Building a Retaining Wall? Don’t Forget What’s Behind It
- Di McQueen-Richardson
- Aug 7
- 5 min read
When people picture a new retaining wall, they usually think about what they’ll see when the job is finished: the blocks, timber, stone, garden beds and landscaping.
But some of the most important parts of a retaining wall are the ones you won’t see at all.
Managing water behind a retaining wall is an important part of a properly designed installation. Depending on the project, this can include drainage aggregate, subsoil drainage and geotextile filter fabric to help water move away while keeping soil and drainage materials separated.
Why is drainage important behind a retaining wall?
Soil naturally contains moisture, and after rain considerably more water can move through the ground behind a retaining wall.
If that water cannot drain effectively, it can accumulate behind the wall.
Water adds pressure and can contribute to problems including saturated soil, movement, staining and deterioration of the surrounding landscape.
That's why drainage shouldn't be something you think about after the retaining wall has been built. It needs to be considered as part of the project from the beginning.
What goes behind a retaining wall?
There's no single construction method suitable for every retaining wall. The correct design depends on factors including the wall's height and type, soil conditions, slope, loads, site drainage and relevant construction requirements.
However, a retaining-wall drainage system may incorporate:
Free-draining aggregate
Subsoil drainage
Geotextile filter fabric
An appropriate drainage outlet
Each component has a different job.
The aggregate provides a free-draining zone. The drainage system collects and carries water. Geotextile can provide filtration and separation between the drainage material and surrounding soil.
What does geotextile do behind a retaining wall?
This is where Bidim Green Filterwrap can come into the picture.
A suitable nonwoven geotextile allows water to pass through while helping prevent fine soil particles from migrating into the drainage aggregate.
Think about what would happen without that separation.
You've created a nice free-draining layer of aggregate, but it's sitting directly beside soil. Over time, fine particles can move into the spaces between the aggregate.
A correctly specified geotextile helps separate those materials while still allowing water movement.
It is important, however, to use geotextile appropriately. It isn't simply a waterproof sheet placed behind the wall. Filter fabric is permeable—allowing water through is part of its purpose.
Is geotextile the same as waterproofing?
No.
This is an important distinction for homeowners undertaking their own landscaping projects.
Geotextile filter fabric and waterproofing perform different jobs.
Geotextile is generally used for purposes such as filtration and separation.
Waterproofing products are designed to prevent water penetration into particular surfaces or structures.
Depending on the retaining-wall design, both may be relevant—but one shouldn't automatically be substituted for the other.
What drainage can be used behind a retaining wall?
Depending on the design, a subsoil drainage product may be installed near the base of the drainage zone to collect water and direct it towards an appropriate outlet.
BelloScapes Supply Co. stocks Megaflo Green 170, a panel-style subsoil drainage system.
Its slim, flat profile makes it an option worth considering where an appropriate panel drainage system is specified for landscaping and retaining-wall applications.
BelloScapes also stocks compatible T & Y fittings, end caps and end outlet fittings, allowing different components of a drainage run to be connected as required.
Where does the water go?
This might sound obvious, but it's one of the most important questions in any drainage project.
A drainage product can collect water, but that water still needs somewhere appropriate to go.
Before installing drainage behind a retaining wall, the entire path needs to be considered:
Where is the water entering?
How will it reach the drainage system?
Where will the drainage system discharge it?
Simply putting drainage behind a wall without considering the outlet isn't a complete drainage solution.
Local requirements regarding stormwater and drainage should also be considered before commencing work.
Can I build a retaining wall myself?
Small landscaping walls may sometimes be suitable DIY projects, depending on their size, construction and local requirements.
However, retaining walls can also be structural elements.
Requirements can vary according to factors such as wall height, location, loads, proximity to boundaries and other structures, so it's important to establish whether approval, engineering or a licensed contractor is required before starting.
If you're uncertain, check the applicable requirements for your project rather than assuming a wall is exempt because it's located in a backyard.
A domestic example: creating a raised garden area
Imagine you're transforming a sloping backyard and want to create a level garden bed using a retaining wall.
It's tempting to focus the entire budget on the visible wall and plants.
But once the wall is complete and the garden filled with soil, accessing the area behind it becomes considerably more difficult.
Planning the drainage first allows the appropriate aggregate, filter fabric and subsoil drainage to be installed during construction.
The finished garden might look exactly the same from the outside—but what happens behind the wall during the next period of heavy rain can be very different.
How much Bidim Green do I need?
The amount required depends on the dimensions and design of the installation.
BelloScapes stocks Bidim Green Filterwrap in several sizes:
600mm × 50m — 30m²
1.0m × 50m — 50m²
1.2m × 50m — 60m²
2.0m × 50m — 100m²
Don't choose solely according to the total square metres. The height and length of the area being covered, required overlaps, and installation configuration can affect which roll width is most practical.
Our upcoming Product Calculator is designed to make estimating quantities easier.
What other products might I need?
Depending on your retaining-wall design, your project could require products from several BelloScapes categories.
Bidim Green Filterwrap – for appropriate filtration and separation applications.
Megaflo Green 170 – for suitable subsoil drainage applications.
Megaflo fittings – for connections, termination and outlets.
Geotextile pins – where required to temporarily secure geotextile during installation.
Having these products available locally also makes it easier if you discover during the job that you need another roll, fitting or box of fixings.
Where can I buy retaining-wall drainage supplies in Bellingen?
BelloScapes Supply Co. supplies geotextile, subsoil drainage, erosion-control products and installation accessories locally in Bellingen NSW.
Our products are available to homeowners and DIY renovators as well as landscapers, builders and other trades.
Order online and arrange click & collect in Bellingen, or contact us regarding local delivery across the Bellingen, Coffs Harbour & Dorrigo regions.
Build for what happens after the rain
A retaining wall isn't finished simply because the front looks good.
What happens behind the wall matters too.
Planning drainage, filtration, and separation before construction can help create a better overall solution and avoid the considerably harder job of trying to address drainage after landscaping is complete.
For technical product specifications, refer to the manufacturer's information. Structural retaining walls and complex drainage installations should be designed and constructed in accordance with applicable requirements and professional advice.
Comments