Six common outdoor drainage slip-ups (and how landscapers can fix them)

The importance of good drainage

In the intricate tapestry of a flourishing garden, good drainage can’t be overlooked – but all too often it’s a case of out of sight, out of mind. Drainage systems are typically hidden underground, and surface water only pools after heavy rains, so problems are easily missed. “It’s not uncommon for homeowners to misdiagnose a poor-performing lawn, thinking it needs more water – when too much water is actually the culprit,” says Andrew Denman, Bunnings landscape specialist team leader.

Unlike a burst pipe, the damage tends to build slowly and can go unnoticed until it becomes severe. Water issues around the home rarely stay on the surface for long. Left unchecked, they can affect not just a garden’s health but the footings and structural integrity of a house. Poor drainage can quietly redirect rainwater toward the foundation, increasing soil pressure and moisture build-up over time. “It often leads to dying plants, lawn failure and long-term structural stress that’s expensive to correct later,” warns Andrew. Valuers will consider these potential remediation costs in the final market value of their property.

By implementing the proper steps in the planning stages of landscaping and hardscaping – or as soon as puddles in the yard, waterlogged grass, muddy messes or moisture pooling around the house are spotted – you can protect your home before it’s too late.

Take a look at these six drainage mistakes commonly seen by landscapers, and how to implement their best practices to ensure smooth sailing of water – in the right direction.

Problem 1: Incorrect fall or zero fall

To keep water in the yard flowing away from a property’s foundation walls, you need an ideal slope. “Proper grading is the first line of defence and the magic number is 1 per cent,” says Andrew. Sculpt the slope away from the house at a minimum rate of 1 per cent (10mm per metre).

Causes

Retrofitting around existing concrete slabs, paving or building foundations can prevent proper, continuous or consistent sloping. Caution should always be taken with any hardscape additions. Skipping surveys and laser checks such as laser scanning for precise grading can also turn out to be a costly oversight.

Quick fixes

There are various options here. You can re-slope the land by adjusting three to five linear metres of soil by adding, removing or shifting dirt to create a positive, downward slope. Alternatively, install a French drain using a slotted agi pipe, wrapped with geofabric and surrounded by graded aggregate. This method creates a forgiving profile that allows water to move through the gravel even if the pipe itself has a minimal fall.

Preventive measures

“New builds give contractors the opportunity to get the garden’s grading right from the beginning,” says Andrew. “From day one, they can establish that ideal 1-in-100 fall [1 per cent].” The ground immediately surrounding the slab should be sloped to fall at least 50mm over the first metre from the building, allowing water to flow towards compliant drainage points.

Problem 2: Installing the wrong pipe

French drains have long been used to keep homes and yards high and dry, yet they’re not foolproof. Finding the right pipe for a site’s specific conditions requires balancing durability, flexibility and water flow for long-term performance.

Causes

The original homeowner may have tried to save money by opting for subpar materials that won’t last underground. Or the drainage may have been installed before grading and hardscaping. “Sometimes homeowners are in a rush to complete all the nice, cosmetic finishes to their garden so they don’t pre-plan and work out where all the water is supposed to go,” says Andrew. Effective drainage requires accurate ground levels, and knowledge of soil types and water flow patterns.

Quick fixes

“Work out where water needs to move and where it needs to be captured to help you select the correct pipes,” advises Andrew. Solid pipes transport water away from the house, while slotted pipes capture groundwater and surface seepage. “And size does matter in high-flow zones and at downpipe connection lines, which call for larger 90–100mm pipes,” says Andrew.

Pro tip

Avoid soil erosion on steep blocks by using solid pipes to allow rapid conveyance. Reserve slotted pipes for horizontal collection lines only.

Problem 3: Skimping on geofabric or quality aggregate

Ironically, agi pipes can cause damage to a home they’re meant to protect if they’re not installed with the correct supports. Without a geofabric sock, silt can clog up the inlet holes of a slotted agi pipe. Backfilling the trench with the wrong type of aggregate can also affect water flow by limiting permeability and drainage.

Warning signs

Symptoms landscapers often see include persistently soggy ground or boggy patches, mud and silt on pathways and patios, or pipes flattening (ovalising) from excessive backfill pressure.

Try this fix

Excavate the impacted areas and retrench. Cover the agi pipe with a non-woven geofabric sock so it can withstand both the weight of the aggregate and the pressure of the flowing water without tearing. Then backfill the trench with 10–20mm washed aggregate, using just the right amount of compaction. “This synergistic strategy increases capture, prevents soil seepage and helps the pipe retain its shape over time,” says Andrew

Problem 4: Using surface solutions for deep problems

Whoops! This mistake can lead to failed, expensive and unnecessary surface-level fixes (like regrading) that don’t address the underlying issue. What may appear to be a rain-runoff problem is dirt deep, causing continual structural damage!

Warning signs

“Customers often complain about water pooling on the lawn, but if this is happening long after a heavy rain event, you know it’s a serious subsurface issue,” says Andrew. And while runoff can cause surface staining, know that subsurface water exerting pressure against foundations often presents as chronic moisture stains. Likewise, water building up beneath a pavement will lift it over time as it has no clear path to drain.

Rapid diagnostic test

“Dig a 200–300mm test pit and if the hole reveals accumulating water or the soil is paste-like, the issue is subsurface,” says Andrew.

Try this fix

Install a wrapped subsurface drainage line parallel to the problem zone and drainage cells behind the retaining walls. This effectively relieves pressure by providing a clear pathway for water to escape rather than relying on potentially clogged, localised weep holes.

Problem 5: Ignoring soil type when designing drainage systems

Not all dirt behaves the same. Before installing subsurface units, you should look into your site’s soil composition. Is it sand, silt, clay or loam?

• Sandy soil has large particles, so it drains quickly but can dry out fast
• Silty soil retains water well and is fertile but is prone to erosion
• Heavy clay soil is dense, and goes hard, promoting pooling and waterlogging. Some clay soils are classified as reactive, changing volume when exposed to wet and dry conditions
• Loamy soil is made of clay, sand and silt, so it’s not too free draining or prone to waterlogging

“Applying a generic fix across different suburbs can result in systems failure – different soils require tailored solutions,” says Andrew. Undersized pipes in reactive clay, for instance, won’t be able to handle heavy rainfall runoff, resulting in an underground pond.

Try this fix

A five-minute soil infiltration test is a quick way to gauge a yard’s drainage capacity and soil type. Loamy soil, which offers an ideal balance between moisture retention and permeability, typically has an infiltration rate of 9–13mm per hour. If your site is composed of heavy clay – draining at about 4mm per hour – you can increase line volume or add secondary lines to effectively move water off the surface. “Using deep, wide trench profiles or higher void-volume aggregate can also prevent perched water tables in poor soils,” says Andrew.

Problem 6: Improperly integrated downpipes dumping water

One of the most overlooked causes of soft zones around a home’s foundation and garden is improper downpipe drainage. Most homes should extend downpipes at least 2–3 metres away from the building’s slab, but this also depends on how the landscape is graded and the type of soil your home sits on. “Proper placement, good connection and secure fittings are all key to controlling stormwater efficiently,” says Andrew.

Try this fix

Connect downpipes to a solid stormwater line feeding rainwater pits or outfalls. Position downpipe adaptors 5–15cm from the pit grate to keep it from being buried by debris or causing splashback. Consider upgrading to higher flow-rate pits when weather patterns indicate extreme rain events. To protect plant zones, install catchment channels or local agi pipe runs. “But always consult a licensed plumber when you’re doing any work involving underground piping or connections to stormwater to ensure compliancy,” advises Andrew.

Want to get your garden’s drainage right?

Contact your local Bunnings Trade team for expert advice and the right products to help you achieve a well-designed landscape that not only looks beautiful but also manages garden drainage effectively.

Frequently asked questions

1. What are the signs that my garden has drainage issues? Common indicators include persistent puddles, waterlogged lawns, boggy patches, moisture stains near foundations or paving that lifts over time due to trapped water.

2. How much fall should my yard have for proper drainage? A slope of at least 1 per cent (10mm per metre) away from the home helps direct water toward compliant drainage points and prevents structural stress.

3. What type of pipe should I use for a French drain? Slotted pipes are ideal for capturing groundwater and seepage, while solid pipes transfer water away quickly, especially important in high‑flow zones.

4. How does soil type affect drainage performance? Sandy soils drain quickly, clay holds water and can cause pooling, and loamy soil offers balanced permeability. Drainage systems should always be designed around the site's soil composition.

5. How far should downpipes extend from the house? Most homes need downpipes extended 2–3 metres away from the slab, depending on soil type and site grading, to prevent soft spots and moisture build-up near foundations.