Plumbing Plan Line Codes, Fixture Tags and Inverts

Why this matters

A plumbing plan carries several piping systems on one floor outline, and it tells them apart with line codes, separates them across sheets by system, and ties every fixture to a schedule by a short tag. The numbers that matter most for underground work, the invert elevations, are written in a form many residential plumbers have never had to read. A shop that misreads a line code prices vent as waste, one that skips the fixture schedule misses a trap primer, and one that reads an invert as a depth from the floor without checking the datum digs to the wrong level. All of it is on the sheet.

The legend slice

Every set's P-001 or general plumbing sheet carries its own legend, and it governs. A typical excerpt:

PLUMBING LEGEND (excerpt) - this set's legend governs

 ----CW----   DOMESTIC COLD WATER
 ----HW----   DOMESTIC HOT WATER
 ---HWR----   HOT WATER RETURN (RECIRCULATION)
 __________   SANITARY WASTE (SAN OR W)
 - - - - -    VENT (V)
 ----ST----   STORM DRAIN
 ----G-----   FUEL GAS
 WC-1         FIXTURE TAG, SEE FIXTURE SCHEDULE
 (FCO)        FLOOR CLEANOUT
 (WCO)        WALL CLEANOUT
 [FD-1]       FLOOR DRAIN, TYPE PER SCHEDULE
 INV 96.25    INVERT ELEVATION, DECIMAL FEET
 (R-1)        RISER TAG, SEE RISER DIAGRAM

Line codes: letters and patterns

Water and gas lines are commonly drawn as a line broken by letters (CW, HW, HWR, G), so the system is spelled out along the run. Drainage is commonly drawn by pattern: a solid line for sanitary waste and a dashed line for vent, sometimes with the letters added. Storm drainage carries ST or SD.

Three cautions that come up on almost every set:

• The pattern is the firm's choice. Some sets draw below-floor waste dashed and above-floor waste solid, and give vent its own dash-dot pattern. Do not carry a dashed-means-vent habit from the last job.
• Letters collide across trades. FD is a floor drain here and a fire damper on the mechanical sheet; CO is a cleanout here and carbon monoxide on a fire alarm or detection drawing.
• Hot water return is easy to miss. On a recirculated system the HWR line runs beside the HW line, often at a smaller size, back to the water heater. A takeoff that counts HW and not HWR is short a full parallel run.

Sizes, flow arrows and cleanouts along a run

• A size note holds until the next one. "3 in SAN" written once on a run applies downstream until another size appears. Where the size changes, the drawing commonly shows the new size just past a branch connection; that point is where the fitting changes, so a takeoff that carries one size the whole length is short the transition fittings.
• Sizes are nominal. 3 in is the pipe's trade size, not its outside diameter, and the outside diameter differs between materials of the same nominal size. Sleeves and core holes are sized from the material the specification names, not from the number on the plan.
• Flow arrows on drainage show direction toward the building drain. A run with no arrow takes its direction from the inverts: flow runs from the higher invert to the lower one.
• Cleanouts are drawn where the code and the engineer want access: at the upper end of a run, at changes of direction, at the base of stacks, at the building exit. Their location is checked at plan review and at inspection, so a cleanout moved in the field to suit a wall is a cleanout the inspector may reject. If one will not land where it is drawn, ask first.

Why waste and vent are often on a different sheet from water

On all but the smallest jobs the plumbing engineer splits the systems: one plan for sanitary waste and vent, one for domestic water, sometimes a third for gas or for below-slab work. Two reasons. The sheets get unreadable with every system on one outline, and the systems are installed at different times: underground waste goes in before the slab is poured and is inspected before it is covered, while water piping usually runs overhead and is roughed later. A crew pricing or roughing from one plumbing sheet should assume another one exists until the sheet index says otherwise. See Sheet Numbering and Discipline Letters on a Drawing Set.

Fixture tags and the fixture schedule

Every fixture on the plan gets a tag (WC-1, UR-1, LAV-2, FD-1, MS-1, EWC-1), and the fixture schedule gives that tag's description, manufacturer basis, mounting, and the connection size for each system it uses: cold water, hot water, waste, vent. Notes in the schedule carry the items that never appear as a drawn line, such as a trap primer for a floor drain, a thermostatic mixing valve at a lavatory, or an accessible mounting height. See How to Read a Schedule on a Drawing Set.

Count fixtures from the tags, and take connection sizes from the schedule rather than estimating them from the drawn line, which may be diagrammatic.

Inverts, datums and depth

An invert elevation is the elevation of the inside bottom of a pipe, the level the flow runs along. On plumbing and civil sheets it is commonly written in decimal feet, as "INV 96.25" or "IE 96.25", against a project datum: a reference elevation stated on the general or civil sheets. Many building projects set the first-floor finish floor at an assumed 100.00, so an invert of 96.25 sits 100.00 minus 96.25, or 3.75 ft, below the floor. That 100.00 is a common convention, not a rule; read the datum note on your set before converting anything. On a set tied to real survey elevations, the floor may be at 742.50, and an invert of 738.75 means the same depth.

Decimal feet are not feet and inches. 96.25 ft is 96 ft 3 in, because 0.25 x 12 = 3 in. An architect's sheet may write the same level as 96'-3". Two inverts on one run, subtracted, give the fall between them; turning that fall into a slope is its own card. See How to Work Out Drain Slope from Invert Elevations.

A worked read: a restroom and its building drain

A plumbing shop is taking off a men's restroom on P-101 (waste and vent) and P-102 (water). The plan shows tags WC-1 twice, UR-1 once, LAV-2 twice and FD-1 once. The 4 in building drain leaves the room toward the building wall, with a floor cleanout at the room end marked INV 97.10 and the wall exit marked INV 96.25. The general notes set finish floor at 100.00.

The fixture schedule on P-601 gives (sizes are this set's, read from its schedule):

Takeoff. Six fixtures (2 + 1 + 2 + 1), so six waste connections and six vents. Cold water connections: two water closets, one urinal, two lavatories and one trap primer line to the floor drain, which makes 2 + 1 + 2 + 1 = 6. Hot water: the two lavatories, plus their mixing valves. The trap primer line is the item most often missed, because on P-102 it may be drawn as a short line or only a note at the drain, and the floor drain itself carries no water line on P-101.

Inverts converted. Room end: 100.00 minus 97.10 is 2.90 ft below finish floor, and 0.90 x 12 = 10.8 in, so about 2 ft 10-3/4 in. Wall exit: 100.00 minus 96.25 is 3.75 ft, which is 3 ft 9 in, or 45 in. The drain falls 97.10 minus 96.25, or 0.85 ft, which is 0.85 x 12 = 10.2 in, along the run.

What the depth means in the trench. The invert is the inside bottom of the pipe, so the trench bottom sits lower by the pipe wall plus the bedding the specification calls for. At the wall end that puts the trench past 45 in and, with a few inches of bedding, past 4 ft. Under 29 CFR 1926.651(c)(2), a trench 4 ft or more deep needs a stairway, ladder, ramp or other safe means of egress within 25 ft of lateral travel for anyone in it, and a competent person inspects it; at 5 ft or more it also needs a protective system under 1926.652 unless it is in stable rock, and a shallower one needs it too when the competent person finds any sign of cave-in potential. Before anyone digs, the utility locate is requested and marked. If the floor is already poured and this is a saw-cut job, cutting concrete releases silica dust, which is controlled with water on the blade and handled under 29 CFR 1926.1153.

What would change the read. If the datum note set finish floor at 98.00 instead of 100.00, the same INV 96.25 would be only 1.75 ft below the floor, and the shallow end would barely clear the slab. The invert number does not change; its meaning does. That is the whole reason to read the datum first.

Where the riser picks up

Vertical piping does not read on a plan, so the plan marks where it happens with a riser tag, such as R-1, at a wall or chase, and the riser diagram or isometric on a 6-series sheet shows that stack floor by floor with sizes, branch connections and cleanouts. When a line on the plan ends at a tag rather than a fixture, it has gone vertical, and the rest of its story is on the riser. See How to Read a Riser Diagram.

Checking your read

Before pricing or roughing: the legend's line codes are confirmed for this set; every fixture tag is counted and matched to a schedule row with its connection sizes; every schedule note (trap primer, mixing valve, mounting) has a line on the takeoff; every invert is converted against the stated datum, with the conversion written beside it; and every line ending at a riser tag is followed onto the riser diagram. Gas piping on the legend is sized, installed and tested under your jurisdiction's adopted fuel gas code, by a fitter licensed for it where your state or locality licenses gas work. Acceptance is a takeoff and a depth figure that each trace to a tag, a schedule row or a datum.

References

• 29 CFR 1926.651 and 1926.652 (excavation means of egress and protective systems) and 29 CFR 1926.1153 (respirable crystalline silica, construction)
• Your jurisdiction's adopted plumbing code and fuel gas code
• See related: How to Read a Riser Diagram; How to Read a Schedule on a Drawing Set
• See related: How to Work Out Drain Slope from Invert Elevations; Line Types and Line Weights on a Construction Drawing