- Domain 9, Gas Appliances and Piping, carries 40 of 100 scored questions - by far the largest domain.
- Domain 7 (Drain, Waste, and Vent Systems) and Domain 6 (Backflow Prevention) rank second and third at 13 and 10 questions.
- You need 70% overall (70 of 100 scored questions) across all 10 domains combined, not per-domain minimums.
- The exam is open book, but only permanent tabs, highlighting, and underlining in approved references are allowed - no handwritten notes.
Exam Format Overview
Before breaking down each content area, it helps to understand the shape of the test itself. The Maryland Master Plumber License Exam - the combined Master Plumber/Gas Fitter exam administered through PSI Examination Services on behalf of the Maryland State Board of Plumbing - is a computer-based, multiple-choice exam consisting of 100 scored questions delivered in a 240-minute window. You need a 70% score to pass, meaning 70 correct answers out of the 100 that count toward your result.
PSI may embed five to 10 additional experimental questions that are not scored. These questions are used to evaluate future exam content, and the time you spend on them does not count against your four-hour allowance. You won't know which questions are experimental, so the practical approach is to treat every question on the screen as if it counts.
If you're still confirming basic eligibility - journey license tenure, supervised training hours, or the backflow certificate PSI requires at check-in - that's covered in detail in MD-MP Requirements 2026: Eligibility, Prerequisites & How to Qualify. This article focuses specifically on what the exam actually tests once you're sitting in front of the screen.
The 10 Domains and Their Weights
The board's official question allocation breaks the 100 scored questions across 10 content areas. Here's the full picture before we go domain by domain:
| Domain | Content Area | Questions |
|---|---|---|
| 1 | Plumbing Fundamentals and Safety | 5 |
| 2 | Materials, Uses, and Specifications | 6 |
| 3 | Traps, Interceptors, and Backwater Valves | 4 |
| 4 | Plumbing Fixtures, Fixture Fittings, and Plumbing Appliances | 4 |
| 5 | Water Supply Systems | 8 |
| 6 | Backflow Prevention | 10 |
| 7 | Drain, Waste, and Vent Systems | 13 |
| 8 | Special Topics | 3 |
| 9 | Gas Appliances and Piping | 40 |
| 10 | Plumbing Mathematics | 7 |
Add those up and you get 100 - the full scored exam. Notice how top-heavy this distribution is: the top three domains (9, 7, and 6) together account for 63 of the 100 questions. That leaves the remaining seven domains sharing just 37 questions between them. For a broader take on how this weighting affects overall difficulty, see How Hard Is the MD-MP Exam? Complete Difficulty Guide 2026.
Key Takeaway
If you only have time to master three domains deeply before test day, make them Gas Appliances and Piping, Drain Waste and Vent Systems, and Backflow Prevention - they represent 63% of the scored exam.
Domain 9: Gas Appliances and Piping (The Big One)
No other domain comes close to Domain 9 in terms of exam impact. At 40 of 100 questions, Gas Appliances and Piping is worth more than the bottom five domains combined. This weighting makes sense given the exam's official name - the Master Plumber/Gas Fitter exam - which reflects Maryland's practice of testing gas fitting competency alongside general plumbing knowledge in a single combined credential.
Domain 9: Gas Appliances and Piping
Candidates should expect deep coverage of gas piping sizing, sizing methods, appliance venting, gas appliance installation clearances, and code requirements drawn from the approved gas code reference. Because this domain is nearly half the exam, weak spots here disproportionately hurt your overall score.
- Gas pipe sizing and material selection
- Appliance connection and shutoff requirements
- Venting systems for gas-fired appliances
- Combustion air and clearance requirements
- Cross-referencing gas fitting rules against the fuel gas code
The board-linked bulletin dated September 1, 2024 lists the 2018 International Fuel Gas Code (IFGC) among the approved candidate-supplied references, alongside the 2018 International Plumbing Code (IPC). Because June 2026 board minutes discuss code revisions tied to a 2024 edition cycle, don't assume the reference edition you studied last year is the one you'll be tested on - confirm the exact booked examination reference edition before purchasing books or committing to a code year in your notes.
Domains 6, 7, and 10: The Next Tier
After Gas Appliances and Piping, three more domains deserve serious study time because together they represent nearly a quarter of the exam.
Domain 7: Drain, Waste, and Vent Systems (13 questions)
This is the second-largest domain and covers the core DWV system design that most master-level candidates already handle daily in the field - but the exam tests it at a code-reference level, not just a practical-experience level.
- Vent sizing and venting methods
- Waste pipe sizing based on fixture units
- Slope, cleanout placement, and trap arm limitations
Domain 6: Backflow Prevention (10 questions)
Backflow prevention earns its own dedicated domain and its own eligibility requirement - PSI requires a current approved 32-hour backflow prevention device testing certificate just to sit for the exam. Expect questions on device types, application, and testing principles beyond simple installation.
- Types of backflow preventers and where each is required
- Cross-connection control principles
- Device testing and certification concepts
Domain 10: Plumbing Mathematics (7 questions)
Math questions are scattered but consistent enough to matter. These typically involve pipe sizing calculations, fixture unit totals, pressure/head calculations, and similar applied-math problems tied directly back to code tables rather than abstract algebra.
- Fixture unit calculations
- Pressure loss and head calculations
- Pipe sizing math tied to code tables
Combined, Domains 6, 7, and 10 add 30 more questions on top of Domain 9's 40 - meaning four of the ten domains account for 70 of the 100 scored questions. That's the exact threshold you need to pass, which is a useful mental anchor when prioritizing study time.
The Lighter-Weight Domains
The remaining six domains are lower-weight individually but still worth understanding, since a candidate can't afford to write off nearly 30 combined questions.
Domain 2: Materials, Uses, and Specifications (6 questions)
Covers approved piping materials, fittings, and their appropriate applications under code - a foundational area for any master-level exam.
Domain 1: Plumbing Fundamentals and Safety (5 questions)
Baseline safety practices, jobsite hazards, and general plumbing principles that set up the rest of the exam's more technical domains.
Domain 5: Water Supply Systems (8 questions)
Water distribution sizing, pressure requirements, and supply system design fundamentals.
Domain 3: Traps, Interceptors, and Backwater Valves (4 questions)
Trap design, interceptor requirements for specific fixture types, and backwater valve installation scenarios.
Domain 4: Plumbing Fixtures, Fixture Fittings, and Plumbing Appliances (4 questions)
Installation and specification requirements for fixtures, fittings, and plumbing appliances beyond gas-fired units.
Domain 8: Special Topics (3 questions)
The smallest domain by question count - likely covering miscellaneous code provisions that don't fit neatly into the other nine categories.
None of these domains should be ignored entirely, but if you're triaging limited study hours in the final weeks before your test date, they're the right place to trim first. For a full walkthrough of the minimum score mechanics across all domains combined, see MD-MP Passing Score 2026: Exactly What You Need to Pass.
Open-Book Rules and Approved References
One structural detail that shapes how you study every domain: the MD-MP exam is open book. That sounds like a relief, but the rules are specific. Permanent tabs, highlighting, and underlining in your approved reference materials are allowed. Handwritten notes, loose papers, and separate study guides are prohibited at the testing station.
This changes the calculus for domain prep. Rather than memorizing every code table for Domains 6, 7, 9, and 10, your real skill to build is fast, confident navigation of the 2018 IPC and 2018 IFGC (subject to the edition confirmation noted above) under time pressure. A well-tabbed code book can save meaningful minutes across a 240-minute exam, especially given how math-heavy and table-dependent Domains 9, 7, and 10 tend to be.
Sequencing Your Study by Domain Weight
Rather than a generic week-by-week template, sequence your review around the actual point value of each domain. A simple, weight-driven approach looks like this:
Domain 9 - Gas Appliances and Piping
- Work through gas pipe sizing tables until they're second nature
- Tab your IFGC reference for venting and clearance sections
- Drill appliance connection and combustion air scenarios
Domains 7 and 6 - DWV and Backflow Prevention
- Review vent sizing and fixture unit tables for DWV
- Reinforce backflow device types using your 32-hour certificate coursework as a base
Domain 10 and remaining lighter domains
- Practice plumbing math problems tied to code tables
- Sweep through Domains 1-5 and 8 for gaps
- Run full-length timed practice sessions to simulate the 240-minute format
For a more detailed prep methodology, including how to structure practice tests and review sessions, see MD-MP Study Guide 2026: How to Pass on Your First Attempt. And if you want a compact reference to keep the domain weights and key numbers in front of you during final review, bookmark MD-MP Cheat Sheet 2026: One-Page Review of Must-Know Facts.
It's also worth remembering that retesting is allowed no more often than every 30 days, and you must apply for the license within one year of passing. Neither fact should change your domain priorities, but both are useful context if your first attempt doesn't go the way you hoped - you can review what the data actually shows in MD-MP Pass Rate 2026: What the Data Shows rather than guessing at your odds.
Once licensed, understand that the general plumbing license runs two years from issuance with no continuing education requirement for renewal, though maintaining insured-to-contract status requires at least $300,000 in general liability and $100,000 in property-damage coverage per occurrence. If you're weighing whether the credential is worth pursuing at all given the study investment described above, Is the MD-MP Certification Worth It? Complete ROI Analysis 2026 lays out the broader picture, and MD-MP Salary Guide 2026: Complete Earnings Analysis covers earning potential for license holders. You can also start building familiarity with the domain structure right now using realistic questions on our practice test platform.
Frequently Asked Questions
Domain 9, Gas Appliances and Piping, is the largest by far with 40 of the 100 scored questions - 40% of the entire exam.
No. The passing standard is 70% overall across all 100 scored questions combined, not a minimum score within each of the 10 domains.
Domain 7 (Drain, Waste, and Vent Systems) at 13 questions and Domain 6 (Backflow Prevention) at 10 questions are the next-largest content areas, followed by Domain 10 (Plumbing Mathematics) at 7 questions.
No. The exam is open book, but only permanent tabs, highlighting, and underlining in approved references such as the IPC and IFGC are allowed - handwritten notes and separate study guides are prohibited.
Yes. June 2026 board minutes discuss code revisions, so confirm the exact booked examination reference edition before buying books, especially for code-table-heavy domains like Gas Appliances and Piping and DWV Systems.