Balconies and terraces

Balcony, Terrace & Walkway Waterproofing

A balcony that fails does not drip into a loft. It drips into somebody's ceiling, which is why this work reaches us as a complaint rather than a maintenance request. Technically these decks are flat roofs: a waterproof layer, falls, upstands and outlets, sitting over occupied space with people walking about on top of it. We treat them as exactly that.

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Why a balcony leak reaches the room below so quickly

On a conventional roof there is a void, a ceiling and often insulation between the failure and anyone who might notice it. Water can move around up there for a long time before it announces itself. A balcony has none of that slack. The waterproof layer, the structural slab and the plaster of the ceiling underneath are stacked tightly together, so the first winter of a failed junction becomes a stain in a habitable room rather than a damp patch nobody sees.

That closeness raises the standard. Diagnosis matters more, because where water shows and where it entered are rarely the same place. So does the finish, because a resident stands on it daily and will notice anything that ponds, lifts or grips badly.

Thresholds and upstands: where these decks actually fail

Ask where a balcony went wrong and the answer is usually the doorway. A door sill sits low so the step out is comfortable, sometimes almost flush for step-free access, and that leaves very little height for the waterproofing to turn up behind it. Wind-driven rain, a blocked outlet or a few centimetres of standing water then have a clear route inside. Original construction that met the standards of its day may leave less turn-up than current guidance expects, and the honest answer is sometimes that the threshold itself has to be altered, not just sealed.

The same logic runs round the perimeter: the wall junction, the balustrade post, the drainage channel across the doorway. Junctions are what a waterproofing specification is really for, so our written proposal names how each one is formed rather than describing the deck as a single surface.

Where the water is supposed to go

Small decks are unforgiving about drainage. One outlet often serves the whole area, so when it silts up with grit, moss or a planter's run-off, the water has nowhere else to go and starts testing the weakest edge. Falls are shallow by design, and a deck laid slightly out of true will hold water in a corner for days after rain.

A survey checks all of that before anyone quotes: where the outlets are, whether they are clear and reachable, where water stands after rain, and whether the falls will still work once a new layer goes on. Where outlets are the root of a recurring problem, keeping them clear belongs on a planned maintenance schedule rather than in an emergency call-out every autumn.

Finishes you can walk on

A roof is walked on by roofers. A balcony is walked on by whoever lives behind the door, in bare feet in July and in wet shoes in November, so the top surface has to do two jobs at once: keep water out and be safe underfoot. Trafficked systems handle this with a hard-wearing top layer and an anti-slip finish worked into it, so the grip is part of the build-up rather than something laid loose on top of it.

The liquid-applied waterproofing we install, HYDRONYLON, suits these decks for a practical reason: it is applied wet, so it runs continuously up a threshold, around a balustrade fixing and into an outlet without seams cut around every obstruction, and it adds very little height to a deck that cannot afford much. Where a terrace is to carry paving, planters or furniture afterwards, the protection layer above the waterproofing gets specified alongside it rather than left as an afterthought for somebody else.

Occupied blocks, podium decks and walkways

At building scale this becomes an access problem. Communal walkways are escape routes and cannot simply be closed. Podium decks sit over car parks and plant rooms, so a failure is discovered by whoever parks there. Balconies belong to individual homes, which means notice, appointments and somebody being in.

For managing agents and asset teams the practical questions are the boring ones: what order, what notice, what is reachable without scaffolding, and what the residents will be told. We answer those in writing before work starts, and the record afterwards is written to be forwarded to leaseholders without editing. The wider version of that approach sits with our commercial roofing work, and in more detail for property and block managers.

When waterproofing is not the answer

Not every leaking balcony is a waterproofing job. Far better to establish that at survey stage than halfway through the work. Adhesion depends entirely on what is underneath and the state it is in: a surface that is friable, damp, contaminated or moving will not hold a new layer however well it is applied, and some substrates need specific preparation before anything can be laid on them at all. Others should not be built on in any circumstances and have to come off.

There are also failures that a new surface would simply hide. Where the insulation inside the build-up is wet, the deck surface is not what failed, and renewing it locks the water underneath. Where a balustrade fixing has cracked the structure, or a slab is moving, the movement will find its way through anything laid over it. Where the water is arriving from a wall, a window or the roof above, a perfect deck changes nothing.

Those calls are made by inspection, not by a sales visit. The condition survey looks at the substrate, the junctions and the drainage before a specification exists. Any guarantee that comes with the system we then specify is recorded in the quotation, not promised in advance on a website.

Frequently asked questions

Can you waterproof over the tiles that are already down?

Sometimes, and the survey settles it. Tiles bedded solidly and still gripping can occasionally be prepared and worked over. Tiles laid loose on sand, lifting at the corners or drumming when tapped are hiding the real surface, and a waterproof layer laid over something that moves will follow it. Where the tiles have to come up they come up, and we say that before you commission anything.

The leak shows in the flat below, but the balcony looks fine. How?

Because the failure is nearly always at an edge rather than in the middle. Water gets past a threshold, behind an upstand, around an outlet or through a joint at the wall, then travels along the structure and appears somewhere that looks unrelated. The visible deck can be in good order while the junction you cannot see does the damage.

Do residents lose the use of the balcony while you work?

For a period, yes, and we plan around that rather than pretending otherwise. The door usually has to stay shut while the new layer is applied and cured, and furniture and planters need moving beforehand. We agree the sequence and the notice with whoever manages the block, and on a multi-balcony job we work in an order that keeps disruption to one home at a time.

Can a whole block of balconies be done as one programme?

That is usually the sensible way to buy it. One survey covers the block, the specification is written once, and the balconies are grouped so access, notice and scaffolding are arranged once rather than repeatedly. It also gives a managing agent a single set of records instead of a folder of unrelated call-outs.

Is a balcony really the same work as a flat roof?

Technically yes, which is why we treat it as our own trade rather than a sideline. It is a waterproof layer over occupied space with falls, upstands and outlets. What is different is the traffic on top of it and the fact that the space underneath is somebody's home.

Balcony staining a ceiling downstairs?

Tell us which floor it is on, what the deck is finished with and where the mark shows inside. That is usually enough to plan a first visit.