Japanese knotweed
A root barrier at the patio is lapped backwards along one edge. Has anyone tested whether roots can pass that lap?
No. This visit did not test whether roots can pass the backwards lap. A root barrier at the patio was seen with one edge lapped the wrong way. That is a visual fault in the joint, not a test. The survey did not dig the barrier out or look for roots crossing that edge. Roots might be held, or they might pass. The inspection has not shown which. The report records the lap and has not tested it.
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 barriers are sheets in the ground, joined by laps that are meant to stay tight. A backwards lap can leave a route between the layers even when the rest of the sheet looks continuous. At a patio edge the surveyor may see only a short length of that joint. Depth, corners and any seal were not proved. A reversed overlap is enough to distrust that edge. It is not a trial of the whole barrier. The finding stops at the lap that was seen.
Ask the seller for the barrier specification, the installer and any knotweed guarantee that depends on this sheet. Your solicitor should treat the backwards lap as a reason to read that paperwork, not as a passed job. A specialist can open the edge and say whether roots use it. This survey has not done so and has not said the patio must come up. Do not tell a lender the barrier works. The joint seen was lapped the wrong way and was not tested.
What should you ask your surveyor?
- How much of the barrier was visible, and which edge was lapped backwards?
- Does a guarantee require a particular lap that the survey has now questioned?
- Is growth visible on both sides of that edge, or on neither?
What are the typical next steps?
- Record the backwards lap as seen and not tested.
- Ask the seller for the barrier specification and any knotweed guarantee.
- If the lap matters, have a specialist expose it. This visit did not test the joint.
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
Can the lap be turned the right way from the surface?
Not reliably. A backwards lap may continue under the patio, and twisting the piece you can reach does not prove the buried joint. The survey has not tested the lap and has not designed a repair. Someone who installs these barriers should re-form the joint if it is wrong. A tug at the surface is not that repair.
Does a guarantee fail automatically because one lap looks wrong?
The survey has not read the guarantee and has not declared it void. A backwards lap is a condition concern. Whether it breaches a warranty depends on the wording and on the installer. Pass both to your solicitor and a specialist. Do not invent a failed policy from a lap the report did not test.