ABS, PVC and the Mixed Drain Joint

Why this matters

Black drain pipe glued straight into a white fitting is one of the most common things found when a wall comes open, and it is usually the mark of a repair somebody made with whatever was on the shelf. Some of those joints hold for decades. Others weep slowly into a wall cavity until the subfloor is soft, and the one person in a position to catch it is the tile setter, the drywall crew or the electrician who opened the wall for something else. Closing the wall over it makes that trade the last one to have seen it.

Before you cut, glue or close anything

Solvent cement and primer. The cements that join plastic drain pipe are solvent blends that are highly flammable, give off vapors heavier than air that collect low in a room, and cause dizziness and headache in an enclosed space. Ventilate with fresh air across the work, keep every ignition source away, including the pilot of a gas water heater or furnace in the same room, which the vapor can reach along the floor, wear the gloves the cement's safety data sheet names, and cap the can between uses. If a closet or a crawlspace cannot be ventilated, the joint waits for someone who can set up forced ventilation.

Opening a drain. A drain or vent line that is cut or pulled apart can release sewer gas and the residue inside it. Gloves and eye protection when opening one, and no face over an open pipe.

Cutting. Cut plastic drain pipe with a saw or a ratcheting cutter, not with heat. Heating PVC releases hydrogen chloride, and burning either plastic gives off toxic smoke.

Telling them apart

Both have been used for residential drain, waste and vent piping since the 1960s and 1970s, and which one a region favored varied, so a single house often carries both after a few repairs. Read the print on the pipe rather than trusting the color alone.

A note on old ABS: a run of ABS drain pipe produced in the mid-1980s was the subject of litigation over cracking, typically showing as circumferential cracks near fittings. A crack in old black pipe is not automatically that problem, but it is a reason to photograph the print.

Why the two are not simply glued together

Solvent welding is not gluing in the way wood glue is. The cement's solvents soften the surfaces of the pipe and the fitting, the softened plastic of the two parts flows together, and as the solvent evaporates they fuse into one piece. Each cement is formulated for the plastic it is meant to soften. PVC cement on ABS, or ABS cement on PVC, softens one side well and the other poorly, and the result can look like a finished joint while being fused only partway around or only part of its depth.

There are two recognized ways to join the two materials, and which one is permitted depends on where you are:

• A transition solvent cement, formulated to join ABS to PVC in non-pressure drain applications, made to ASTM D3138 and usually colored green so an inspector can see it was used.
• A mechanical joint, typically a shielded transition coupling: a rubber sleeve sized for the two outside diameters, wrapped by a stainless steel shield and clamped with bands, listed for joining the two materials.

The model plumbing codes handle the ABS-to-PVC joint in their section on joints between different materials, and they do not all treat it the same way: some permit a listed transition cement only in certain parts of the system, and some expect a mechanical joint. Read the joints-between-different-materials section of the plumbing code your jurisdiction adopted, and the cement's own label, which states where it may be used. For any trade other than plumbing, the practical answer is simpler: you do not make this joint, you recognize it.

How to recognize a bad mixed joint

Look for any of these where black meets white:

• Purple primer and clear or gray cement on both sides, with no green cement and no mechanical coupling. That is a PVC joint made on an ABS pipe.
• A plain rubber coupling with no metal shield, clamped onto the two materials inside a wall or ceiling. Unshielded couplings are made for specific uses and are generally not what the codes expect for a concealed transition.
• Signs of leakage: a mineral trail or stain running down from the joint, darkened framing below it, a soft or discolored subfloor, a sewer odor in the cavity.
• A joint that has started to pull apart, showing a gap at the fitting's edge or a ring of bare pipe that used to be inside the hub.
• Movement. A joint that shifts when the fixture above is used is failing. Do not twist or pull a joint to test it; watching it while someone runs the fixture tells you the same thing without breaking it.

None of these is your diagnosis of the joint. Each is a reason to stop and call the plumber before the wall closes.

What other trades find, and what they do

• Tile and tub surround crews open a wall behind a tub or shower valve and find the tub's drain tied into an older stack.
• Drywall and remodel crews find a mixed joint in a wet wall behind a kitchen or a laundry.
• Electricians and HVAC crews drilling or running a line set through a wet wall find the drain in their path.

The rules are the same for all of them. Do not tape it, wrap it, coat it in epoxy putty or caulk the stain. Do not strap your own work to the drain or lean anything on it. Do not close the wall over a weep or a stain. Photograph the joint with the print and the cement colors visible, note its position, and route to plumbing. When telling the customer, say what you saw and why the wall stays open; the conversation itself has its own card.

Worked example: a tub wall in a 1978 house

A tile crew is replacing a tub surround in a 1978 house. With the old surround and the backer off, the wall cavity shows the tub's drain assembly in white PVC running into a black ABS drain line.

What they see. The PVC tub drain is cemented directly into an ABS fitting. Purple primer shows on both the white pipe and the black fitting, and there is no green cement and no coupling. A mineral trail runs from the bottom of the joint down the black pipe, and the subfloor around the drain is darker than the rest of the cavity floor.

The joint sits about 14 inches above the subfloor, and the stain runs 9 inches down from it. That leaves 14 minus 9, or 5 inches, of clean pipe between the bottom of the stain and the subfloor, so the stain does not reach the floor along the pipe, which suggests the darkened subfloor came from water dripping off the pipe rather than running down it. The crew lead presses the subfloor with a thumb rather than probing it: one patch about 6 inches by 8 inches feels soft. Nobody runs the tub to test the joint: a dry minute would prove nothing, and a weeping joint would only add water to a floor that is already soft.

The call. The crew does not close the wall, does not seal the joint, and does not try to remake it with the cement on their truck, which is the wrong cement for this joint anyway. They photograph the joint with the purple primer, the absence of green cement, the stain and the soft patch, write "PVC tub drain appears to be solvent-welded into ABS fitting with PVC primer, no transition cement or coupling visible, staining below joint, soft subfloor about 6 by 8 inches," and call the plumber.

What happened next. The plumber cut out the joint and replaced it with a shielded transition coupling, which was what the local code permitted for that location, and the soft subfloor patch was cut out and replaced. The tile schedule slipped one working day.

What the other path would have cost. Had the crew closed the wall, the joint would have kept weeping into the cavity under a new tile surround, and the next repair would have meant cutting out new tile to reach a joint that was in plain view on demolition day.

What would have changed the call. Green cement at the joint, with the pipe and fitting fully seated and no stain, is a transition joint made the intended way; the crew photographs it and carries on, leaving the question of whether it suits that location to the plumber's knowledge of the local code. A shielded coupling, clean and dry, is likewise a recognized method. A black-to-white joint below a basement slab or outside the house is outside what an interior trade sees, and the same routing applies.

How to check you got this right

• The materials are identified from the print, not only the color.
• The photos show the cement colors at the joint, since green, purple or none is the evidence a plumber or inspector needs.
• Nothing was taped, coated, remade or tested by twisting.
• The wall stayed open until the plumber had seen the joint.
• Every solvent can in the room was capped and every flame source was off while cement was in use.

References

• ASTM D2661 and D2665 (ABS and PVC drain, waste and vent pipe), ASTM D2235 and D2564 (ABS and PVC solvent cements), ASTM D3138 (transition solvent cement for ABS to PVC non-pressure joints).
• The joints-between-different-materials section of the plumbing code your jurisdiction has adopted.
• The solvent cement's safety data sheet, for ventilation, ignition and glove requirements.
• See related: Recognize, Record, Route: The Rule for Legacy Materials; Telling a Customer About an Old Material Without Scaring or Understating.