How to make a root barrier and the tools needed

So, you have identified that you need to redirect or block roots and want to make a root barrier? You may be asking the question 'what can I use as a root barrier?' Before we discuss how to make a root barrier and suitable materials, we would stress that it’s essential that you understand how roots grow. Roots are opportunistic, just as an animal hunts for food, plant roots hunt for food in the form of nutrients and water. What this means is that roots are designed to exploit situations where there is a high chance of water or nutrients present.

Knowing how roots grow is important because placing any physical object in the ground such as root barrier can increase the concentration of water and nutrients in that locality. Root barrier, as with other manmade structures like foundations, can restrict water movement creating a wet side to a barrier or structure. When this happens roots will sit in the soil profile close to the barrier and proliferate on the wet side. This is where a mass of fibrous roots can develop, and as they do, they will find weakness in a structure or barrier if they exist.

Points to consider when looking to make a root barrier

The first thing to do is to avoid creating a place near a barrier that will allow roots to accumulate. Using a permeable barrier to reduce the dampness of soils on the wet side of a barrier is clearly advantageous. However, regardless of the type of barrier used, a quick win is to instal the barrier on the plant side of the trench, so the softer backfill is not given to roots to grown in. The soft trench backfill will initially be easier for roots to grow in and will hold more water.

Soil movement is a key factor, particularly with heavy soils which will expand and contract  in line with the seasons. This means that any material used must be flexible to movement.

Weaknesses in a barrier must also be avoided. If there are penetrations for service pipes for example, this must be fully sealed and protected against roots.

What are root barriers?

Before we look at what can be used to make a root barrier, it is pertinent considering exactly what a root barrier is. It’s worth making a differentiation between weed mats and root barriers. A weed mat is normally a porous non-woven geotextile which will suppress garden weeds. A root barrier though must block roots completely. In addition, a barrier must resist soil movement and the puncture by sharp objects. It needs to be durable both below and above ground.

Simply put, a root barrier must be durable, flexible and potentially be able to last as long as the life of the plant you’re trying to block. In the case of a tree this can be many, if not hundreds of years.

What can I use as a root barrier?

Below is a simple summary of the different materials that you can use as a root barrier. Some will work, while others are often too costly or complicated to work.

Concrete	Reinforced concrete engineered to withstand soil movement would be the base requirement. Any joints in the concrete would need to be sealed and root proofed.
Metal	Sheet Piling may be used but for it to be successful the joints would need to be sealed and root proofed.  Steel would have be of a grade to resist damp conditions for long term durability.
Plastics	Geotextiles sheets made for polyethylene have become more popular as root barriers due to the comparative cost and therfore being far cheaper to purchase that installing an engineered metal or concrete structure.

What should I avoid as a root barrier?

Now you know what you’re up against when considering how to make a root barrier, it’s best that you disregard the following:

• Old carpet - it will rot.
• Corrugated metal sheeting - it will not stand the test of time either.
• Anything made of wood - even hard wood, it is unlikely to last long.
• Paving slabs laid vertically - this will not work as due gaps between them will be exploited.

In reality, there’s a large range of geotextiles that have been specifically developed as root barrier which are affordable and will probably work better than anything that you may wish to make yourself.

Here’s what you’ll need to instal the different root barrier types

To make a root barrier work it is really important that manufacturer installation instructions are followed, and that those installing a root barrier are suitably experienced. This is especially true when installing root barrier to stop indirect damage such as tree related subsidence or when installing root barrier to block Japanese knotweed. This is because in both cases a deep installation is often needed (over 1 metre in depth). In the following sections there is a summary of the different tools and equipment that you will need to use, and the type of barrier that you may wish to consider using.

Root barrier for trees

When looking to block roots from established trees it is important that the depth of root barrier and its extent around the rooting area is sufficient for the root barrier to work. The advice of a professional arboriculturist would make most sense here, as the tree species and soil type, together with the specific property situation would need to be considered. 

For new tree plantings, a root barrier can be used to redirect tree roots away from built surfaces and structures, in some cases landscape architects will specify suitable products.

Whenever geotextile is used there are some other considerations that are often overlooked. The joining and sealing of root barriers to structures, or around service penetrations, does need proper attention as these could provide potential weaknesses. In addition, aligning the root barrier so that it protrudes above ground, or is built into the finished surface, is equally important as roots of trees are known to hop over barriers.

There are broadly two options for the type of geotextile barrier that can be used :

1. Non-permeable geotextile which can be formed as laminated woven textile made from HDPE sheet.
2. Permeable geotextile which can be formed as composite of woven and non-woven HDPE, with a cooper foil layer sandwiched between the geotextiles to avert root growth through the coppers ion charge which is not tolerated by root tips of plants.

Composite root barrier is being increasingly chosen by specialist contractors, due to it being permeable which is seen by Arboriculturists as a prerequisite for root barriers when resolving tree related subsidence.

Permeable root barrier is also suitable for new tree planting although there are more non permeable root barriers on offer which tend to be cheaper.

One issue with root barriers when used with new plants is that the barrier can cause roots to follow the wall of the barrier, and this is known to create root spiralling which can have a detrimental effect on tree development. To overcome this some non-permeable barriers, have vertical ribs to help re-direct roots downward. It’s also thought that permeable barriers may inhibit spiralling as its less likely for moisture to build up on the barrier wall.

Root barrier for Bamboo

Bamboo plants have vigorous, shallow rooting rhizomes which really are a force to be reckoned with. Therefore, a non-permeable barrier is the best form of geotextile membrane to consider using for this situation. They can be installed to a depth greater than 400-500mm and must completely surround a plant or sufficiently extend past all rhizome to work on their own. In many cases, this can be achieved with root barrier installation along with monitoring and herbicide control by professional specialists. 

As with trees, there are some other considerations when using root barrier for Bamboo. These including joining and sealing of root barriers to structures or around service penetrations, because as with trees these areas will be weak points in a root barrier installation. Likewise, Bamboo has also been known to go over root barrier, therefore ensuring that the root barrier sits above ground or is incorporated into paving, paths, roads is critical.

Weed membrane

When installing a weed membrane it’s probable that a non-woven material will be used. It’s important that a quality product is used as some of the garden centre and DIY store offerings are of poor quality and will not last for long.

Securing a weed mat can be done is several ways. Plastic or metal pegs can be used, or you can rely on the weight of bark, mulch or gravel to hold the mat down. Again, sealing weed membrane to walls, kerbs and the like can help minimise weed growth at the edges of the mat which in effect is a weak point for a horizontal geotextile barrier.

Joining, sealing and welding root barrier geotextiles

To really make a root barrier work it’s imperative to ensure that geotextiles are joined, welded, or correctly sealed. Where there is a penetration or where the geotextile ends or must be joined to another sheet of geotextile, a potential weakness is created. Removing these potential weaknesses will improve the performance of any root barrier system you make.  

Using root barrier tape

Using a specialist tape for root barrier will be advantageous. These tapes are often much wider than normal gaffer tape and will lead to a quicker install and improved grab adhesion over a greater surface area.  Root barrier geotextile sheeting can be over lapped and if used in such a way, should ideally be taped both sides.

The prayer fold

Alternatively, a prayer fold can be created prior to taping which is preferable if it’s not possible to tape both sides of a barrier (this can be the case when a barrier is being installed horizontally).  A prayer fold is where an upstand of root barrier is created and then folded into itself. Root barrier tape is then used to hold the prayer folded root barrier upstand together. This method of folding barrier rather than just overlapping is seen to be a superior method of joining root barrier.

Using geotextile membrane glue

When looking to bond root barrier to foundations, kerbs or even concrete gravel boards to fences it is essential that this is done so that a good seal is made between both barrier and the structure you’re bonding to.  The same applies when you are using a geotextile membrane glue to join root barrier.

To get glue to work best, surfaces which are dry and clean are best with glue applied evenly over the entire areas that are to be bonded. For vertical barriers it may be sensible to temporarily peg or pin a barrier above the bonding point. Pegs and pins must not be used within the profile, where the barrier must block roots.

Welding geotextile

Using a heat welder will produce the most reliable join, as they effectively melt two geotextiles together. There are many types of geotextile welding machines, with some automated for large scale applications. Applying heat so that plastic melts but is not distorted is a skill, and normally requires a specialist contractor with all the appropriate tools and equipment. For smaller projects a heat gun, roller and picker may be all that’s required.

Tools, equipment and protection of root barrier

Consideration also needs to be given to the excavation of root barrier trenches, the preparation of surfaces for horizontal barriers, manoeuvring and handling of root barrier.

It’s normal to use an excavator for deep trenches, however access is often an issue. Access will be a deciding factor to weather a root barrier can be installed. Deep barrier over 2 metres in depth will need to be dug using a reasonable sized excavator requiring a defined width of access and operating area. For shallower trenches a soil saw can be used or other specialist trenching machine.

To make sure a barrier is not punctured it is often worth laying sand binding below and above the barrier or to use a protective fleece. A protective fleece is much the same as a weed mat - being made from a non-woven polyethene - it will better resist impact and sharp objects. A protective fleece can also be used vertically.

Finally, manoeuvring root barrier into position can be a logistical challenge, particularly when a barrier has been specified in the large format. The weight and size of a root barrier will need to be assessed so that the right lifting equipment is used to prevent anyone from injuring their back.

Getting started

In conclusion, you should consider the following when starting your root barrier project:  

• Designing a root barrier solution that actually works, is often best left to the professionals.
• Selecting the best product/s for your specific application is fundamental if a root barrier is to work.
• Installation should use the right tools and equipment.
• Carefully joining, bonding and sealing a root barrier is critical to minimise any weakness in root barrier performance.

If you found this useful, but still need help feel free to give our experienced team a call.