ICC P2 common mistakes
ICC P2 · Understand the traps and how to avoid them.
Answering questions on appendices, referenced standards, and local amendments as if every appendix chapter were mandatory code text, without first checking the scope and adopt-option language in Chapter 1.
Most study outlines reproduce appendix tables (frost depth, rainfall, sizing) side by side with main-body chapters, so candidates memorize the numbers but never learn that an appendix is enforceable only when the jurisdiction adopts it.
What to do instead
For every appendix-derived item, mentally preface the question with 'in a jurisdiction that has adopted this appendix.' Locate and reread the Chapter 1 scope and the appendix adoption statement once, then practice flagging which tables live in the body versus an appendix.
Treating 'readily accessible' and 'accessible' as synonyms when deciding whether a cleanout, valve, or appliance connection may be hidden behind a panel, lid, or covering.
The two terms read as ordinary English, and trade practice often buries cleanouts under finished covers, so experience-based instinct conflicts with the defined term.
What to do instead
Memorize the definitions as a pair with the differentiator: readily accessible means reached without removing a panel, lid, or similar obstruction and without use of tools; accessible permits removal of an access panel. On any question naming one of the two terms, underline it and test the answer against that single criterion.
Selecting a temperature and pressure relief valve discharge arrangement that reduces the pipe size below the valve outlet, traps the line, threads or caps the termination, or terminates more than 6 inches above the floor or outside.
Candidates remember 'drain it somewhere safe' as a concept but not the individual restrictions, and photos in field guides show insulated or downsized terminations that look normal.
What to do instead
Build a five-point checklist and run every relief-valve option through it: full size of the valve outlet, gravity flow with no traps, material rated for the discharge temperature, no threaded or capped end, and termination not more than 6 inches above the floor or to the exterior. Eliminate any answer that breaks one point.
Computing required fixture counts by applying the wrong occupancy row, or by using rule-of-thumb ratios such as 'one lavatory per 25 people' instead of working from the occupancy classification and the ratio tables.
Occupancy classification is a Chapter 3 definitions exercise that feels like a preliminary detail, so candidates skip straight to the fixture table and grab the row that best matches the building's name, not its actual use.
What to do instead
Always classify the occupancy first and write it down, then select the row for that occupancy, then apply the male/female split and any accessibility-driven addition. If two answers seem plausible, re-derive the count from the classification rather than from the building's title.
Mixing the flush-tank and flush-valve columns of the supply fixture unit table, or converting fixture units to gallons per minute without accounting for which fixture type dominates the building.
The two columns sit adjacent, the numbers look interchangeable, and a building with a handful of flush valves is easy to misclassify as a tank-only system even though the flush-valve column drives the demand.
What to do instead
Before touching the table, inventory the fixtures and split them into tank-type versus flush-valve-type. Total each column separately and use the higher-demand result. Practice with a blended building (mostly tanks, one flush-valve water closet) until the column choice becomes automatic.
Choosing a backflow preventer that cannot survive the installation conditions, such as an atmospheric vacuum breaker on a line under continuous pressure or with a downstream shutoff valve, or a device rated for a lower hazard than the actual risk.
All the device names sound protective, and candidates match them to the fixture (hose bibb, boiler feed, irrigation) instead of to the two governing variables: backpressure versus backsiphonage, and hazard level.
What to do instead
Answer in two steps: first decide whether the condition is backpressure or backsiphonage, then decide whether the hazard is a health hazard. That pair of answers narrows the device to one family. Reject any option whose device cannot be under continuous pressure when the question states continuous pressure.
Applying a single drainage slope to every pipe size, most often grading a 2-inch or 1 1/2-inch branch at 1/8 inch per foot, or flattening a 4-inch line below its minimum.
The 1/4-inch-per-foot figure is the one repeated in every training class, so the size-based break points get compressed into a single remembered number.
What to do instead
Memorize the slope table as a staircase keyed to pipe size, and rehearse it aloud: steeper slope for small pipe, shallower for large. On each slope question, state the pipe size first, then recall the slope; never recall the slope and assume the size.
Assuming a fixture drain can run indefinitely to its vent, or approving a trap arm length based on total fixture drain run rather than on the distance from the trap weir to the vent connection at the required slope.
The trap arm rules sit in the venting chapter but the measurement is a drainage dimension, so candidates look for the limit in the wrong chapter and never find one.
What to do instead
Redraw any trap arm question as two labeled points: the trap weir and the vent fitting. Measure only between them, apply the size-and-slope limit for that trap size, and ignore the rest of the drain run. If the option measured from the fixture outlet to the stack, discard it, because that is the wrong span.
Sizing roof drains, leaders, and horizontal storm piping with drainage fixture units or pipe-diameter rule of thumb instead of using the roof area multiplied by the design rainfall rate.
The sanitary chapters dominate study time, so the fixture-unit habit carries over into storm questions even though roof drainage is driven by area and rainfall intensity, and those two inputs appear only in the storm chapter.
What to do instead
Write the storm formula on scratch material before starting storm problems: roof area times rainfall rate gives the flow, and the flow selects the leader and drain size. Check that the question supplies a rainfall rate; if an answer option uses fixture units for a roof area, it is a distractor.
Answering secondary or overflow roof drainage questions as though the overflow system may connect into the primary system, terminate where the discharge is not visible, or be omitted when the primary drains are sized generously.
Because the secondary system carries no water in normal service, candidates treat it as optional detailing rather than as a separate code-required system with its own termination and independence rules.
What to do instead
Treat primary and secondary roof drainage as two separate systems on every question: separate piping, separate discharge, and discharge located where its flow is visible to occupants or maintenance staff. Any option that ties the two systems together is wrong regardless of how the sizing numbers look.