A drain belly is a localised low spot where a section of pipe has sagged below the original design gradient, creating a dip that holds water and sediment between uses. Causes include settlement of bedding material, ground movement, poor original installation and undermining by leaks elsewhere. The dip slows flow through the affected length, allowing solids to settle and giving rise to recurring blockages even after clearance. Building Regulations Part H1 and BS EN 752 specify minimum gradients for foul and surface water drainage to maintain self-cleansing velocities. Engineers usually identify bellies during a CCTV drain survey by observing standing water and reverse fall on the camera footage. Repair options include localised excavation to reset the gradient, or partial relining where the surrounding bedding has stabilised.
Drain bellies (low spots in pipe)
Localised low section in a drain where the pipe sags below the design gradient.
