Threshold Drains for Apartments and Balconies: Specification Guide

Threshold drains solve a specific problem: keeping water out of a building at the point where inside meets outside. Balcony doors, apartment terraces, ground floor sliding doors, and level entry access points all need water managed right at the door line, and standard off the shelf drains rarely fit the space.

This guide covers what a threshold drain is, where it's required, what the waterproofing standards say, and how to specify one correctly for an apartment, townhouse, or commercial building project.

What is a threshold drain?

A threshold drain is a narrow linear drain installed directly in front of a door opening, running the full width of the threshold. Its job is to intercept water before it can cross the door line and enter the building.

You'll find them at balcony sliding doors, terrace access doors, level entry showers, and commercial entries where the external surface sits at or near the internal floor level. Any time a design calls for a flush, step free transition between inside and outside, a threshold drain is what makes that possible without inviting water in.

Why level thresholds need dedicated drainage

Traditional construction kept water out with a step up at the door, usually 50mm or more. Modern design has moved away from this. Accessibility requirements, and the simple fact that level transitions look better, mean step free thresholds are now standard in new apartment buildings and high end residential work.

Remove the step and you remove the barrier. Wind driven rain against a balcony door, ponding water on a terrace, or overflow from a blocked balcony outlet will sit directly against the door track. Without a drain at the threshold, that water finds its way inside, into the slab, or into the unit below.

Water ingress at balcony thresholds is one of the most common defects in apartment buildings. It's expensive to fix after completion because the repair means removing finishes, re waterproofing, and often rectifying damage in the apartment underneath. A correctly specified threshold drain designed in from the start costs a fraction of that.

What the standards require

AS 4654.2¹ covers waterproofing membranes for external above ground use, and AS 3740 covers waterproofing of domestic wet areas. Between them, they set out the requirements for managing water at external door thresholds, including membrane termination heights and drainage provision at level transitions.

The practical takeaway for builders and certifiers is this: where a threshold is level or near level, the water needs an engineered path away from the door. A threshold drain connected to the stormwater system, with the membrane dressed correctly into the drain body, is the accepted way to achieve compliance.

Your hydraulic consultant or certifier will confirm the specific requirements for your project. What we can tell you is that the drain itself needs to suit the detail: correct width for the rebate, correct depth for the membrane connection, and an outlet positioned where the plumbing actually is.

Why threshold drains are almost always custom

Here's the reality of threshold drainage: no two door openings are the same. A standard length drain almost never matches the width of the opening, and a threshold drain that stops short of the full door width leaves an unprotected gap at exactly the point water will find.

A threshold drain needs to be made to the opening. That means:

• Length matched to the exact door or opening width, manufactured to the millimetre
• Channel width and depth to suit the rebate detail and membrane termination
• Outlet position placed to line up with the riser or stormwater connection, not wherever a standard product happens to put it
• Grate style selected for the finish, wedge wire for a clean linear look, or tile insert so the drain disappears into the threshold tiling

Strip Drains threshold drains are manufactured in our Sydney workshop to the exact dimensions of each opening. Every drain is fabricated from 316L marine grade stainless steel, with the channel body, grate, and outlet all in metal, no plastic components. For coastal apartment projects, this matters: 316L handles salt air exposure that will corrode cheaper 304 grade steel.

Threshold drains in apartment buildings

For multi residential projects, threshold drains are typically specified across dozens or hundreds of openings, and the openings vary. Balcony doors on one facade might be 2400mm, corner apartments might have 4800mm sliding stackers, and ground floor terraces something else again.

We manufacture custom drains to a schedule. Builders send us the opening dimensions and outlet positions across the project, and we fabricate each drain to its opening. Single piece lengths are available up to 3640mm, and longer openings are handled with joined sections aligned on site.

If you're pricing a project, the most useful thing you can send us is the door schedule with widths and the waterproofing detail for the threshold. A PDF or photo upload is useful here. From that we can quote the full package.

Getting the detail right on site

A few practical points that make the difference between a threshold drain that works and one that causes callbacks:

• The drain must run the full width of the opening. Stopping 100mm short at each end defeats the purpose.
• The membrane needs to dress into the drain flange, not just butt up against it. This is where most threshold failures happen.
• The external surface should fall toward the drain, even a 1 percent fall is enough to keep water moving away from the door.
• The grate should sit flush with the finished surface on both sides for a safe, level transition.

Specifying a threshold drain for your project

Whether it's a single balcony door on a renovation or a full apartment building schedule, the process is the same: measure the opening, confirm the rebate and membrane detail, locate the outlet, and have the drain made to suit.

If you're working on a project that needs threshold drainage, get in touch with the Strip Drains team. We manufacture to order from our Sydney workshop, we've supplied threshold drains for projects from single doors to full residential developments, and we can advise on the right specification before anything is fabricated.