A balcony sliding door looks simple: a large pane of glass that moves sideways. It is also one of the most demanding details on a facade. The door has to keep out rain and wind, carry people across a lowered threshold, accept dust and grit every day and still glide. Most complaints trace back to the track and the threshold, not the glass. This article explains how they work, where water goes, and what to check at design, installation and in use.
Why a balcony door is a hard case
A window sill sits above the floor and is out of the way. A balcony door sill sits at floor level, so rain, washing water and dust collect against it. The track is a long, shallow channel that is walked over, swept and exposed to wind pressure. The door also needs to be as slim and level as possible so that access is easy. These demands conflict, and the detail has to resolve them.
Exposure matters too. A balcony on a high floor takes stronger wind and driven rain than a ground-floor terrace, and a deep recess or overhang changes how much rain reaches the door. Ask what the supplier's system has been tested for, and how the test relates to your site. The air leakage guide explains what that kind of test measures.
The track and how it drains
A sliding door runs on rollers in a bottom track. Water can enter the track from rain, washing and condensation, so the track is designed as a drained channel with drainage slots that let water leave to the outside. Behind the outer drainage line a raised rib or upstand sits between the track and the room, forming the protected inner line. The track should not be a place where water stands.
Drainage slots need to stay open. They can be blocked by dust, paint, mortar or a tile edge. Sections and shop drawings should show the slot positions, the cover or flap if provided and the route from the track to the open air. The window drainage article follows the same path in more detail, and the principles apply here.
The threshold and the balcony floor
The balcony finish is the part of the detail that the door supplier often does not control. A balcony floor should fall away from the door, so surface water runs toward the drain and not into the track. A tile layer, screed or timber deck can raise the finished level and cover the outlets. Coordinate the finished balcony level with the door sill early, and fix a clear step or upstand between exterior and interior.
Some projects ask for a low or flush threshold for accessibility. This is possible, but it asks more of the whole detail: a drained channel at the threshold, a waterproofing upturn behind it and a carefully sealed end. Treat a flush threshold as a bespoke detail to be reviewed with the waterproofing designer and the facade consultant, not as a catalogue option.
Gaskets, brush seals and the inner line
Sliding doors use brush seals or gaskets at the meeting stiles, the jambs and the head. Brush seals manage wind and dust but are not a watertight barrier alone. The air and water line is usually completed by a compression gasket at the closed position and by the upstand under the door. Check what each seal does, and whether its replacement is a simple job.
At the corners of the track, the end caps and sealing between the track and the frame need particular attention. A small gap here can let water pass the upstand. A sample door or a mock-up will show the joints more clearly than a drawing. Ask who seals the track ends on site and what material is used, since a wrong sealant can block a drain.
Wind, pressure and big panels
Large panels bend under wind pressure, and bending moves the seals. A very large door may need heavier sections, a stronger interlock or a different rail. Wind load is applied to the panel and to the frame, and the system should be selected for the exposure and the size. Do not choose a sliding system by its catalogue look alone.
Large doors are also heavy. The rollers, the track and the head rail carry the weight and are the parts that wear. Ask for the roller type, the weight the system is intended for and how a roller can be replaced. Sightlines are another trade: slim interlocks look elegant but give less room for seals, and a thicker section can be the better engineering answer.
Installation: what goes wrong
Many leaks begin on site. The frame is set out of level, the sill is bedded on mortar that blocks the drain, or the balcony waterproofing is finished after the frame is fixed and ends against the wrong face. Protect the track during finishing, and keep the drainage slots open until handover. The installation sequence guide explains the order of trades.
- Frame level and plumb, checked before it is fixed.
- Sill supported evenly, with no mortar in the drain path.
- Waterproofing upstand coordinated with the door supplier's drawing.
- Track ends sealed with a compatible sealant, without closing the drains.
- Tracks protected from tile adhesive, paint and grinding dust.
- A water check carried out on the first completed door.
Open the door, then look at the whole balcony
A door is only as dry as the balcony it opens onto. Look at where the balcony drains, how fast water can leave in heavy rain and what happens if the outlet blocks. If the outlet clogs with leaves, the water level rises toward the threshold, and no sliding door can be expected to cope with standing water. Deck outlets, scuppers and overflow routes belong in the same conversation as the door.
Look also at what sits above. A slab edge, a parapet or an upper balcony can send water down onto the door head. A drip, a projecting hood or a deeper recess often does more than any seal. Ask the architect to show the water route from the slab above through to the balcony drain on one section, and check that the door sits comfortably within it.
Finally, consider the cleaning of the balcony. Residents wash balconies with a hose or bucket, and the water runs toward the door unless the fall and the upstand deal with it. A short note in the occupant guide, explaining that the balcony should be washed away from the track, avoids a surprising number of complaints.
Living with the door
A sliding door lasts if the track is kept clean. Residents should sweep or vacuum dust and grit, wipe the track and make sure the drainage slots are clear. They should not use sharp tools or pour strong chemicals into the track, which can damage seals. Rollers wear faster in a dirty track, so cleaning is also maintenance.
Facilities teams can add the door to the regular facade inspection list: roller condition, seal wear, track cleanliness and any stain at the lower inside corner. Report water marks promptly, since early cleaning of a blocked drain is simple, and a long-running leak can damage finishes. See also the facade maintenance checklist.
Heat, glass and comfort at the door
A balcony door is usually a large area of glass, so it affects heat and comfort as much as water. The glass build-up, the frame and the thermal performance of the sections decide how the room feels near the door. The insulated glass units guide explains what is inside the glass, and the thermal breaks guide describes how the frame is separated.
Condensation can also show at the bottom of a door in cool weather, where the track is exposed. This is not always a leak, so check the signs before opening up the detail. The condensation guide explains how to tell the difference.
Before approving the door, put these questions to the supplier and the installer.
- What is the track section, and where does the water leave it?
- What upstand height separates the track from the room, and how is it sealed at the ends?
- What balcony finish level does the drawing assume?
- How are the rollers and seals replaced?
- What exposure and size is the system intended for?
- What is the first-unit check on site?
General guidance for orientation, not a specification. Applicable standards, values and details are confirmed per project with the facade consultant.
