Root barriers near buildings, drains and highways

A root barrier deflects roots downwards or sideways away from something you want to protect. It does not make a tree safe to build next to, it does not remove the need for a foundation designed for the ground and the tree, and installing one badly does more harm than leaving it out. That is the honest starting point, because root barriers appear on drawings as a one-line note far more often than they are properly designed.

Where they do work, they work for a narrow set of problems: keeping roots out of a shallow drain run, keeping roots from lifting a footway or a driveway build-up, and directing new roots downwards where a tree is planted close to a structure. Where the problem is clay shrinkage under a foundation, a barrier is the wrong tool and the foundation is the answer.

What a root barrier can and cannot achieve

A barrier is a vertical membrane, normally a high-density polyethylene sheet or a purpose-made ribbed product, installed in a trench to a designed depth, with sealed or overlapped joints, and terminated so roots cannot cross over the top or grow round the ends.

What it can do:

• stop shallow roots entering a service trench or a granular build-up
• protect a permeable paving layer from root ingress
• guide the roots of a newly planted tree downward past a wall or pavement
• keep roots out of a soakaway

What it cannot do: prevent soil moisture movement. This is the point most often misunderstood. In shrinkable clay, foundation damage is caused by the tree drawing moisture from the soil, and water moves through the clay whether or not a sheet of plastic is in the way. Barriers do not prevent clay shrinkage, and warranty provider standards set foundation depths against tree distance and species water demand for that reason.

Nor can a barrier make an over-close planting position acceptable. A large-species tree three metres from a wall is in the wrong place, and a barrier only postpones the conflict.

Deflection against containment

Two different intentions, two different designs.

Deflection sends roots downwards past the thing being protected, using a barrier that is deep enough that roots do not simply grow beneath it and back up. It is the normal approach for new tree planting beside a footway or a wall, and the depth needed depends on soil conditions and the species, commonly in the range of 600 millimetres to a metre for street planting, deeper where the granular layer being protected is deep.

Containment encircles a rooting volume, usually a tree pit, to keep roots within a designed area. Containment is more demanding, because it restricts the volume available to the tree and can produce a root-bound specimen that fails in a decade. Where containment is used, the enclosed soil volume has to be sized for the tree at maturity.

Say on the drawing which of the two you are doing. A barrier drawn on plan with no depth, no termination detail and no stated purpose tells a tree officer that nobody has thought it through.

Depth, material and jointing requirements

The recurring specification items, all of which belong on a detail rather than in a note:

• material and thickness, by product reference, with the manufacturer's installation requirements
• depth below finished level, with the reason for that depth
• top termination: to finished level, or slightly above, so roots cannot cross over
• bottom termination: to a stated depth or onto a firm stratum
• joint detail: overlap dimension or welded joint, with the sealing method
• ends: returned or extended far enough that roots do not grow round the end, with the return length stated
• relationship to services, because a barrier crossing a drain run needs a detail at that point rather than a hole

Ribbed products work by deflecting roots that meet them rather than by being impermeable, so the orientation of the ribs matters and installing the sheet the wrong way round is a real failure mode on site.

The most common construction failure is much simpler: backfilling against the barrier with topsoil on both sides and no compaction control, so the barrier leans and the top edge ends up 100 millimetres below finished level. Roots go over it within two seasons.

Installation within a root protection area

Here is the contradiction at the heart of the subject. To install a barrier near an existing tree, you have to dig a continuous trench, and digging a continuous trench inside a root protection area severs the roots you are trying to protect.

BS 5837:2012 treats excavation within a root protection area as something to be justified and controlled rather than assumed. Where a barrier is proposed near a retained tree, the submission needs:

1. The barrier position shown against the root protection area on the tree constraints plan.
2. An arboricultural method statement covering how the trench is excavated, which will usually mean hand digging or air spade work with roots retained and pruned cleanly where cutting is unavoidable.
3. A limit on the proportion of the root protection area affected, agreed with the arboriculturist.
4. Arboricultural supervision at the excavation stage, with a record.
5. Timing, avoiding the hottest part of the growing season where possible.

If the honest answer is that the trench cannot be dug without unacceptable root loss, the barrier should be abandoned in favour of a different solution. Our note on trees, soakaways, sewers and root ingress covers the drainage alternatives, which usually involve sealed flexible-jointed pipework rather than a barrier.

Effect on tree stability and health

Cutting roots on one side of a tree removes anchorage on that side. For a tree with a significant crown, that changes its response to wind loading in the direction that matters, and the effect is not theoretical.

Two further consequences. Severing roots reduces the water and nutrient supply, which shows as crown dieback one or two seasons later, by which point the site is finished and nobody connects the two. And a wound on a severed root is an entry point for decay organisms, which is why clean pruning cuts rather than machine tearing matter.

An arboriculturist should state, in writing, the proportion of the root protection area affected and the expected effect on the tree. Where that statement cannot be made positively, the design should change. An illustrative rule of thumb some practitioners use is that severing roots along more than a quarter of the circumference on the windward side needs a stability review rather than a method statement. Treat that as illustrative only, because the real judgement depends on species, crown size, soil and exposure.

Where engineering the foundation is the better answer

For most house-next-to-tree problems, the foundation is the solution and the barrier is a distraction.

The options, in the order an engineer usually considers them:

1. Trench fill to a depth set by the volume change potential of the clay and the distance to and species of the tree.
2. A piled foundation with a suspended ground floor and a void or compressible layer to accommodate heave.
3. A reinforced raft, in the right conditions.
4. Heave precautions to slabs and to the faces of foundations where a tree has been removed.

Removing a tree in shrinkable clay creates its own problem, because the soil recovers moisture and swells, sometimes for years. Heave precautions are therefore part of the design wherever mature trees have been felled on a clay site, and that has to be in the geotechnical and structural documents. The Geotechnical Assessment should give the plasticity results that drive these decisions, and the Tree Survey should give the species, position and mature dimensions that the foundation design needs.

For drainage, the better answers are flexible sealed joints, ductile or structured-wall pipe with sealed connections, rodding access, and routing the run outside the rooting zone altogether. For footways and driveways, a cellular confinement no-dig build-up spreads load without excavation, which protects both the roots and the surface.

What to show on the drawings

An application that proposes a root barrier should show, at minimum:

• the barrier on the site plan, with a distinct line type and a reference to a detail
• a cross-section through the barrier at each different situation, with depths, levels and the top and bottom terminations
• the barrier shown against the root protection area and against retained tree positions
• the extent of hand excavation, hatched
• the joint and end-return details
• a note identifying the supervising arboriculturist and the stages requiring inspection

Then expect a condition requiring the method statement to be approved before any excavation, and a condition requiring supervision records. Our note on discharging tree protection conditions without delay sets out what the discharge submission needs to contain.

This is general guidance, not legal advice; check the position with your local planning authority, a qualified arboriculturist and a structural engineer before relying on it. Arboricultural work of this kind is normally expected from a suitably qualified author, and professional review is strongly advised.

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