Sloping Block Drainage in Allambie Heights: Stopping Stormwater Pooling Against the House

The situation

This Allambie Heights property sits below a sloping block, with a hill and a neighbouring house further up the rise. Every time it rained, that runoff had nowhere to go but down, straight through the backyard and up against the house. The owners had already built a retaining wall along the back of the property to hold the slope, but the wall on its own wasn’t dealing with the water; it was just holding back the ground while the water kept finding its way through and pooling against the house.

This is one of the most common drainage problems on sloping blocks across the Northern Beaches, and it’s not something a retaining wall alone was ever designed to fix. Once you’ve got a wall in, the water still needs somewhere purpose-built to go.

What we found

Walking the block, the pattern was clear: water coming off the hill and the house above was travelling downhill, hitting the retaining wall, and having nowhere to go but pool against the structure and work its way toward the house. A retaining wall holds soil back, but it doesn’t manage water, so without proper drainage behind it, that water just banks up and looks for the next weak point.

The fix

We installed a subsoil drainage system behind the retaining wall, built to catch the water before it could pool:

• Ag line (a slotted pipe wrapped in fabric, laid in a trench) behind the wall, to intercept the runoff coming down the slope.
• Blue metal packed around the ag line, so water can move freely through it into the pipe instead of pooling in the soil.
• Geotextile fabric wrapped around the blue metal, so silt and fine soil can’t wash in and clog the drainage over time.
• Grated drains at the surface, to pick up water before it ever gets a chance to pool.

Everything ties into a single collection pipeline that carries the water from behind the wall all the way to the front of the property, well away from the house, instead of letting it sit in the backyard.

We sized the whole system to handle a 1-in-100-year storm, meaning it’s built to cope with the kind of heavy, rare downpour that puts the most pressure on a drainage system, so the owners have confidence in it during the storms that matter most, not just everyday rain.

The outcome

The backyard now has a proper path for stormwater to follow: caught behind the retaining wall, filtered so it doesn’t clog up, and carried straight out to the front of the property instead of pooling against the house. It’s the difference between a retaining wall that just holds the slope back and a full system that deals with the water too.

Got a sloping block with water pooling somewhere it shouldn’t? Our stormwater drainage team handles subsoil drainage behind retaining walls across Allambie Heights and the rest of the Northern Beaches, sized properly for the storms that actually test it.