Finishing a basement converts the cheapest space in a house into living space, and it also converts a sewer backup from a wet concrete floor into a destroyed room. The drain under that floor and the lateral beyond it do not get easier to inspect or repair once there is flooring, framing and drywall over and around them. A camera inspection before the work starts is the cheapest line in the finishing budget and the one most often left out.
Quick answer: before finishing a basement, camera the building drain and the lateral to the main, inspect every floor drain and its trap, locate the drain route under the slab, and check the condition of any cast iron stack base that will be enclosed. Fix bellies, channeling and root intrusion before the floor goes down, consider a backwater valve if the main has ever surcharged, and leave accessible cleanouts and an access panel at the stack. Record everything as the baseline.
What a backup costs in a finished basement
An unfinished basement that takes a backup is cleaned, disinfected and dried. A finished one loses flooring, the bottom of every wall, insulation, trim, doors and whatever furniture was on the floor, and it may need mould remediation. Insurance often excludes sewer backup unless a rider was bought. The inspection that would have found the cause costs a few hundred dollars. See what an inspection proves for insurance.
The building drain
Camera from the main cleanout toward the street and, where there is access, back toward the stack and the floor drains. In a house old enough to have an unfinished basement worth finishing, the drain is likely cast iron, and the camera looks for channeling along the bottom, scale, cracked hubs and offsets. A drain in poor condition should be replaced while the floor is bare concrete and a saw cut is cheap. Cameras for cast iron and cast iron vs PVC on camera explain what the picture shows.
The floor drains
Every floor drain is a path for a backup into the room. Pull the grate and camera the trap and the branch to the building drain; a cracked trap, a branch full of debris or a drain that was never connected are all common. Decide which floor drains the finished layout keeps and make sure each has an accessible cover. Inspecting a floor drain covers the method and reading a floor drain backup the picture.
The lateral
Camera to the main. Roots, offsets and bellies in the lateral are what cause backups, and the finished basement is where the backup arrives. Any defect that is repair-grade should be repaired before the finish, because the alternative is repairing it after the first flood with the room already lost.
The main and the backwater question
If the camera reaches the tap and the main is seen running full, or if the house or the street has a history of surcharge flooding, a backwater valve belongs in the plan and it goes in while the floor is open. Inspection before a backwater valve covers the decision.
Locating the drain route
Use the sonde to paint the building drain’s route on the slab before framing. Walls should not sit over the drain where a future repair would need the floor cut, cleanouts must stay reachable, and an access panel belongs wherever the stack meets the floor. Photograph the painted route before it is covered and keep the photograph with the house records.
The stack base
A cast iron stack rusts from the inside at its base, where it turns into the building drain and where it is wettest. Once it is behind drywall it is invisible until it leaks. Camera it from the cleanout at its base or from above, and if it is thin, replace the base section before enclosing it.
Sump and drain tile
If the basement has a sump, confirm it is not connected to the sanitary drain, which is prohibited almost everywhere and which the camera can show. A sump that discharges into the sewer will push the sewer back into the room when the main is full.
Leaving access
Every cleanout stays accessible through a flush cover or an access door. The stack base gets a panel. The backwater valve, if fitted, gets a lid in the floor. The future camera inspection needs a way in, and so does the plumber who will one day need to clear a line without cutting the finished floor.
What to record
The footage of the drain, floor drains and lateral with distances, the painted route photographed, the depth marks, the condition of the stack base, what was repaired and the after footage, and the location of every cleanout and panel under the finished surfaces. This is the folder that makes the finished basement inspectable and repairable later.
Common questions
Should I inspect the sewer before finishing a basement?
Yes. A backup after the finish destroys the room and is often uninsured; the inspection costs a fraction of one square of flooring.
Do I need a backwater valve?
If the main has ever surcharged into the house or the street floods in storms, strongly consider it, and install it while the floor is open.
Can I inspect it myself?
With a reel, yes, and a sonde makes the route mapping possible. The buy vs hire calculator helps decide.
Related guides
See inspection before a basement bathroom, inspection after a basement flood, and floor drain and main line inspection for basement homes.