedicating real practice time to isometric drawings pays off.
Most plumbing domains reference IPC 2018 with Oklahoma revisions, while Fuel Gas references IFGC 2018 with Oklahoma revisions. Authorized plumbing-mathematics and OSHA construction references also apply where relevant, particularly for the Safety domain.
Candidates testing between October 8, 2026 and January 8, 2027 can choose the previous or newly adopted 2024-code version for the full Plumbing Journeyman and Natural Gas exam. Starting January 9, 2027, the 2024-code version becomes mandatory, so confirm your selection matches your study materials across all 10 domains.