Garage Flooding in Wheeler Heights: Fixing 30 Years of Water Seepage Behind the Slab

The situation

A Wheeler Heights garage had been taking on water every time it rained, for 30 years. Behind the garage sits a steep, rocky wall that sheds a lot of rainwater and sediment during rain events, plus a constant trickle of seepage from the hill above even between storms. On paper, the garage had been designed with drainage for exactly this: ag-line and blue metal behind the wall. In practice, it had never worked, and the garage had sat unusable for storage for as long as the owner could remember.

What we found

The ag-line and blue metal system was there, but it had been built without geotextile fabric wrapped around it. Geotextile fabric is what keeps silt and dirt out of the blue metal so water can keep moving through it into the pipe. Without it, this kind of system is only good for a year or two before it clogs solid, and that’s exactly what had happened here: the whole system was full of sludge and dirt, doing nothing.

That wasn’t the only issue. The brick and slab the garage was built from had never been waterproofed, so even without the drainage failing, water sitting against it had a way in. And the old pipework had choke points that stopped water getting through to drain away freely, which meant water was pooling up behind the garage instead of moving on. Between a blocked drainage system, no waterproofing, and nowhere for the water to go, it was pooling high enough behind the garage to push in through the slab every time it rained.

The fix

Given the hill constantly sheds sediment, any new pipe system behind this garage was always going to silt up again eventually, just like the old one did. So rather than replace the ag-line and blue metal with the same type of system, we took a different approach:

• Removed the old ag-line, blue metal and pipework behind the garage entirely, along with the choke points that were stopping water draining away.
• Jackhammered away the concrete that was blocking the water’s path, so it can drain freely and the water level behind the garage can never rise above the slab, which is the point where it was getting into the garage.
• Waterproofed behind the garage from the bottom of the slab up above the brick wall, sealing the structure itself as a backup to the drainage.
• Left the drainage path open rather than piping it, on the basis that a once-a-year clean-out of debris is cheaper and more reliable than another pipe system that will silt up without constant maintenance.

The outcome

The garage no longer has a pipe system that can silt up and fail again. Water draining off the hill now has an open, clear path away from the slab, backed by waterproofing on the structure itself, and the owner only needs to clear debris once a year to keep it that way. It also came out cheaper for the client than installing a new drainage system would have. After 30 years of a pool of water sitting behind the garage, the owner now has usable storage space that was never possible before.

Got water finding its way into a garage, shed or room built against a slope or retaining wall? Our stormwater drainage team looks at the whole picture, not just the pipe, so you don’t end up paying for a drainage system that clogs again in a year or two. Get in touch and we’ll come take a look.