Wet-edge, infinity and vanishing-edge pool construction

The photograph people send a designer is almost always a wet edge. Water meets sky, or water meets a valley, and the far coping disappears. The Mauritius horizon in /images/infinity-mauritius.jpg is that wish in a single frame. What the photograph does not show is a catch trough, a balance tank, an auto-fill that must not flood the downstairs slab, and a level tolerance that would make a cabinetmaker nervous.

The weir is a few millimetres of honesty, not a mood. Resort pictures — including the Alila-style infinity frames you will find under /images — are lighting and a still morning. They are not a specification.

A wet edge, a vanishing edge and an infinity edge are the same family. One side of the pool is a precise lip. Water goes over it as a sheet, is caught, and is pumped back. The rest of the pool may still have a skimmer, or the weir may be doing all the surface work. Either way you have left the ordinary backyard circuit. The construction hub treats that as structure and plant, not as a lifestyle add-on.

Vanity or a response to the site

There are two good reasons to build a wet edge in Australia, and several expensive bad ones.

The first is the ground. If the block already falls, if the living floor wants to look over water into a view, and if a downhill coping would only ever be a fence in the scenery, a weir is a site response. Excavation, retaining and engineering will be doing heavy work anyway. Hill suburbs of Sydney and Brisbane, coastal escarpments and Perth ridges are the honest geography.

The second is a reflecting court meant to be read as a single plane — a dark interior, a still morning, a knife of water rather than a bullnose. That can sit on a flatter site if the owner understands they are buying theatre and a tank.

The bad reasons are fashion and a display village. A wet edge aimed at a paling fence on a flat lot is a second pool’s worth of hydraulics for a view of treated pine. The money usually wants to go into the interior, a decent plant and a garden. If you still want the sound of a sheet, a short weir onto a trough you can stand in is more proportional than a full vanishing side.

The weir is a level problem

The lip must be level enough that, in still air, the sheet is continuous. The useful tolerance is millimetres over the length of the edge. A weir that is out by the thickness of a few coins will run as a waterfall at the low end and a dry tile at the high end. You will see it from the kitchen.

That is why wet edges and reactive clay are a tense marriage, and why a weir on timber sleepers is a bet you will lose. The structure that holds the lip has to be stiffer than a suburban garden wall. Settlement after the shoot, a trough that rotates, a tile bed packed by eye: each one writes itself on the sheet.

Form the weir as a dedicated detail. A cut-down coping stone is not a weir. A pebble arris is not a weir. Tile or a precision-cut stone with a designed profile is the usual language. The setter needs a level reference that is not “looks all right from the spa”.

Catch troughs are the second pool

Water that goes over the weir has to land somewhere that is not the downhill neighbour. A catch trough — a channel under or just beyond the lip — collects the sheet and wind-blown spray. It must be wide and deep enough for a party surge, and finished as a wet structure, not a garden drain.

Troughs leak into the view if they are an afterthought. They need waterproofing that matches the shell, outlets that cannot block with a palm frond, and access to clean them. A trough you cannot get a hand into becomes a compost trench. A trough that overflows because the outlet is a single 50-millimetre pipe is a stain on the stone below and a conversation with whoever owns the downstairs ceiling.

On a steep site the trough is also retaining. It has weight and water, and it can surcharge the downhill face. Draw it with the engineer, not with the landscaper after the shell is cured.

Balance tanks and the volume nobody sees

Bathers climb in. Wind stacks water at one end. The sheet runs. The trough cannot hold every excursion, so a balance or surge tank sits in the circuit — a buried chamber, a generous trough, a room under the deck. Its job is to keep the weir pump flooded when the main pool dips, and to take the surge when people jump.

Size is a design number, not a leftover crate. Too small and a northerly empties the tank and the sheet dies. Too large and you are heating a hidden pond. The tank needs a legal overflow, a reliable auto-fill, and a lid a person can open. If nobody can say where the tank is, the quote is not finished. Buried tanks under paving are serviceable only if someone left a pit.

Auto-fill is a safety device for the pump

A wet-edge system loses water on purpose. Evaporation from two surfaces, windage, splash, a leak in a trough joint — the tank level moves. An auto-fill puts mains water back before the pump runs dry. It must fail in a way that does not flood a habitable room: a tundish or air gap where the water authority wants one, a high-level overflow, and a stop a household can find. Do not rely on a person with a hose. The first week away in January is when the sheet was left on and the tank died. Interlocks that shut the weir pump on low level are ordinary, not precious.

Wind is a design load

Australian sites that want wet edges are often the windy ones — coastal headlands, ridges, river escarpments. Wind thickens the sheet on one end, throws water out of a shallow trough, and can empty a marginal tank in an afternoon.

Practical answers: a deeper trough on the weather side, a tank with genuine surge, a weir that is not the longest possible edge, a control that lets you turn the sheet off in a gale, and planting that takes the first punch without becoming a climbable barrier. Coastal durability then decides the metal in the trough grates. If the site is a wind tunnel, a full vanishing side may be the wrong vanity. A shorter weir will run more often.

Tiled edges, interiors and the line you cannot hide

The weir is where tile and pebble stop being decoration and become a gauge. A full mosaic court can take a weir if the setter can hold plane and the grout or epoxy is right for immersion. A pebble interior usually wants a different material on the lip — a tile band, a stone nosing — because a rounded pebble edge will not throw a clean sheet.

Dark interiors make the horizon trick more convincing and show every tooth in the weir. Light interiors forgive a little; they also look less like the Mauritius photograph.

Lights under a sheet are a second essay. Lighting a custom pool should be read before anyone cores a niche in the downhill wall. A wash on the catch face, rather than a glare on the weir, is usually what the night wants. Those Alila-style night frames people collect are lighting design, not proof a backyard LED puck will do.

Running costs are a second pool

A wet edge adds a pump, or a second speed, that may run whenever you want the look. It adds evaporation, chemical demand and water. In a dry Perth or Adelaide summer that is a bill and a social fact. In a tropical wet season the rain can help the tank and hurt the chemistry.

You can design for restraint: a switchable sheet, a smaller edge, a tank that is not a cave. You cannot design a free horizon. If the running cost was not in the brief, it arrives as a surprise in February. Troughs want cleaning. Level probes want checking. The weir wants a person who will not “just grind a bit off” without an engineer.

Construction is less forgiving than a skimmer rectangle

The sequence in a custom build still applies, with tighter hold-points. Set-out must lock the weir height to the finished floor and the view. Steel and shoot must hold that height. The trough and tank are part of the rough-in. Commissioning means a still morning, a windy afternoon and a party-sized surge test, not a single pretty photograph.

If the site is a slope, the retaining is the pool. If the site is a roof, the waterproofing under the tank is a habitable-space problem. If the site is coastal, every grate is a salt specification.

None of this is an argument against wet edges. It is an argument against buying one from a skimmer-pool drawing with a polyline on the downhill side. When the ground falls away and the horizon is the point of the garden, a weir earns its mythology. When the ground does not, spend the mythology on a better interior and a plant room you can stand in. The water will still look like water.