DWV Fittings for Each Change of Direction

Why this matters

The wrong fitting in a drain does not leak on the day it goes in. It strands paper where a branch crosses a run, it backs a stack up into the lowest fixture during a big discharge, and it fails the rough-in inspection when the inspector sees a sanitary tee lying on its back. The rule is short and almost every plumber knows it in outline: the fitting depends on which direction the flow is turning. This card states it as a rule card from IPC Table 706.3 and UPC Section 706, as adopted by your jurisdiction, explains why each pairing exists, and runs it against a real bathroom layout. Fitting names and shapes are on 'Common Plumbing Fittings Reference'; this card assumes you know a sanitary tee from a combo.

The three turns

Every change of direction in a drain is one of three, named for the direction of flow before and after the fitting.

 HORIZ TO VERT       VERT TO HORIZ       HORIZ TO HORIZ
 sanitary tee OK     long sweep, or      wye + eighth bend
                     wye + eighth bend   or combo
      |                   |
      |                   |                  \  branch
 -----+ branch            |                   \
 arm  |  curve turns      \__                  \
      |  flow down           \__          ------+------>
      v                         ----->       main run
    stack                     drain

Horizontal to vertical is a horizontal branch entering a vertical stack or drop: a lavatory arm into the stack in the wall, a toilet's horizontal drain into the stack. A sanitary tee is permitted, as are wyes, combos and long and short sweeps.

Vertical to horizontal is a stack turning to run horizontally at its base: the stack foot in a basement, a drop under a floor turning toward the building drain. A long sweep, a wye with an eighth bend, or a combination wye and eighth bend (combo) is permitted. A sanitary tee is not.

Horizontal to horizontal is one horizontal pipe joining another, or a horizontal run changing direction in plan: a kitchen branch joining the building drain, a drain turning a corner in a crawlspace. A wye with an eighth bend, a combo, a long sweep or eighth and sixteenth bends are permitted. A sanitary tee is not.

The one-line card. A sanitary tee is a horizontal-to-vertical fitting only. Lying on its back it is not a horizontal fitting, and rolled so its run is horizontal it is not a stack-base fitting.

Reading the adopted table rather than the card

IPC 2021 Table 706.3 lists each fitting against the three directions, with footnotes that allow some short-radius fittings (quarter bends and short sweeps) in certain directions only in limited sizes, for example small fixture drains. UPC 2021 Section 706 states the same three cases in prose. The rule card above is where both codes agree; the footnotes are where they and their editions differ, and they decide whether a short sweep may be used for a given turn at a given size. Read the footnote for the size in front of you rather than extending a short-radius allowance from a 2 in fixture drain to a 3 in branch.

Back-to-back fixtures. Both codes restrict a double sanitary tee (a sanitary cross) receiving back-to-back fixtures, because the discharge from one shoots straight across into the other. The IPC's exception for back-to-back water closets turns on the developed length between the toilet outlet and the fitting; read the current edition's figure before using one. A double combination wye and eighth bend is the usual answer on back-to-back toilets.

Vent piping. A vent that never carries water is not a drain, and the drainage fitting pattern does not govern a true dry vent in the same way. A vent that can carry water (a wet vent, the part of an island vent below the flood level rim) is built with drainage fittings and drainage slope.

Why each pairing exists

Horizontal to vertical, sanitary tee allowed. Flow in a horizontal branch is slow and shallow. The sanitary tee's curved inlet turns it downward into a pipe where gravity takes over, and the vertical run below has room for the discharge to fall without backing into the branch. The tee's short radius costs little here because velocity is low going in.

Vertical to horizontal, long pattern required. Water falling down a stack picks up speed. At the base it must turn and slow to the flow depth of a horizontal pipe at 1/4 in per ft or less, and a short turn makes that transition abruptly: the flow piles up at the turn and can momentarily fill the pipe, pushing air back up the stack and pulling water from trap seals on the lowest fixtures. A long sweep or a wye and eighth bend spreads the turn out and keeps the pipe from running full at the turn. This is also why a basement fixture tied in right at the foot of a busy stack is the one that bubbles during a second-floor flush.

Horizontal to horizontal, sanitary tee banned. Laid on its back, the tee brings the branch in square to the main. Branch flow shoots across the main and against the far wall instead of joining it, and when the main is running, its flow drives across the branch opening and up into it. Solids stall at that junction, which is where a recurring kitchen line stoppage in an old house often turns out to be. A wye joins the branch at 45 degrees so both flows travel the same way.

Worked example: a second-floor bath to the basement drain

A 1950s house getting a second-floor bath over the kitchen. A new 3 in stack drops through a closet to the basement, then runs horizontally to join the existing 4 in building drain, which already carries the kitchen line. Fixtures upstairs: lavatory (1-1/2 in trap, 2 in drain), tub (1-1/2 in trap), toilet (3 in).

Change of direction by change of direction:

1. Lavatory arm into the stack. Horizontal to vertical. 3 in x 2 in sanitary tee in the wall, its vent leg continuing up as the stack vent. Allowed.
2. Tub arm into the stack. Horizontal to vertical, below the lavatory. 3 in x 1-1/2 in sanitary tee. Allowed.
3. Toilet into the stack. Horizontal to vertical. 3 in sanitary tee, single, not back to back. Allowed.
4. Stack base turning horizontal in the basement. Vertical to horizontal. A 3 in long sweep quarter bend, or two eighth bends. The draft drawing showed a 3 in short quarter bend here; that is replaced.
5. Horizontal run turning 45 degrees around a basement column. Horizontal to horizontal. 3 in eighth bend. Allowed.
6. New 3 in branch joining the existing 4 in building drain. Horizontal to horizontal. 4 in x 3 in wye with a 3 in eighth bend, or a 4 in x 3 in combo. The helper had cut in a 4 in x 3 in sanitary tee on its back; that comes out.
7. Cleanout. At the base of the stack, in the run just downstream of the turn, accessible.

How the tub and toilet are vented on a stack that also carries the lavatory above them is a separate venting check (a vertical wet vent in the codes' terms), not a fitting question; see 'Wet Venting a Bathroom Group Under IPC and UPC'. The fitting at each junction is the same either way.

The count. Seven items, six of them fittings that turn the flow (items 1 to 6) plus the cleanout. Two of the six as first drafted were wrong, and both were on the two turn types where the sanitary tee or short bend is not allowed (items 4 and 6). The three sanitary tees in the stack, which look the most like mistakes to a homeowner, are the correct ones.

Fit check before glue. A wye and eighth bend at item 6 takes more length along the drain than the sanitary tee did, so the existing 4 in pipe is cut back further. Lay the fittings out dry, mark both, and only then cut; a combo is shorter than a separate wye and eighth bend if space along the drain is tight.

What would change the answer. If the building drain were 3 in rather than 4 in, item 6 becomes a 3 in x 3 in wye and eighth bend, and the bigger question becomes whether 3 in carries the total load, which is sizing, not fittings (see 'Drainage Fixture Units for a House and the Bathroom Group'). If the new toilet had been placed back to back with an existing one on the far side of the closet wall, item 3 becomes a double fitting and the back-to-back restriction above applies.

Where the field gets this wrong

• The tee rolled up "a little" for fall. A sanitary tee at the stack, rotated so its branch points slightly uphill to give a horizontal arm more slope, puts the fitting partly on its back. Set the tee plumb and get the slope from the arm.
• The combo used vertical to horizontal and installed upside down. A combo's long curve has to face downstream. Installed with the curve facing upstream it is a short turn against the flow.
• The stack foot in a slab that nobody saw. A short quarter bend under a slab at the base of the main stack is invisible until the lowest fixture in the house starts gurgling. On an old house with that symptom, camera the base of the stack before assuming venting.

How to verify you got this right

• Every change of direction on the rough-in is labeled with its turn type and the fitting used, and each pairing is one the adopted table allows at that size.
• No sanitary tee anywhere receives flow in a horizontal-to-horizontal or vertical-to-horizontal direction.
• Any short-radius fitting outside the horizontal-to-vertical case has a footnote in the adopted table allowing it at that size.
• The cleanout at the stack base is accessible after finishes.

References

• IPC 2021 Table 706.3 and Section 706, as adopted by your jurisdiction.
• UPC 2021 Section 706, as adopted by your jurisdiction.
• See related: Common Plumbing Fittings Reference.
• See related: Drain Pipe Slope + Venting Quick Reference; Drainage Fixture Units for a House and the Bathroom Group; Wet Venting a Bathroom Group Under IPC and UPC.