When an addition puts more fixtures on a house, someone has to answer whether the existing drain and lateral can carry them. The answer is a code calculation in fixture units against pipe diameter and slope, and a plumber or engineer does it on paper. A sewer camera does not do the calculation. What it does is supply the facts the calculation depends on and reveal the conditions in the pipe that make a textbook answer wrong.
Quick answer: the camera establishes the lateral’s actual diameter, whether the bore is reduced by scale, roots or deformation, whether bellies hold water, where the slope appears flat, and the condition of the material. It cannot measure slope or flow rate. A 4 in lateral in good condition handles most residential additions on paper; the camera tells you whether yours is in good condition, and that is the question the paper cannot answer.
Confirming the diameter
Records say 4 in; the pipe may be 3 in cast iron at the house, 4 in clay in the yard and 6 in at the tap. The camera shows the transitions and, with a known reference such as the head’s own size or a fitting, lets the diameter be read. Residential pipe diameters covers what to expect, and camera head sizes explains how the head’s size gives a scale.
Bore reduction
A 4 in pipe with an inch of scale on the bottom and roots at every joint is not a 4 in pipe for capacity purposes. The camera shows how much of the bore is open. Heavy scale in cast iron, tuberculation, grease in a kitchen line and a liner that was installed thick all reduce effective diameter, and a plumber calculating capacity needs to know. Interpreting footage helps judge the fraction.
Bellies
A belly is a section with no effective slope that holds water and solids. More fixtures discharging into a bellied lateral means more solids settling in it and a faster path to blockage. The camera finds and measures bellies; a plumber decides whether the addition’s load makes re-laying that section necessary. Detecting a belly shows the picture.
Slope evidence
The camera cannot measure grade, but it sees the evidence of poor grade: standing water, sediment on the floor of the pipe, a water line high on the wall. Where those appear along a stretch, the slope is suspect and a plumber may want to verify it with a level at the cleanouts or by excavation.
Condition
Capacity assumes a pipe that will stay a pipe. A clay lateral with offsets and root intrusion, or channeled cast iron, is going to fail regardless of load, and adding load brings the failure forward. The camera’s condition findings are what tell an owner that the addition’s budget should include a lateral, not just a bathroom. What the inspector cannot tell you sets the limits.
What the camera does not do
It does not measure flow, slope or fixture units, and it does not say whether the main in the street has capacity for the addition, which is the utility’s question. Capacity sizing is a plumber’s or engineer’s job using the code tables, and the camera report is one of their inputs.
Typical outcomes
A sound 4 in lateral with normal slope and no bellies carries a second bathroom or a rental suite without comment in most codes. A 3 in section at the house may need upsizing for a large addition. A lateral with heavy scale is often descaled and relined, which restores bore and condition together. A bellied or failing lateral is replaced, and the new one is sized for the whole house as it will be.
When the main is the limit
In older neighbourhoods the public main can be at capacity, and the utility may require a capacity letter before a permit for an addition. The camera at the tap shows whether the main is running full, which is a useful early warning, but the utility’s own records decide.
What to record
Diameter at each transition with distances. Material. The percentage of bore open at the worst points. Bellies with start, end and depth of standing water. Slope evidence. Condition findings. This is the report the plumber sizes from and the one the permit office may ask for.
Reading diameter from the footage
A camera head of known size gives a scale. A 23 mm head fills about a fifth of a 4 in pipe’s width and nearly a third of a 3 in pipe’s, and an operator who has seen both learns to tell them apart at a glance. Fittings help more: a wye or a cleanout tee has a standard geometry, and a change in material usually comes with a change in diameter that the camera shows as a step or a reducer. Where the diameter matters for a capacity decision and the footage is ambiguous, a plumber can measure the pipe at the cleanout with a tape and work from there.
Timing the inspection
Inspect before the architect finishes, not after the permit is applied for. A capacity question raised by the permit office after drawings are done costs a redesign; one answered by footage beforehand costs an inspection.
Common questions
Can a sewer camera tell me if my sewer can handle another bathroom?
Not alone. It tells you the diameter and condition; a plumber does the fixture-unit calculation. A sound 4 in lateral usually can.
Does scale in cast iron reduce capacity?
Yes, and the camera shows how much. Descaling and lining restore it.
Who decides if the public main has capacity?
The utility, from its own records. The camera at the tap can show whether the main is running full.
Related guides
See inspection before an ADU, inspection before a basement bathroom, and sewer cameras for home inspectors.