I was under a house in Chartwell two winters ago, doing a pre-purchase crawl-space check, when I found a soakpit dug straight into peat. Whoever built it had backfilled the hole with the same black, greasy soil that came out of it — no scoria, no geotextile wrap — and the downpipe feeding it had clearly been overflowing onto the lawn for at least one whole wet season, judging by the moss line creeping up the fascia. From the street the section looked flat, tidy, easy. Underneath, the roof water had nowhere real to go.
What a subsoil drain is actually doing
A subsoil drain and a soakpit do two different jobs, and buyers often assume one covers both. The subsoil drain is a slotted pipe, wrapped in geotextile fabric and bedded in clean scoria or metal, laid to intercept groundwater moving through the soil before it reaches a footing or a subfloor. A soakpit is a separate structure at the end of a downpipe or a stormwater line, built to accept a volume of water and let it disperse into the surrounding ground. Confuse the two, or connect them together without thinking about volume, and you get exactly what I found in Chartwell: a system that looks like drainage but behaves like a bathtub with a slow leak.
Sizing the pipe and the metal around it
For a standard suburban dwelling, a 100 mm slotted subsoil pipe is usually adequate for perimeter collection, stepping up to 150 mm where several runs join into one collector line heading to an outlet or a soakpit. Fall matters more than diameter — a minimum of 1 in 200 is the figure I look for, and flatter than that on a Waikato section with barely any natural grade is asking for silt to settle in the pipe rather than flush through it. The trench itself should carry at least 150 mm of clean drainage metal below and around the pipe, wrapped in geotextile to stop fines migrating in and clogging the slots within a few seasons. Installed properly, this kind of perimeter drainage on an average section runs somewhere from $80 to $150 per linear metre, depending on trench depth and how much existing paving or planting has to come out first.
Why peat changes the sizing sums
A soakpit sized off a rule of thumb for clay does not behave the same way once it sits in Waikato peat. Peat holds water like a sponge rather than letting it percolate away, and it compresses under load over time, which can twist a rigid pipe run feeding into the pit. Before sizing a pit on a Chartwell or Rototuna section, I want to see a simple percolation test done in the actual hole, not an assumption carried over from the last job. Where percolation is slow, the answer is usually a bigger pit — commonly 1 to 3 cubic metres of voids, built up with scoria or purpose-made soakage crates and wrapped in geotextile — rather than trying to force water through ground that will not accept it at the rate the roof delivers it in a heavy Waikato downpour.
What I look for before anyone signs
On an inspection, I walk the section after checking the roof, not before, because the two tell one story. Moss or dark staining low on the fascia, a lawn that stays soft in the same spot for days after rain, or a soakpit lid sitting slightly domed from water pressure underneath are all signs the sizing was wrong from day one. I also check how close a soakpit sits to the foundations — anything closer than about 3 metres from a footing on soft Waikato ground worries me, since saturated soil that close can soften bearing capacity under a slab or a pile. None of this is a reason to walk away from a section, but it is a reason to get the numbers checked before settlement rather than after, and often that means bringing in a Hamilton drainage contractor to run a proper percolation test and confirm the existing pipework is actually sized for the roof area it serves.
Frequently asked questions
How do I know if a soakpit is undersized for the roof it serves? Persistent ponding near the pit after moderate rain, a lawn that stays soft for days, or water backing up the downpipe during a downpour are the usual signs it cannot absorb water as fast as the roof delivers it.
Can a subsoil drain and a stormwater downpipe share the same pipe? They can be connected into a shared soakpit if it is sized for the combined volume, but running groundwater and roof water through the same slotted subsoil pipe usually overloads it faster than either flow alone would.
Does peat always mean a bigger soakpit? Not always, but it usually means a percolation test rather than a standard size, since peat’s ability to accept water varies a lot across a single Waikato section depending on depth and compaction history.