Drainage, falls and stormwater around a Melbourne deck

Melbourne rain is not always heavy. It is persistent. A deck that is almost flat will hold a film of water in every low board, grow a slick in August, and wick damp into a weatherboard. Drainage is not a pit you hide behind the barbecue. It is the geometry of the whole platform.

Think of three waters. The rain that hits the boards. The water that runs off the roof. The ground water that sits in clay. Each needs a path.

Fall on the walking surface

Boards should not be a billiard table against a house wall. A fall away from the cladding, small enough that a table does not wobble and large enough that a storm does not sit at the sill, is the aim. You can build the fall in the joists or in a tapered packing on a waterproof deck. You cannot rely on “timber will warp into a fall”.

Against sliding doors, pair the fall with a grate or a channel. Driven rain on a north-westerly will test the sill even if the deck is perfect. See waterproofing when a room is below; the same channel helps when the room is beside.

Gaps between boards are drainage. Paint them shut with a film finish and you have built a tray. Hidden-clip systems that force boards too tight will cup and hold water at the edges. Leave the gap the species and the season need. See merbau and spotted gum.

Under the deck

Water that gets through the boards has to miss the bearers’ worst face and leave the undercroft. A plastic sheet on soil that holds a pond is worse than bare earth that can breathe. Grade the ground under a low deck so it does not sit in a bath. Keep garden sprinklers from running into the frame.

If you have downpipes, do not let them dump onto the new timber. Tie them into stormwater. A pretty chain from a verandah roof onto a merbau deck is a tannin fountain.

On clay sites, consider agricultural drain on the uphill side so the winter soak does not rise through the footings. This is ordinary site drainage, not luxury.

Neighbours, easements and the legal path of water

You cannot solve your damp by sending a concentrated flow over the boundary. Victorian civil law and council assets both care. If there is an easement, do not bury it under a solid deck without consent. A grated section or a clear access can be the compromise.

A sloping block will move your water faster. Design the receiving end: a drain, a rain garden that is meant to be wet, or a connection. Do not invent a waterfall onto a neighbour’s sleep-out.

Surfaces that stay walkable

Fall plus cleaning is slip resistance. Oil on a wet, mouldy south side is a skating rink. A deck that drains and is washed in spring is safer than a deck with a miracle anti-slip additive over dirt. See mould after winter.

Pool decks have their own splash story; read poolside decking. Balconies have a coffee-cup outlet; read apartment balconies.

If you remember to give water a way out, the timber has a chance. If you remember only the species, you have bought a nice lid for a pond.

Gutters, tanks and the deck as a catchment

Once you add a pergola roof, the deck is downstream of a small building. Size the gutter for a Melbourne dump, not for a polite drizzle. An overflow that lands on the barbecue is a design failure. An overflow that lands in the neighbour’s light well is a legal one.

Tanks under decks are fashionable and useful if the overflow is piped and the tank can be serviced. They are not useful if they sit in a sealed undercroft and ferment.

At the house wall, a linear drain can protect a low sliding door. It needs a fall, a grate you can lift, and a destination. A drain to nowhere is a planter.

Clay, sand and the ground story

Bayside sand will swallow a lot of water and then surprise you with a high water table. Eastern clay will sit wet beside a footing all July and crack away in January. The same deck detail does not serve both. On clay, keep water off the pad. On sand, watch for footings that were never deep enough because “it drains”.

If you are unsure, a plumber who understands stormwater is a better Friday spend than another litre of oil. Dry timber is the finish that lasts.

Testing the fall before you celebrate

On the day the boards are down, put a hose at the house wall and watch. Water should walk away, not toward the slider, not into a low corner you will call “character”. If it ponds, pack or plane before the furniture arrives.

After the first real storm, look again. Melbourne will find the dip. A five-minute visit with a broom after that storm will tell you more than the drawing did.

Drainage is not a hidden extra. It is the difference between a deck and a lid on a puddle.

The first winter as a defects period

Treat the first serious July as a test, not as a surprise. Walk the deck in the rain. Look at the house wall, the neighbour’s side, the undercroft, the outlet. A small pond is a packer. A stain on a soffit is a different phone call.

Take photographs. If you are still in a defects period with the builder, this is the evidence. If you built it yourself, this is the list.

Water is patient. The fall you fudged will still be wrong in year five. Fix it while the furniture is still in boxes.

A leaf-filled gap is a dam the width of a pencil and the length of the deck. Scooping is drainage. Put a plastic gap tool next to the broom and you will use it. The fancy drain will not save a surface you never clear.