MT Journeyman Plumber common mistakes
MT Journeyman Plumber · Understand the traps and how to avoid them.
Sizing a whole drainage system from the fixture-unit table alone, without checking the building drain and building sewer against the slope-based capacity tables and the minimum-size rules for the building sewer.
Candidates memorize the drainage fixture-unit table and treat its total as the final answer, forgetting that the developed length of the building drain/sewer and its slope govern capacity, and that the building sewer may never be smaller than the building drain.
What to do instead
Work every sizing problem in a fixed order: count fixture units by fixture type, apply the correct trap and branch rules, total the stack, then size the building drain and building sewer from the slope-based capacity table at the actual slope, and finally confirm the minimum size rule. Practice with a three-story example so the sequence becomes automatic.
Treating every fixture unit as interchangeable when converting between drainage fixture units and total fixture units for water supply sizing.
The two systems use different tables and different unit values (a water closet is one drainage fixture unit but several supply fixture units), and candidates cross-apply the drainage table to supply pipe sizing or vice versa.
What to do instead
Keep two separate reference sheets and label them clearly: drainage fixture units and supply fixture units. On supply problems, convert to total fixture units using the supply table, then apply the demand-weight or probable-demand method with the correct meter and building supply curve. Never carry a drainage total into a water sizing calculation.
Installing a trap arm at the wrong slope or length, or omitting the vent connection within the required distance from the trap weir.
Candidates focus on the trap itself and forget that the trap arm length and slope are what keep the trap from siphoning; they also confuse the vent-to-weir distance limit with the developed length of the trap arm.
What to do instead
For each fixture, identify the trap, the trap arm, and the vent takeoff, then verify the trap arm slope and that the vent connects within the maximum developed length allowed from the trap weir. Draw the trap arm on every plan review problem and mark the vent takeoff point before choosing pipe sizes.
Using the wrong joint method for a given material, such as solvent cementing ABS to PVC, threading plastic pipe, or using a mechanical joint where the code requires a specific listed transition fitting.
The joint-method tables are dense and candidates rely on field habit rather than the code, missing that dissimilar plastics need a listed transition fitting and that some materials are prohibited from threaded joints.
What to do instead
Build a materials-to-joints matrix from the code: for each material (ABS, PVC, cast iron, copper, PEX, galvanized steel) list the permitted joint methods and the prohibited ones. Review the transition rules for dissimilar materials separately, and drill with questions that show a joint and ask whether it is permitted.
Misapplying fixture clearances and installation dimensions, such as water closet centerline distance from sidewalls and compartment size, or lavatory and shower spacing.
These dimensions are scattered across the fixture chapter and its tables, and candidates answer from memory of a typical bathroom layout instead of the code minimums.
What to do instead
Make a one-page fixture dimension sheet: water closet centerline to sidewall and to adjacent fixtures, compartment width and depth, lavatory spacing, shower and bathtub access dimensions, and drinking fountain height. Rehearse each dimension against the code table until the numbers are exact, and pay attention to whether the question asks for a minimum or a maximum.
Answering Montana licensing and administrative-rule questions using general trade knowledge instead of the specific statute and rule provisions, such as who may pull a permit, journeyman supervision ratios, and continuing education requirements.
Candidates study the plumbing code intensively and skim the state law and administrative rules, assuming the questions will be general; the exam tests specific statutory definitions, license categories, and disciplinary provisions.
What to do instead
Read the Montana plumbing statutes and the Board of Plumbers administrative rules directly, and outline them by topic: license types, qualifications, examination and renewal, supervision and apprenticeship ratios, permit and inspection requirements, and grounds for discipline. Convert each provision into a one-line fact and self-test on the facts rather than on general impressions.
Confusing vent types and their sizing or installation limits, such as an individual vent, a common vent, a wet vent, a circuit vent, and a stack vent, and applying the wrong rules to each.
Vent terminology overlaps and candidates memorize definitions without attaching the installation limits, size rules, and prohibited arrangements to each type.
What to do instead
Create a vent-type comparison table with columns for definition, permitted use, sizing rule, slope and connection requirements, and common prohibitions. For each practice question, name the vent type first, then apply that column of the table. Pay special attention to wet vent sizing and to the rules for combining vents above the highest fixture.
Ignoring the distinction between minimum and maximum requirements and between permitted and prohibited installations when reading a question stem.
Candidates read for the topic rather than the qualifier, so a question asking for the maximum developed length or the largest permitted size is answered with the minimum value they memorized.
What to do instead
Underline or mentally flag every qualifier in the stem: minimum, maximum, shall, shall not, permitted, prohibited, and the specific fixture or material named. Before selecting an answer, restate the question to yourself in one sentence that includes the qualifier, then check the code table for that exact direction.
Sizing water supply piping without accounting for pressure loss from developed length, elevation, and fixture pressure requirements, or skipping the check that the residual pressure at the most remote fixture meets the minimum.
Candidates size from fixture units and stop there, forgetting that the supply system must deliver a minimum residual pressure at the furthest fixture after friction and static losses are subtracted.
What to do instead
Use a four-step method on every supply problem: determine demand in supply fixture units, select the meter and building supply size, compute friction loss over the developed length plus elevation loss, and verify the residual pressure at the most remote and highest fixture meets the code minimum. Show the arithmetic so the final check is never skipped.
Failing to verify that materials and fixtures are listed and approved, and missing the code requirement that installations follow the manufacturer's listing and the adopted standard.
Candidates treat material questions as pure identification and forget that approval, listing, and third-party certification are part of the code requirement, so they choose an otherwise plausible but unlisted material.
What to do instead
For every material or fixture question, add two checks before answering: is the product listed and approved for the application, and does the installation follow the listing and the referenced standard? Review the code sections on approval of materials and on prohibited materials, and practice identifying answers that are technically possible but not listed.