Surface Water is the Enemy Number One
I've seen it a hundred times on sites from the Sunshine Coast down to Gippsland. A bloke gets his kit delivered, the TRUECORE steel frames are looking sharp, and he's so keen to get the roof on that he forgets about the dirt beneath his boots. Then the first big east coast low hits. Suddenly, that nice flat pad becomes a dam. Water doesn't just sit there. It gets under your slab, it messes with your soil moisture, and if you're on reactive clay, your house starts moving before you've even picked out the kitchen tiles.
Most owner builders think drainage is just about gutters and downpipes. Wrong. It starts with the fall of your land. You want at least a 50mm drop over the first metre away from your house walls. If you've got a flat site, you've got to create that fall. Don't just trust the excavator guy to get it right. Get out there with a laser level yourself. Because water is lazy. It'll find the easiest path, and usually, that's right into your footings.
The AG Pipe: More Than Just a Plastic Tube
Groundwater is a silent killer for residential slabs. You need agricultural (AG) pipes. But don't just chuck them in a trench and call it a day. I saw a project in Toowoomba where the owner builder buried a 100mm socked AG pipe in straight clay. Total waste of time. The clay just smeared over the sock and blocked the water from getting in within six months.
Proper subsoil drainage needs a 1 in 100 fall. You lay your pipe, you surround it with 20mm blue metal or recycled concrete aggregate, and you wrap the lot in geofabric. This creates a filter. The water moves through the rock, into the pipe, and away to your discharge point. And always, always install inspection points at the corners. If that line gets silty in five years, you'll be thanking yourself when you can just stick a hose or a ferret down there to clear it out instead of digging up your finished landscaping.
Connecting the Dots with Stormwater
Once your roof is on, you're dealing with massive volumes of water. A standard 150 square metre roof sheds about 1,500 litres for every 10mm of rain. That's a lot of weight hitting your pipes at once. Most councils have strict rules about where this goes. Usually, it's to the street or a dedicated easement. But as an owner builder, you need to make sure your plumber isn't cutting corners on the pipe class.
Use 100mm DWV (Drain, Waste, Vent) pipe for your main stormwater lines. Some guys try to use the thin-walled stuff because it's cheaper at the trade desk. Don't do it. It cracks if a truck drives over it during the build. Also, make sure your downpipe offsets are clean. Round downpipes on steel frame kits look great, but they need to be secured with proper clips so they don't rattle when a storm rolls in at 2am. It sounds like a small detail until it's keeping you awake.
Silt Pits and Why You Need Them
If you're building on a slope, you're going to have surface runoff from the high side of the block. You need a cut-off drain. This is usually a shallow trench filled with rock or a concrete spoon drain. It catches the sheet flow before it hits your house. But here's the trick: don't just pipe that dirty surface water straight into your main lines. Use a silt pit.
A silt pit is just a plastic box with a grate on top and the outlet pipe sits a bit higher than the bottom. This lets the sand and muck settle at the base so it doesn't clog your underground pipes. You just lift the grate once every few months and shovel the gunk out. Simple. If you're building in a place like the Dandenongs or the Byron hinterland where the rain is heavy and the soil is loose, this isn't optional. It's survival.
Retaining Walls and Hydrostatic Pressure
A lot of kit home sites require some level of cut and fill. If you've built a retaining wall to level out your site, that wall is a giant dam. Without drainage, the weight of the sodden earth behind it will eventually push it over or crack it. I don't care if it's timber sleepers or Besser blocks. You need a dedicated AG line at the base of that wall, behind the footer, covered in at least 300mm of drainage gravel. This relieves the hydrostatic pressure. It lets the wall breathe. It’s the difference between a wall that lasts fifty years and one that leans like it’s had too many schooners after six months.
The Slab Edge and Weep Holes
When you're finishing your site works around your steel frame, watch your slab edge. The NCC (National Construction Code) says you need to keep your weep holes clear. If you build up your garden beds or paths too high, you're literally inviting termites and moisture into your wall cavity. Even with a termite-resistant steel frame, you don't want water sitting against your bottom plate. It’s messy and leads to mould. Keep that 75mm clearance between the ground and your weep holes. It’s there for a reason.
Plus, if you're using a waffle pod slab, drainage is even more vital. Waffle pods are great, but if the soil around the edge gets saturated, the moisture can get under the ribs and cause the slab to heave. You want a concrete apron or at least a high-quality clay cap around the perimeter to shed water away from the house's footprint.
Final Check Before the Landscaping
Before you toss the grass seed down or lay the pavers, do a bucket test. Actually, use a hose. Run it on high for ten minutes in different spots around the house. See where the water goes. If it's pooling anywhere near the house, fix it now. It’s ten times harder to move dirt once the garden is in. Building a kit home gives you heaps of control, but that means you're responsible for the stuff that isn't in the box. Drainage is the boring, invisible part of building, but it's the bit that saves your house from falling apart. Make it a priority.
