Japanese knotweed
What did the visit record about a knotweed barrier, about a lap joint at the path, and about growth the membrane edge might be missing?
The visit recorded a knotweed barrier, a lap joint at the path, and growth where the membrane edge might be missing. A barrier is a sheet meant to separate ground on one side from growth on the other. A lap joint is where two sheets overlap, and that overlap was noted at the path. Growth by an edge that looks uncovered is a visual record. It does not prove rhizomes pass the joint, and it does not prove the sheet is complete underground. The survey has not excavated the barrier and has not decided that the membrane works.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
Root-barrier sheets rely on the overlap being continuous and properly made. A lap at a path is a place people walk, so the joint can be the point that shows at the surface while the rest stays buried. Growth at an edge may mean the sheet stops short, or it may be vegetation sitting on top of a covered edge. Looking cannot tell those apart, and this visit did not dig. The record is the barrier, the lap and the growth. It is not a map of rhizomes.
Give the solicitor the note of the barrier, the lap at the path and the growth at the edge. A knotweed specialist can say whether the membrane needs a closer look, which this survey has not done by excavating. Do not treat the visible lap as proof the whole barrier is sound, and do not treat the growth as proof it has failed everywhere. The papers should keep the three observations as things seen, with the buried sheet still unchecked.
What should you ask your surveyor?
- Where was the lap joint, and was it only seen at the path surface?
- Was growth at the membrane edge identified only by eye?
- Did the visit leave the buried part of the barrier unexcavated?
What are the typical next steps?
- Record the barrier, the path lap and the edge growth as seen, not proved.
- Tell the solicitor the buried membrane was not checked.
- Ask a knotweed specialist before treating the barrier as sound or failed.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
If no growth shows in the lawn, is the barrier a success?
No. A quiet lawn does not prove a knotweed barrier is complete. The visit recorded a lap at the path and growth where an edge might be missing, and it did not excavate the rest. Buried sheet and buried growth can both stay hidden. Absence of stems in the grass is not a test of the joint.
Does a visible lap mean the barrier was badly laid?
Not by itself. A lap joint is how sheets of a barrier meet, and seeing one at the path only shows that a joint is there. The survey did not uncover the overlap to judge how it was made. The note is a location. It is not a verdict that the membrane has failed or that it is intact.