Balcony Flashing: Where the Deck Meets the Wall

Survey the fall you inherited, then price the screed

A fall that exists on a section drawing is a design fall, and what survives onto a finished deck is always less: construction tolerance takes some, the screeder takes some flattening the last metre to meet an edge trim, and a cantilever slab takes the rest by deflecting and then creeping for years afterwards. BS 6229 handles that by designing to roughly twice the fall you want to end up with, so that the deck still drains once everything has moved. AS 4654.2 approaches external above-ground work from the same direction with a stated minimum fall in the region of one in a hundred. Both are worth reading in the current edition rather than quoting from memory, and both assume somebody checks the achieved fall rather than the drawn one.

On existing balconies, measure before you promise anything. A rotary laser and a rule, readings at the frame, at a metre out, at the nose, and at both ends, all written down on a sketch that goes in the file. Older balconies were frequently built falling back towards the building on purpose, so that runoff did not drip onto the pavement below, and that decision is survivable right up until somebody replaces the doors with a level threshold. If your survey finds a deck falling inward, the flashing conversation is over until the fall is fixed, and fixing it usually means a screed rather than a coating.

Falls on a balcony are also rarely a single plane. A deck draining to one corner outlet falls two ways at once, and a deck with a threshold channel along the building edge falls away from the building along its whole length and then along the channel to the outlet. Set the high point at the door, the low point at the outlet, and check the diagonal, because a plane that is correct on both edges can still pond in the middle if it was built as two triangles that do not meet.

For pricing, the useful property of a screed laid to falls is that it tapers to nothing at the drain, so its volume is well under what a uniform layer at the deep end would cost — but how far under depends on where the falls meet. A single plane running down to a channel along one side is a wedge and averages half its maximum depth. Four planes meeting at a central gully make a shallow upside-down pyramid, which holds a third of its box, so they average two-thirds, and a gully deck ordered at half comes up a quarter short.

The field coat is the part that usually goes right

Cold liquid-applied membranes win on balconies for a geometric reason rather than a chemical one: a small deck is nearly all edge. A four-by-two balcony has under nine square metres of field and something like thirty metres of perimeter, upstand, corner and penetration, and a system that arrives as sheets has to be cut, welded or dressed at every one of them. A system that arrives in a tub follows the shape. The cost per square metre looks alarming next to sheet materials right up until you notice how little of a balcony is actually field.

The number that governs the order is dry film thickness, and the number that governs the tub count is solids content, because everything that is not solids evaporates during cure. A product at eighty per cent solids has to go down noticeably thicker wet than the thickness you want dry, and applicators who work to a coverage rate printed on a lid rather than to a wet film gauge routinely land under specification in the middle of a roller pass where nobody is looking. Use the comb, use it on every coat, and record the readings.

Specify the right family for the exposure while you are at it. ASTM C836 covers a cold liquid-applied elastomeric membrane intended to sit under a separate wearing course, ASTM C957 covers one with an integral wearing surface designed to be walked on directly, and ASTM C898 is the guide to using the first of those properly. Choosing a buried-membrane product and then leaving it as the finished floor of a balcony is one of the commonest specification errors on this detail, and it fails by ultraviolet degradation and abrasion rather than by leaking, which means it fails slowly and out of warranty. ASTM C1305 is the crack bridging test to ask about when the substrate is a concrete slab with a history.

Everything that is not field is a seam, and seams are what you buy

In a liquid system the field coating is reinforced only where it needs to be, and every place it needs to be is a detail: the perimeter upstand, the threshold, every internal and external corner, the outlet flange, movement joints, day joints, substrate cracks, balustrade bases, and the penetrations somebody added for deck lighting. Each of those takes reinforcement fabric or a flashing strip embedded between coats, and each is measured in linear metres.

Run the numbers on a small deck and the proportions surprise people the first time. Take that four-point-two by one-point-six balcony: two long edges, two returns, the threshold across the door, four corners detailed individually, an outlet, a movement joint at the slab junction and four balustrade bases. The field is under seven square metres. The detail length is comfortably north of twenty-five metres. On a balcony, the reinforcement is not a rounding item on the order, it is the order.

Corners consume more than their length suggests, because fabric will not turn a three-way corner without being cut and lapped, and every splice needs overlap. Fabric also has to be fully wetted out — laid into wet base coat and worked through with a roller until the weave disappears — which takes more material than laying it dry and coating over it, and a great deal more than the theoretical length. A waste allowance in the high single figures is normal, and low double figures is realistic on a deck with a lot of corners.

Sequence matters as much as quantity. Details go in first, cure, and then the field coat runs over them, not the other way around, because a detail applied over a cured field coat depends on adhesion to a surface that has already skinned. On a threshold specifically, the reinforcement runs continuously from the deck, up the upstand, and returns into the door reveal or onto the pan, in one piece where the geometry allows it, so that the most loaded part of the system contains no splice.

Measure the detail run off the plan before the material is ordered, not off the deck on the day. The tub count and the fabric count are bought from different parts of a supplier's catalogue and arrive on different lead times, and a crew standing on a scaffold with enough coating and not enough scrim will detail the easy runs, leave the corners, and come back to them after the base coat has cured.

1. Grind out and fill cracks, honour any moving joint as a joint rather than filling it, and prime to the system's own specification.
2. Detail the outlet first, dressing fabric into the flange so the field coat later laps onto detail work rather than the reverse.
3. Detail internal and external corners individually, cutting and lapping fabric rather than forcing a single piece to turn.
4. Run the threshold in one continuous piece from deck to upstand to reveal or pan, avoiding any splice within the door width.
5. Detail the perimeter upstands and any balustrade bases, then let the detail coat cure before the field goes on.
6. Apply the field coats to the wet film thickness the gauge shows, and record the readings against the areas they were taken in.

Where the water leaves, and where it goes when it cannot

Most balconies drain through one outlet, which means most balconies have a single point of failure with leaves, moss, cigarette ends and a child's tennis ball all queuing to find it. BS EN 12056-3 covers roof drainage layout and calculation and BS EN 1253 covers gullies for buildings; between them they tell you what size the outlet should be and what the gully has to do, and neither of them can help once the grating is under a pot plant.

So the question worth asking on a balcony is not what happens when the drain works, it is what happens when it does not. On a roof the answer is a mandated secondary drainage path — the International Building Code chapter on roof assemblies and the plumbing code between them require an emergency route, sized from tributary area against the local design rainfall intensity, discharging somewhere visible so that a blockage announces itself. Balconies frequently sit outside that requirement, and yet a balcony with a level threshold and a blocked outlet floods a bedroom rather than a roof void. Borrow the roof logic even where nothing compels you to.

The practical version is a weep or scupper through the front fascia, set slightly above the deck at the low corner, discharging clear of the elevation below. It stains, and it is meant to: a mark on the render that appears three days after a storm is a maintenance call, whereas the same water going the other way is an insurance claim. Set the invert of the overflow above the deck rather than level with it so the primary outlet does the ordinary work and the overflow only ever runs in anger.

The finish the client wanted is the layer that hides the evidence

Whatever goes on top of the membrane is a wearing surface and a client requirement, and it has no waterproofing duty at all. Treating it as though it does is how balconies end up tiled with a rigid bonded finish over a moving cantilever, with the grout cracking at the perimeter within two winters and everyone blaming the tiler.

Bonded tile over a liquid membrane can be done, but only inside a system rated for it — ANSI A118.10 describes the load bearing bonded waterproof membrane that thin-set tile is allowed to sit on, the TCNA Handbook carries the exterior deck methods, and BS 5385-3 covers external ceramic floor tiling practice. Three things then have to be right: perimeter and intermediate movement joints that actually pass through the tile and the bed, an adhesive rated for external freeze-thaw exposure, and drainage at the bedding layer rather than only at the tile surface, because water gets into the bed, sits on the membrane, and freezes there.

Pedestal-supported pavers dodge most of that by keeping the finish level and dry-laid over a falling substrate, with the water running away underneath at membrane level, and by leaving the membrane accessible rather than entombed. They introduce their own arithmetic — support grid, height range, wind uplift on a raised edge — which the pedestal terrace guide covers rather than this one.

Either way the membrane needs protecting from the trades that follow it, and that protection is measured over the field and the upstands together rather than over the floor plan. Scaffold boards, balustrade drilling swarf, a dropped tile cutter and paver edges all land on a cured film only a couple of millimetres thick. Protection board is cheap, and the alternative is a repair executed underneath somebody else's finished floor.