Layout, measuring, testing and inspection

By her third month, Maya was trusted to rough in a suite bathroom on the seventh floor of Kingsway Tower with Luis checking her work. She laid out the fixture locations from the drawings, set the closet flange and the drain branch, ran the PEX supplies from the manifold, put the system under test and called Luis to look. He checked her measurements against the drawings, sighted the slope on the branch, read the gauge and signed the test sheet. Then he told her that she had just done, in miniature, everything a plumber does on every job: lay it out, measure it, cut it, join it, support it, test it and record it.

This final lesson brings the course together. It covers layout from drawings, measuring and fitting allowances, rough-in dimensions, and the pressure and leak tests that every plumbing system must pass before an inspector will approve it.

Laying out from drawings

Plumbing layout starts with the drawings and the building's reference lines. On a large project, the surveyors mark gridlines and benchmark elevations on every floor, as TRD-103 described. Fixture locations are measured from gridlines, finished walls or column centers shown on the architectural drawings, and drain inverts and pipe elevations are measured from the floor benchmark.

A good layout routine is to mark the fixture centers and pipe routes on the floor or wall, check them against the architectural drawings (not just the plumbing drawings, which may be schematic), check for conflicts with framing, ductwork and other services, and confirm with the foreman before cutting holes. Laser levels and plumb lasers make it easy to transfer points from floor to ceiling and to check slope over long runs.

Remember that walls and floors have finishes that will be added later. A closet flange set to the bare slab will be too low once tile goes down; a fixture rough-in measured from a stud will be off once drywall and tile are on. Drawings and manufacturer's rough-in sheets say whether a dimension is from the finished floor or wall, and the plumber must allow for the finish thickness.

Rough-in dimensions

Every fixture has a rough-in sheet from its manufacturer showing where the drain and water supplies must be. The rough-in is the work done before walls are closed: drains, vents and supplies brought to the right place, capped and tested, ready for the fixtures to be set after finishes. Typical rough-in dimensions include:

Water closet: the outlet center is commonly 12 inches from the finished back wall, with the floor flange set level and at the right height for the finished floor. Side clearances from the center to a wall or other fixture are set by the code; the IPC requires at least 15 inches from the center to a side wall or partition.

• Lavatory: the drain stub in the wall, the supply stubs on either side, and heights that suit the vanity or wall-hung basin, all from the rough-in sheet.
• Bathtub and shower: the drain location from the tub manufacturer's sheet, and the mixing valve and showerhead heights from the specifications, adjusted for accessible units.
• Kitchen sink and dishwasher: drain and supply locations coordinated with the cabinet layout.

Accessible suites have different dimensions for heights, clearances and controls, set by the accessibility requirements in the building code and by the project. On Kingsway Tower, a number of suites are designed as barrier-free, and their rough-in sheets are kept separate so nobody uses a standard suite's dimensions by mistake.

Measuring pipe and fitting allowances

Pipe lengths are measured in a few standard ways. Center-to-center is the distance between the center lines of two fittings, which is what drawings and layouts usually give. End-to-end is the actual length of pipe. Between the two is the fitting allowance, also called take-off or make-up: the part of the center-to-center distance taken up by each fitting, after allowing for the depth the pipe goes into the fitting.

For socket fittings such as soldered copper, pressed copper or solvent-welded plastic, the take-off of a fitting is its center-to-end dimension minus the socket depth that the pipe enters. The cut length is then:

where C is the center-to-center distance and t is the take-off of each fitting. Fitting manufacturers publish take-off dimensions, and many plumbers keep a chart of the fittings they use most. For threaded pipe, lesson 8 showed the same idea using thread engagement.

Offsets, which move a run sideways or up and down around an obstacle, use the right-triangle math from TRD-102. For a 45-degree offset, the travel (the center-to-center length of the diagonal piece) is the offset multiplied by about 1.414; subtract the take-offs of the two 45-degree fittings to get the cut length. Rolling offsets, which move a run in two directions at once, find the true offset first from the two directions, then apply the same rule.

import math
from fractions import Fraction
# Cut lengths from center-to-center measurements and fitting take-offs (inches)
def s16(x):
    f = Fraction(round(x * 16), 16); w, r = divmod(f, 1)
    return f"{w} {r}" if r else f"{w}"
takeoff = {"3/4 90": 7/16, "3/4 45": 3/16, "1/2 90": 5/16, "1/2 tee": 5/16}
pieces = [
    ("riser tee to first 90", 30, "1/2 tee", "1/2 90"),
    ("supply run",            72, "3/4 90", "3/4 90"),
]
for name, c, f1, f2 in pieces:
    print(f"{name:22s} cut {s16(c - takeoff[f1] - takeoff[f2])} in")
# a 45-degree offset of 10 in around a beam flange
offset = 10
travel = offset * math.sqrt(2)
print(f"45-degree offset travel {s16(travel)} in, cut {s16(travel - 2 * takeoff['3/4 45'])} in")

riser tee to first 90  cut 29 3/8 in
supply run             cut 71 1/8 in
45-degree offset travel 14 1/8 in, cut 13 3/4 in

Measure twice, cut once is more than a saying in plumbing. A pipe cut short is wasted, and a pipe cut long is forced, which strains joints and supports. Mark cut lines clearly, keep measuring tapes in good condition, and work from the same reference throughout a run so small errors do not add up.

Testing new plumbing

Every new plumbing system is tested before it is concealed and before it is put in service. Tests prove that the joints are tight and the system will hold, and an inspector usually witnesses them or checks the record. Tests are done in stages: underground drainage before backfill, rough-in drainage and water piping before walls and ceilings are closed, and a final check when fixtures are set and the system is working.

Drainage and vent piping is commonly given a water test: all openings are plugged, and the system is filled with water to a head of at least 10 feet above the highest joint being tested (or to the top of the section), held for at least 15 minutes, and checked for any drop in level or leaks. An air test at 5 psi held for 15 minutes is allowed as an alternative for some materials. Tall buildings are tested in sections so the lowest joints do not see too much pressure from the water column.

Water supply piping is given a pressure test, usually with water, at not less than the working pressure, held for a set time with no drop on the gauge and no visible leaks; the IPC also allows an air test of at least 50 psi for piping other than plastic. The specifications often set more: on Kingsway Tower they call for 150 psi for two hours, recorded on a test sheet.

Air testing deserves particular care. Compressed air stores a lot of energy, and if a fitting or plastic pipe fails under air pressure it can fail violently, throwing fragments. Many plastic pipe manufacturers prohibit testing their pipe with compressed air or gas, and safety rules on many sites forbid it. Use water where possible, follow the manufacturer's instructions, use a pressure relief valve on the test rig, never exceed the specified pressure, and keep people clear of plugged openings and test caps.

Test plugs and caps must be rated for the pressure and installed correctly. A test plug blown out of a pipe under a water column can injure someone and flood the floor below. Gauges must be accurate and readable at the test pressure, and many specifications require calibrated gauges.

When a test fails, find the leak before doing anything else: walk the piping, look and listen at every joint, and for water piping watch the gauge while isolating sections. Fix the cause, which might be an unsoldered joint, an unpressed fitting, a cracked fitting or a joint not given its cure time, and test again. Never adjust a gauge, top up a water test or shorten the time to make a failing test look like a pass. That is falsifying a record, and the leak will still be there when the ceiling is closed.

# Check a water supply pressure test log: any drop over the test period fails
log = [  # (minutes, psi)
    (0, 150.0), (15, 150.0), (30, 149.8), (45, 150.0), (60, 150.0), (90, 150.0), (120, 150.0),
]
start = log[0][1]
lowest = min(p for t, p in log)
tolerance = 0.5          # gauge reading tolerance in psi (example)
print(f"start {start} psi, lowest {lowest} psi over {log[-1][0]} minutes")
print("PASS" if start - lowest <= tolerance else "FAIL: find the leak before retesting")

start 150.0 psi, lowest 149.8 psi over 120 minutes
PASS

Temperature changes affect pressure tests. Water in a pipe exposed to the sun or a cold draft changes temperature during the test, and the pressure rises or falls with it even with no leak. Testers allow the system to stabilize before starting the clock, and note the conditions on the record.

Inspection, flushing and disinfection

When a stage is complete and tested, the inspector is called. Have the system under test before the inspector arrives, the permit card and approved drawings available, pipe markings visible and access to every part of the work. The inspector checks materials, sizes, slopes, venting, supports, protection, firestopping and the test, and either approves the stage or lists deficiencies to correct. Only after approval may the work be covered.

Before a new water system is put into use, it is flushed to remove debris, flux and cutting oil, and in larger buildings often disinfected with a chlorine solution held for a set time and then flushed, with water samples sent for testing. The IPC, the local health and building authorities and the specifications set the procedure, often by reference to recognized standards for disinfecting water piping.

Finally, the plumber's records go into the turnover package for the owner: test reports, inspection approvals, as-built drawings showing any changes, valve tags and charts showing what each valve controls, and manufacturers' manuals and warranties for fixtures and equipment. A future service plumber trying to find a shut-off at two in the morning will be grateful for every accurate record.

Bringing it together

PLB-120 builds on this course with drainage and venting design, and PLB-130 with water supply. The materials, joints, supports and tests you have learned here are the foundation for both.