{{ propApi.closeIcon }}
Our industry
Our industry $vuetify.icons.faArrowRight
Economic research and forecasting Economics Housing outlook Tailored market research Economic reports and data Inspiring Australia's building professionals HOUSING The only place to get your industry news Media releases Member alerts Submissions See all
Business support
Business support $vuetify.icons.faArrowRight
Become an apprentice host Hire an apprentice Why host a HIA apprentice? Apprentice partner program Builder and manufacturer program Industry insurance Construction legal expenses insurance Construction works insurance Home warranty insurance Tradies and tool insurance Planning and safety services Building and planning services How can HIA Safety help you? Independent site inspections Solutions for your business Contracts Online HIA Tradepass HIA SafeScan HR Docs Trusted legal support Legal advice and guidance Professional services Industrial relations
Resources & advice
Resources & advice $vuetify.icons.faArrowRight
Building it right Building codes Australian standards Getting it right on site See all Building materials and products Concrete, bricks and walls Getting products approved Use the right products for the job See all Managing your business Dealing with contracts Handling disputes Managing your employees See all Managing your safety Falls from heights Safety rules Working with silica See all Building your business Growing your business Maintaining your business See all Other subjects COVID-19 Getting approval to build Sustainable homes
Careers & learning
Careers & learning $vuetify.icons.faArrowRight
A rewarding career Become an apprentice Apprenticeships on offer Hear what our apprentices say Advice for parents and guardians Study with us Find a course Get your builder's licence Learn with HIA
HIA community
HIA community $vuetify.icons.faArrowRight
Join HIA Sign me up How do I become a member? What's in it for me? Get involved Become an award judge Join a committee Partner with us Get to know us Our members Our people Our partners Mates rates What we do Mental health program Charitable Foundation GreenSmart
Awards & events
Awards & events $vuetify.icons.faArrowRight
Awards Australian Housing Awards Awards program National Conference Industry networking Events Building and Renovation Home Show HOMEFEST
HIA products
HIA products $vuetify.icons.faArrowRight
Shop @ HIA Digital Australian Standards Contracts Online Shipping and delivery Purchasing terms & conditions Products Building codes and standards Hard copy contracts Guides and manuals Safety and signage See all
About Contact Newsroom
$vuetify.icons.faMapMarker Set my location Use the field below to update your location
Change location
{{propApi.text}} {{region}} Change location
{{propApi.successMessage}} {{region}} Change location

$vuetify.icons.faPhone1300 650 620

Avoiding risks when building near trees

Building near existing large (or soon-to-large) trees can create problems during construction and, more importantly for slab-on-ground houses, can cause problems over the life of the home. So what can be done up front to help minimise the danger from tree roots next to a house?

Tree roots can easily crack slabs if proper precautions aren’t taken at the construction stage. The best advice is to get expert advice from someone with knowledge of trees and how their root system works. However, the following tips may help you ask the right questions and ensure that both the trees and the home you build can live happily together for many years to come. 

Root physiology

It’s important to recognise that traditional methods of trenching and backfilling with concrete or aggregate may not work as root barriers or root deflectors.

To be able to predict what roots may or may not do, a knowledge of where they can and can’t grow is needed before considering how to deflect them. 

Tree roots need five important ingredients to grow: 

  • Water
  • Aeration
  • Food 
  • Lack of contamination
  • Most importantly: room to grow. 

This means that most roots will grow within topsoil layers and spread out well beyond the canopy limit.

Root growth is far more dependent on soil conditions than species. Roots will take advantage of every opportunity to go rampant where soil conditions are suitable.

Therefore, the best way to prevent roots growing next to a slab is to manipulate and/or eliminate some of the important ingredients for root growth. 

Providing conditions that will allow for better growth away from the house is one way to prevent problems. 

Trees and building can live together happily

Eliminating root growth near the house can, of course, best be achieved by mechanical means – i.e. root barriers or root deflects, which are the ultimate eliminators. However, the trick is to ensure that all the ingredients for best growth be eliminated permanently in the first instance so as to continually maintain the ‘eliminator’ in the long term.

Barriers and root deflectors

A cut-off wall, consisting of a 300mm wide trench about 1–2m deep filled with concrete or aggregate, has traditionally been used for root barriers. However, it can have the following drawbacks:

  • It can be expensive 
  • It can be difficult to provide a complete barrier where service pipes cross 
  • It can be ineffective – either by not being deep/long enough or by being cost prohibitive 
  • It can eventually encounter concrete cracking, allowing root penetration 
  • Cause silting up of aggregate trenches or, if aggregate is too small in the first place, will exclude air voids
  • Promote root growth 
  • Lead to a lack of maintenance – landscaping over the trench 
  • Fail to address any of the soil conditions that may make root growth suitable near the house. 


A case study

An engineer specified a backhoe root barrier drain 300mm by 1200mm deep for 5m with 20mm aggregate to allow the house to be built 2m away from the nearest tree. Five years later, due to silting and landscaping, roots as thick as an arm had broken through the barrier along the entire length of the trench to a depth of about 1m and massed towards the house for 4m in all directions. 

Two years ago the aggregate barrier was extended for a further 10m to protect a new extension. 

When the recent barrier restoration work was undertaken the most recent barrier was filled with a mass of hairy roots across the 300mm width and down the 1m i.e. – heading for ideal conditions under the extension’s slab. 

The solution? A proprietary plastic sheet was installed using a narrow width excavator (75mm) to a depth of 1200mm. The plastic sheet needed to be able to deflects any roots and not allow penetration at any time. Products on the market are robust enough for installation (approximately 1mm thick) and have mouldings to create ideal deflection barriers. 

One particular plastic root barrier system uses sodium bentonite layered in the trench once the plastic root barrier is installed. The material is fine clay and with any moisture it sets like plasticine. This should prevent roots from growing under the bottom of the barrier where the excavator has aerated the soil at the bottom of the trench. Any pipe penetrations of the barrier are treated in the same way with the bentonite on both sides of the barrier where the pipe has penetrated. 

Moisture barriers

A technique used for root barriers or root deflectors can be used for moisture stabilisation of problem sites – i.e. highly reactive or wet sloping sites. The zone of influence is generally accepted as having a depth of 1.5m on the coast and 3m in highly reactive soils. 


Tree root problems can be avoided and managed so that trees and buildings can live happily together. It is better to encourage root growth away from the house (without cutting the tree down) and make conditions next to the house as unsuitable as possible. Barriers or deflectors can be installed to protect against damage to foundations in the short term. 

Tree root science is not an exact science and caution should be taken in all cases where there is any doubt as to the effectiveness of a barrier. Tree arborists are the best qualified to give you advice on tree roots and the location of trees before you build. 

To find out more, contact HIA’s Building Services team.

Email us

Share with your network:

More articles on:

{{ tag.label }} {{ tag.label }} $vuetify.icons.faTimes
Find guides, how-tos, resources and more

Building it right topics


Can’t find what you need, check out other resources that might be closer to the mark.

Business support


Supporting building professionals with custom built services and products.

  • Legal support
  • Contracts Online
  • Host an apprentice
  • Insurance services
  • Managing safety

Explore Business support

2019 National Construction Code - Volume 1

Volume 1 of the National Construction Code (NCC) relates to commercial buildings – Class 2 to Class 9. The code details technical provisions for the d...

2019 National Construction Code - Volumes 1 & 2 (BCA)

This pack contains Volume 1 (commercial buildings – Class 2 to Class 9) and Volume 2 (residential buildings – Class 1 and Class 10) of the National Co...

2019 National Construction Code - Volume 2

Volume 2 of the National Construction Code (NCC) relates to residential buildings – Class 1 and Class 10. The code details technical provisions for th...