Green Roof Root Barriers: [Your Complete Guide]

Green roofs can be an excellent way to help manage stormwater runoff, provide insulation and improve the appearance of any building.

However, if not designed or installed properly they can also be a major contributor to roof leaks.

One of the most important components of a successful green roof installation is the root barrier. Because without it, the plants on the roof will quickly outgrow their designated area and begin to damage the roof membrane.

What is a root barrier?

A root barrier is a layer of material that’s placed between the growing medium and the roof membrane to prevent plants from growing through the membrane and damaging the roof.

It comes in many forms, but the most common are plastic sheeting made of high-density polyethylene (HDPE), polypropylene (PP). Some root barriers are reinforced with metal mesh to make them more durable.

How does a root barrier work?

Plants grow forever as long as they have sun, water, and nutrients. Their roots also grow and spread in search of more water, nutrients, and space.

This means that if the roots aren’t confined to a space, they will eventually push through the roof’s waterproofing membrane and cause leaks.

Some types of roofs are more likely to experience this problem than others.

For example, an intensive green roof with shrubs and trees will have a higher chance of failure than an extensive green roof with only grasses and flowers.

A root barrier will stop the roots from growing through the membrane by blocking them with an impenetrable barrier.

When a plant’s roots reach the root barrier, they have nowhere left to go. The barrier forces them to grow laterally instead of vertically and helps to keep them confined to the planting area.

It is important to note here growing medium is not layered directly on top of the root barrier.

Instead, the root barrier is laid on the top roof membrane and followed by a drainage or retention plate that provides temporary water storage.

The drainage plate is covered with a geotextile that allows water to pass through while stopping the growing medium from seeping through.

Typically, the growing medium is only applied to the roof after the drainage plate and geotextile sheet have been installed on top of the root barrier.

What materials are used for root barriers?

Root barriers are made from a variety of materials, the most common of which are plastics (polypropylene, HDPE) or natural fabrics such as bamboo geotextile.

To improve puncture resistance, some root barriers are reinforced with additional grid material such as metal meshes. These nets are usually made from copper, galvanized steel, and in some cases aluminum.

No matter what material is used, the primary function of any root barrier is to block roots from growing through it.

However, there are some other characteristics that you should look for when choosing a root barrier for your project.

How thick a root barrier is?

The thickness of the root barrier varies depending on the material of the barrier, plantation plan, and the product’s specifications.

A root barrier can be as thin as 0.03 inches or as thick as 0.4 inches depending on the product and the planned application.

Most root barriers are sold in 100-feet rolls and can be cut to fit any roof. The depth of the barriers is typically 12, 24, 36, and 48 inches.

How long does a root barrier last?

A high-quality root barrier should last at least 10 years or more. Because you cannot often remove a root barrier to replace it with a new one, it is important to select a product that will last.

Talk to your supplier and read the product specifications to find out how long the root barrier you are considering is expected to last.

What characteristics should a high-quality root barrier have?

We know that a root barrier’s most important job is to stop roots from growing through it. This is the bare minimum.

However, there are some other considerations that you should keep in mind when choosing a root barrier for your green roof. These include the following:

High tensile strength and puncture resistance

A good root barrier should be able to withstand the stress of a root that is pushing through it. If the root barrier doesn’t have much strength, then there’s a chance that roots will puncture or tear holes in it.

To avoid this, look for root barriers that are puncture-resistant and have high tensile strength ratings. These will be able to withstand a lot of pressure from the roots without tearing.

Flexible

Although it’s important that the root barrier is able to block roots, it should also be flexible enough so that workers can lay it flat on top of your roof membrane.

If your root barrier isn’t flexible enough, then you will have a hard time installing it and may end up with wrinkles or creases in the barrier. Root barrier material should be flexible enough to follow the contours of your roof.

Chemically inert

Since a root barrier is in direct contact with the soil, it’s important that it doesn’t chemically interact with the soil or the growing medium.

Many root barriers are made from inert plastics or fabrics that don’t leach chemicals into the soil and/or will not disrupt the pH of the soil.

Resistant to biodegradation 

Biodegradation is the breakdown of organic matter by microorganisms such as bacteria and fungi.

Since a root barrier is placed in the soil, it needs to be resistant to biodegradation so that microorganisms don’t break down its fibers and compromise their strength.

Most root barriers are made are built not to degrade over time by the activity of the microorganisms. However, you shouldn’t assume that all root barriers will be resistant to biodegradation and you should always look for proof of this before purchasing.

UV Stable

UV radiation causes some materials to degrade over time. This can also affect the barrier’s strength and cause it to deteriorate more quickly than expected.

Therefore, a good root barrier should be UV stable and able to withstand years of exposure without breaking down.

Resistant to Mold Growth

Root barriers are placed in the soil and are often wet for extended periods of time. This can create an environment that is conducive to the growth of mold.

Mold can affect the structural integrity of a root barrier and compromise its ability to block roots. It can also spread to other areas of your roof, which is very difficult to remove.

Therefore, a good quality root barrier should be resistant to mold growth.

Environmentally Friendly

We want to create sustainable green roofs, so it’s important to choose root barriers that are recyclable. 

Because at some point the root barrier will need to be replaced, it’s important that the material can be recycled and reused.

As we have earlier mentioned root barriers are made from different materials. So, the recycling methods and recyclability rates of those materials will vary. Therefore, check with the manufacturer to learn if their products are recyclable.

It’s also a good idea to use root barriers made of recycled materials. If you can find a root barrier made from recycled materials, you will not only be helping the environment, but you will also likely save money on your project.

Don’t forget to check the embodied energy of the root barrier product.

Since embodied energy is a measure of “the total amount of energy consumed in all stages from raw material extraction to final disposal of the product”, it’s important that you purchase a barrier with low embodied energy.

Also, if possible, purchase from a local supplier. If you live in North America, it makes little sense to import German root barriers when there are many good-quality root barriers made in North America.

Sourcing materials from afar has a negative impact on the environment. Because it takes more energy to transport the materials than it does to produce them locally.

Are root barriers used only for horizontal applications?

No, although root barriers are most commonly used for horizontal applications, they can also be used in vertical applications such as slope protection and retaining walls.

Also, surrounding trees with a root barrier can help reduce the competition for water and nutrients, which will also improve their health.

Are root barriers a one-time use product?

No, although root barriers are designed to be a long-term solution, they will eventually need to be replaced.

The lifespan of a root barrier depends on the type of material it is made from, how well it is maintained, and the environment in which it is used.

Do root barriers have a negative environmental impact?

Typically no. However, if the root barriers are made from sustainable materials and are recyclable, then the environmental impact should be minimal.

However, if the root barriers are not made from sustainable materials, and/or leach compounds that are not healthy for the environment, then there can be a negative environmental impact.

Do root barriers affect the drainage of a green roof?

No, root barriers do not affect the drainage of a green roof. In fact, they are often used to improve drainage by preventing roots from clogging up the drains.

The only time root barriers can adversely affect drainage is if they are installed incorrectly.