Building Work and the Drains

The open branch

The Single Commonest Cause

When drainage is altered — a bathroom moved, a kitchen relocated, an extension built over an old run, an outbuilding disconnected — the old branch usually stays in the ground. It is capped at the building end, because that is the end anybody can see, and left connected at the other.

That leaves a pipe running from the live drainage out into the ground under your property, open at the far end, going nowhere. It is a direct route from the system into the sub-floor, and it is completely invisible.

Ten or twenty years later somebody has rats under the floor with no entry point outside and no idea why.

If you are having work done, the thing to ask for is that redundant branches are disconnected and sealed at the junction, not just capped at the building. It costs almost nothing at the time and it is expensive to find later.

During the work

The Temporary Open System

While drainage is open it is open to the sewer, and rats notice quickly — days rather than months. A run left uncapped over a weekend is an invitation.

Reasonable things to expect of a contractor: open ends bagged or capped overnight, chambers covered, and the system not left open across a shutdown. This is ordinary good practice and most will do it without being asked, but it is worth asking.

If you get a sudden rat problem during building work, the drainage is the first place to look and the timing is the evidence. It is usually fixable on the spot at that stage, which is the one advantage of finding it early.

Building over drainage

Chambers Under Floors

Extensions frequently get built over existing runs and existing chambers. There are proper ways to do that — diverting the run, or sealing and venting a chamber that ends up enclosed — and there is the way it often happens, which is a chamber under a screeded floor with a cover somebody has concreted over.

Two consequences. You now have a section of drainage that cannot be inspected, rodded or surveyed, which matters when something goes wrong. And an inadequately sealed buried chamber is a route from the drainage straight into the floor void.

If you are planning work over a run, this is worth getting right at design stage. If it was done years ago and you now have rats under that floor, a survey from the next accessible chamber is where to start.

After the work

A Short Checklist

• Are redundant branches sealed at the junction, not just capped at the building end?
• Is every chamber still accessible? Lift each cover once, now, while you can still get the contractor back.
• Are new pipe entries sealed properly — mesh and mortar rather than a bead of foam?
• Were airbricks reinstated, and are any of them now buried by a new path, raised border or patio?
• Is the junction between old and new closed, at ground level and in the roof?
• Has the ground level changed relative to the damp course and the airbricks?

None of that is difficult while the scaffolding is still up. All of it is awkward afterwards.

Images on this page are illustrations only.

Short answers

Drain Blocker Questions

Going further

Drain and Rat Guides