The Ground Always Wins
I’ve seen too many owner builders get fixated on the shiny bits. They spend months debating whether to go with a Shale Grey or Deep Ocean finish on their roof, but they haven't spent five minutes thinking about where the rain goes once it hits that metal. Here is the cold, hard truth: water is the most destructive force on your job site. If you don't respect the fall of your land, your beautiful new steel frame home is going to have some serious issues before you even get to the lock-up stage.
Building a kit home means you're the boss. You're the one coordinating the site cut. If the bobcat driver leaves a massive dip right where your tanks are supposed to sit, that's on you. Most people assume the plumber just magically makes the water disappear. It doesn't work like that. Drainage starts with the very first bucket of dirt you move. Because if your site isn't graded correctly, you're just building a very expensive dam.
The Critical 1:20 Rule
You need to get familiar with the National Construction Code (NCC) Volume 2. It’s not exactly a page-turner, but it spells out the requirements for surface drainage. Generally, you’re looking for a fall of 50mm over the first metre away from the building. That’s roughly a 1:20 gradient. It sounds small. But on a rainy Tuesday in Gippsland when the clouds open up, that little bit of slope is the only thing keeping your slab from being undermined.
Don't trust your eyes. Use a laser level or a dumpy level. I’ve met plenty of blokes who reckon they can 'eye-off' a site, and they're usually the ones wondering why their garage is flooded three months after the hand-over. You want that water moving away from the footings as fast as possible. If you’ve got a slab-on-ground construction, ponding water is your worst enemy. It can cause the soil to heave, and even the strongest TRUECORE steel frame can't stop a slab from cracking if the ground starts moving under it.
Swales and Spoon Drains: Your Best Friends
If your block has even a slight slope coming down toward the house, you need a cut-off drain. A swale is basically a shallow, wide ditch lined with grass or rocks. It’s simple, cheap, and effective. You're basically creating a creek bed to intercept the water before it hits your house. Some people hate the look of them. I reckon it looks a lot better than a wet mess under your floor joists.
For those tighter spots, a concrete spoon drain is the go-to. It's a shallow 'U' shaped channel that carries water around the perimeter. We see this a lot on tight blocks in NSW where there's not much room between the house and the boundary. Whatever you choose, make sure the outlet actually goes somewhere. I’ve seen owner builders run a beautiful drain that just ends at the corner of the house. That's not a drainage solution. That’s just relocating the problem.
The Mystery of the Agi Pipe
Sub-soil drainage is where most DIYers get confused. We're talking about agricultural pipe, or 'agi pipe' as everyone calls it. It’s that flexible, slotted black pipe that usually comes wrapped in a 'sock' of geofabric. Its job isn't to take the water from your downpipes. Never, ever connect your roof water to your agi pipe. If you do, you're literally pumping hundreds of litres of water into the ground right next to your foundations. I’ve had to explain this more times than I care to admit.
Agi pipes are for picking up ground moisture. You bury them in a trench filled with 20mm blue metal or recycled aggregate. The pipe sits at the bottom, slots facing down (yes, down, so the water rises into it as the trench fills), and it carries the sub-surface water away to a legal point of discharge. If you're building on the side of a hill, you definitely want an agi line on the high side of your site cut. It stops the 'weeping' effect you get when you cut into a slope.
Why it Matters for Steel Frames
One of the big wins with steel frames is that they don't rot. Termites won't touch them either. But even though your TRUECORE frames are tough as nails, the rest of your house isn't invincible. Constant moisture leads to mould in your plasterboard and can eventually compromise your floor coverings. Plus, if the ground moves because of poor drainage, your doors will start sticking. There is nothing more annoying than a door that won't swing right because your site prep was lazy.
Handling the Roof Runoff
When your kit arrives, you’ll have a pile of roofing iron and gutters ready to go. On a standard 150sqm house, an inch of rain equates to nearly 4000 litres of water. That's a lot of weight. You need to have your storm-water pipes in the ground and ready to connect as soon as the roof is on. Don't leave the downpipes flapping in the wind, dumping water at the base of your walls. Get some temporary flexi-pipe from the hardware store and run it at least three metres away from the slab until the final plumbing is hooked up.
Council Plumbing and Legal Points of Discharge
You can't just dump your water onto the neighbor's property. That’s a one-way ticket to a council fine and a very awkward conversation over the fence. Every council has a 'Legal Point of Discharge' (LPOD). This might be a kerb-and-channel at the street, or a dedicated storm-water main at the back of the block. You need to find this out during your planning phase. If the LPOD is at the front of the block and your block slopes to the back, you’re looking at a pump pit or a very deep trench. That's the kind of stuff you want to know before you start digging footings.
The Retaining Wall Trap
If your site involves retaining walls to create a level building pad, pay attention. Every retaining wall over about 600mm needs its own drainage. Without it, the hydrostatic pressure (that's just fancy talk for the weight of the water in the soil) will eventually push the wall over. Put an agi line behind the wall, cover it with gravel, and make sure it has 'weep holes' so the water can escape. Most of the retaining wall failures I’ve seen weren't because the wall was weak, but because it was holding back a mountain of wet mud with nowhere to go.
Practical Tips for the Owner-Builder
- Check your site during a heavy downpour before you even start building. See where the water naturally pools.
- Don't skimp on the gravel in your drainage trenches. It's the cheapest insurance you'll ever buy.
- Keep your silt fences up during construction. If you let mud wash into the council's storm-water system, they'll be on you like a ton of bricks.
- Always call before you dig. Finding a communications cable with an excavator is an expensive way to spend a Wednesday.
- Keep a photo record of every pipe you put in the ground before you backfill. If there's a problem later, you'll want to know exactly where that T-junction is located.
At the end of the day, drainage is about being smarter than the water. It’s not the most glamorous part of building your own home, but it’s what keeps the whole thing standing for the next fifty years. Get it right at the start, and you can sleep easy when those summer storms hit. Get it wrong, and you'll be spending your weekends with a shovel and a very wet pair of boots.
