You’ve picked the spot for your dam, but you suspect the ground won’t hold what you put in it. Maybe there’s sand under the topsoil, or the neighbour’s dam dries out every summer. Most guides on how to build a dam assume clay that seals itself, but that’s not always the case.
A lined dam is a different build. However, it doesn’t need to be a hard one. You can cleanly split the job into specific tasks you can do yourself and jobs that need a machine or a crew. This guide covers what you need to know.
How to Build a Dam: Approvals and Site Checks Before You Dig
Paperwork can easily stop a project dead in its tracks, so any plan for how to build a dam should start here. Approvals and water licences vary by state and council. Ring your local council before any excavation, and ask your state water authority whether you need a licence to store or take water.
Next, call Before You Dig Australia on 1100 to check for buried services. Then walk the site. A small catchment may never fill the dam, and a spot downhill of bare paddocks or heavy stock traffic collects silt and manure. Keep large trees well back, because roots crack walls and leave voids as they rot. Also make sure a machine can reach the site.
Check for groundwater too. At Mount Tamborine, engineered rock held a 1 mm Rollgum liner down against groundwater uplift. The rock also gave algae a large surface to grow on, and sludge built up between the rocks. Find groundwater and site constraints before the liner goes down.
Finally, test the soil. Dig down 20 cm – 50 cm, put a sample in a jar of clean water and leave it for several days. The sand, silt and clay settle in layers, and our dam construction guide says clay should make up at least 20% of the total.
Do You Need a Liner When You Build a Dam?
Natural clay is the simplest answer to how to make a dam hold water, provided there’s enough of it and the operator compacts it properly. If your test comes in under 20% clay, you have two choices:
- Use a liner
- Sealing treatment such as bentonite or a polymer
Our guide to how to seal a dam compares both methods in detail. If you’re working out how to make a dam on sandy, gravelly or fractured ground, a liner is usually the simplest option.
One caution: a liner stops water reaching the soil, but it doesn’t fix a badly built wall. Compact the embankment as you would for any dam. The liner also shapes the design. We recommend a 3:1 slope (three metres across for every metre of fall) with rounded corners. Tight corners and steep walls fold and strain the sheet. Rollgum comes in rolls up to 15 m wide and 30 m long, and sizing the excavation to suit the roll reduces wastage and cost.
Dam Building: What You Can Do Yourself and What to Hire
There are dam building jobs that need a machine or a crew and jobs most people can do themselves. Here’s roughly which jobs fall into which category for most of our customers:
| Job | DIY or hire | Why |
| Site choice and approvals | DIY | You know the land and the catchment. |
| Excavation and shaping | Hire | You need a machine, an operator and a good eye for slopes. |
| Clearing the base | DIY | Picking out stones and roots by hand is slow but simple. |
| Laying the underlay | DIY with helpers | It’s light, but wind can catch it. |
| Placing the liner | Team | The sheet is heavy and an excavator helps. |
| Anchoring and filling | DIY | Backfilling the trench and watching the water rise suits one person. |
Can I install a dam liner myself?
On a small dam, yes, with helpers. On a larger one, you’ll need a team. A 10 m by 15 m sheet of 1 mm EPDM weighs roughly 170 kg. Rollgum recommends an excavator and a team of people to place it.
It’s also much easier on a calm day, because wind can have your liner flying all over the place.
Hire a professional when the dam is large, the timeframe is tight or you can’t round up a team. We install in the Melbourne area, travel for larger projects and can refer you to a local installer elsewhere.
How to Make a Dam With a Liner, Step by Step
We have a full Rollgum installation guide covering every step in detail for those who want a full breakdown. Here’s the short version of how to build a dam with a liner (plus the most common mistakes at each stage):
- Prepare the site. Shape gentle slopes with rounded corners, dig the anchor trench and clear sharp objects. Common mistakes are an unstable base and skipping the groundwater check.
- Measure depth and contours, not just the water surface, and add 0.5 to 1 m of overhang on every side for seams and anchoring. Measuring only the surface is the classic way to end up with the wrong size.
- Lay the underlay. Overlap the sheets generously and protect sharp areas. Gaps and thin overlaps are the usual slips.
- Place the liner from the top down, with your team and an excavator. Don’t drag it over hazards or stretch it tight. Let it sit into the excavation.
- Seam the liner, following your seam system’s instructions. Dirty or wet surfaces, incorrect primer preparation, creases and inadequate rolling are the common mistakes.
- Tuck the edge into the trench and backfill. Don’t trim the liner too early, and don’t secure the edge so tightly that filling loads the liner.
- Fill slowly. Watch for the liner spanning gaps, pulling tight or shifting, and check the edges once the dam is full.
- Add plants and rocks last. Never drop sharp or heavy material onto unprotected liner, or pile weight in one spot.
What Dam Building Costs and What People Forget
Dam building can be affordable if you do it sensibly. However, many fail to budget for machine hire and operator time, underlay and extra liner for the overhang and anchor trench, the inlet and spillway.
It’s the costs most people forget that often end up causing the most trouble:
- Spoil needs somewhere to go
- A spillway has to carry a big storm so the wall doesn’t wash out
- A lined dam is slippery, so build a gentle slope or a rope ladder to give animals a way out
- Plants and decorative rocks come last, because rocks need extra protection underneath and both can encourage algae.
Plan the Dam Around the Liner
Knowing how to build a dam on poor soil comes down to deciding on the liner type you’re going to use early on, and shaping the dam to suit it. Hire the machine for the digging, do the base by hand and bring helpers for the liner. Fill slowly. Dam building looks daunting until you split it into jobs, and then it’s clear which ones you should take on yourself.
At Water Quality Solutions, we’ve been helping dam, lake and lagoon owners across Australia with liners and aeration for over fifteen years. Send us your dam’s dimensions, and we’ll be happy to work out the liner size and quote it.