What is a drainage settlement compensator?

A drainage settlement compensator is a movable connection between the drainage inside the building and the drain outside it. It makes sure no strain is put on the pipe when the ground outside settles relative to the building itself.

The Dutch term is polderstuk, named after the situation it was devised for: soft polder ground where the surface sinks while the piled house stays put.

What it looks like

The usual arrangement is two 90-degree bends with a length of pipe of about half a metre between them. The joints are made with slip couplings: rubber sleeves in which the pipe can slide and articulate slightly.

The result is a loop that takes up vertical movement. When the external drain settles, the assembly moves with it instead of snapping off at the wall.

When it is used

• On soft, settlement-prone ground — clay and peat
• Where the building is piled but the surrounding ground is not
• On short runs where there is little length over which to distribute the movement
• Where the drain is fixed to the foundation or the slab and therefore follows the movement of the building

Practical points

• Pre-set it high. If significant settlement is expected, the assembly is deliberately laid slightly high so that after settlement it still falls correctly and does not sag.
• Fall still matters. A loop that forms a water trap after settlement causes odour and blockage. Build the expected residual settlement into the level difference.
• Passing through the wall. Use a sleeve or an oversized opening through the wall or foundation; a pipe built in tight will fracture within the first few centimetres of settlement.
• Access. Provide a rodding eye, preferably outside or in the void. Bends are always the first place a blockage lodges.
• Do not clamp it. The assembly has to move. Do not bracket it rigidly and do not cast it in.

Related terms

• Polder
• Settlement
• Below-ground drainage
• Above-ground drainage
• Ducting sleeve
• Rodding eye

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