Tree roots in drains: how they get in and what actually stops them
Cutting roots out restores flow and changes nothing. The entry point is still open, and that is what decides whether they come back.
Roots do not break into sound pipes
This is the part that surprises people. A root does not force its way through intact clay. It finds an opening that already exists: an open joint, a hairline crack, a badly made connection. Moisture and nutrients leak out, the root grows towards them, and once a fine root is through the gap it thickens inside the pipe.
So root ingress is always evidence of a second problem. The roots are the symptom; the defect is the cause.
Why London is full of it
Most of the capital drains through vitrified clay laid in short jointed lengths, much of it Victorian or Edwardian. A century and a half of ground movement has opened a great many of those joints. Add mature street trees and established garden planting, and the two meet.
Clay subsoil makes it worse, because seasonal swelling and shrinking works joints apart year after year. That is why Dulwich, Wimbledon and Richmond produce so much of it.
What cutting achieves, and what it does not
Root cutting clears the mass and restores flow. That is genuinely useful when a drain has stopped and someone needs a shower tonight.
What it does not do is close the entry point. Roots regrow towards the same moisture, and a run that needed cutting last spring will need it again. If you are on an annual cycle, you are paying maintenance costs for a repair you have not done.
What ends it
Lining the affected section. Cut the roots out, clean the pipe wall so the resin bonds, install a liner and cure it in place. The liner forms a continuous new wall inside the old pipe with no joints for roots to use.
Two things to check in any quote: that branch connections covered by the liner will be reinstated with a robotic cutter, and how much of the run is being lined. A repair to one section does not change the condition of the next. More in lining or digging.
Do you have to remove the tree
Usually not, and often you cannot: many London trees are protected or belong to the council. Sealing the pipe is the practical route, and it is also the one that does not involve a planning application.
Where a tree genuinely sits on top of a run, a survey establishes that before anyone starts a conversation about felling. See trenchless repairs for what the work involves.