Before no-hub couplings and rubber gaskets, cast iron drain pipe was joined by hand: the spigot of one pipe went into the hub of the next, oakum, a tarred hemp rope, was packed into the gap with a yarning iron, and molten lead was poured on top and caulked tight. Done well, a lead-and-oakum joint lasts as long as the pipe. Done less well, or after the oakum has dried and shrunk over seventy years, it loosens, leaks, admits roots and lets the pipe sections move. A camera sees these joints from inside, and the signs of a failing one are specific.
Quick answer: a sound lead joint shows on camera as a smooth step at the hub with a continuous ring of lead or packed oakum visible in the gap, no root, no stain and no water entry. A failing joint shows loose or missing oakum fibres hanging into the bore, a dark gap between the lead ring and the pipe, root hair entering at the ring, a stain or scour mark below it, and sometimes a visible offset where the spigot has moved in the hub. Failed lead joints are the usual root and infiltration entry in cast iron, and the usual repair is lining, which seals every joint, or a no-hub coupling at a single bad joint.
What a sound joint looks like
The hub is a bell at the end of one pipe; the next pipe’s spigot sits inside it. From inside, the camera sees the pipe wall step up to the hub and back down to the next pipe, with the joint material filling the annular gap between them. In a sound joint that filling is a continuous ring: grey lead at the outer edge, sometimes with a darker line of oakum inside it, flush with or slightly behind the pipe wall. There is no gap, no fibre, no root and no stain. A scaled cast iron pipe often hides this entirely under tuberculation, and the joint is then judged by what passes through it rather than what is seen. The tuberculation guide covers that case.
How the joint fails
The oakum dries, shrinks and loses its seal. Ground movement works the spigot in the hub and the lead ring loosens or cracks. Water finds its way out through the loosened packing, carrying soil and undermining the pipe, and in through it when the ground is wet. Roots follow the moisture and grow into the oakum, which they find easy to penetrate. The sections begin to move relative to each other and an offset develops. In a horizontal line under a slab this can go on for decades; in a vertical stack the joint usually holds longer because there is less movement and no soil load. The offset guide covers the displacement stage.
The signs on camera
Loose fibre: strands of oakum hanging into the bore at the joint, pale or dark, often mistaken for root hair. Root hair: fine roots entering at the lead ring, which is where the seal has failed. A gap: a dark crescent between the lead and the pipe wall. A stain: discolouration or a white deposit below the joint where water has been moving. A scour: a clean patch where water has washed the wall. Displacement: the inner circle off centre. Any one of these is a joint to note; two or more is a failing joint. Dim the LEDs so the gap and the fibre show against the grey lead, and use a self-leveling head so the position of the failure around the joint is recorded.
Oakum fibre or root?
Both are strands at a joint. Oakum is dark or tar-coloured, lies flat or hangs limply, does not move in the flow and ends at the joint. Root hair is paler, waves in the flow, branches, and often continues along the pipe. The distinction matters because loose oakum is a seal failure that lining fixes and roots are a seal failure that has already let something in. The root guide shows the stages.
Repair options
A single failed joint in an accessible line is repaired by cutting the pipe and installing a no-hub coupling or a new section. Multiple failing joints in a buried or under-slab line are the classic case for lining, which seals every joint at once without excavation, provided the pipe is round and not channeled. Re-caulking a lead joint is possible in exposed pipe and rarely done. And a stack with failing joints at every floor is usually replaced, because lining a vertical stack with branches is complex. The camera pass that counts the failing joints and records their distances is the specification for whichever route is chosen. Repairing cast iron drains walks through the options.
What to record
The counter reading at each joint showing fibre, root, gap, stain or displacement, graded sound, suspect or failing. The count of failing joints is what decides between a single coupling and a liner, so every joint belongs in the record, not just the worst.
Common questions
Do lead joints leak?
When the oakum dries and the lead loosens, yes, outward in dry weather and inward in wet.
Are lead joints a lead hazard?
Not in a drain line. The lead is on the waste side and nothing from it enters the water supply.
Can roots get through a lead joint?
Through the oakum once it has dried, yes. The lead ring itself resists them; the packing behind it does not.
Related guides
See the best sewer cameras for cast iron pipes, inspecting a cast iron stack from the roof, and cast iron vs PVC on camera.