Roof Terrace and Balcony Waterproofing

A deck in a garden lets water through and nothing happens. A deck over a bedroom lets water through and something happens.

That single sentence is the whole subject. Every difference in specification, sequence, cost and risk between a garden deck and a roof terrace follows from it, and most terrace failures we are called to come from somebody treating the second job as though it were the first.

A terrace over a room is a flat roof with a floor on it

The deck is the walking surface. The roof is the layer underneath, and it is doing exactly the job a flat roof does anywhere else on the building.

That reframing tells you which questions matter. Nobody asks what colour a flat roof is before asking whether it drains, but on a terrace the visible part is the boards, so the conversation starts in the wrong place: composite or timber, grey or brown, wide or narrow. Those are real decisions and they are the last ones that matter.

What matters is what is under the boards, and the reason it matters is that it is about to become invisible. A flat roof you can see is a flat roof you can inspect. A flat roof with a deck on top has been deliberately covered up, so anything wrong with it now announces itself on the ceiling below rather than on the roof above.

The order of the layers, and why the order is the job

From the structure upwards: deck or slab, insulation, waterproofing laid to a fall, support for the boards, then the boards.

Taking those in turn.

The structural deck or slab carries the load. A terrace in use carries more than a roof that nobody walks on, and it carries it in a different way, so whether the existing structure is adequate is a question to settle before anything else. That is a survey question, not an assumption.

Insulation, where there is a heated room below. On a terrace this normally sits above the structural deck and below the waterproofing, which keeps the structure warm and takes the condensation risk out of it. Getting the insulation on the wrong side of the waterproofing is one of the ways a technically watertight terrace still produces water on a ceiling.

The waterproof layer is the roof. It has to be continuous across the field, turned up at every wall and upstand, taken correctly into the door threshold, terminated at every edge and dressed into the outlet. And it has to be laid to a fall, because water that is not moving is water that is waiting for a defect.

The support for the boards holds the deck clear of the waterproofing. Over a habitable room that support should bear on the surface rather than penetrate it, for a simple reason: every fastener through a waterproof layer is a hole, and holes are a maintenance liability for as long as the terrace exists.

The boards go on last. By that point every decision that affects whether the room below stays dry has already been made.

This is why the sequence is not a scheduling detail. The waterproofing has to be complete, inspected and signed off while it is still fully visible, because that is the last moment anything about it can be corrected without lifting a finished terrace. Our page on roof terraces, balconies and decking sets out how that sequence runs as a job.

Falls and outlets: where the water actually goes

Water leaves a terrace through an outlet, moved there by falls built into the layer underneath, not off the edge of the boards.

This surprises people, because the deck surface is normally laid level for comfort and to stop furniture rocking. So the visible surface of a terrace is flat, and everything that makes it drain is happening out of sight.

Three things go wrong here regularly.

No fall, or a fall that no longer works. Roofs settle, decks deflect and extensions move. A fall that was adequate when the roof was built can flatten over decades, and water starts standing where it used to run. On an exposed flat roof you would see the ponding. Under a deck you see nothing at all until something else tells you. Our article on ponding water on a flat roof covers what standing water does over time.

An outlet that is too small, badly placed or blocked. A terrace has no free edge to shed water over, so every drop that lands on it has to leave through a hole. Block the hole and the terrace becomes a tank with a floor over it. Outlets also collect exactly what a terrace generates: leaves, planter compost, general debris pushed between the boards.

An outlet that cannot be reached. This is the avoidable one. If clearing the outlet means dismantling the deck, it will not be cleared, and the design has guaranteed the failure it was supposed to prevent.

Why balconies leak more than their size suggests

Because a balcony is almost entirely perimeter, and the perimeter is where the difficulty is.

Think about the details packed into three or four square metres of a typical London balcony: a door threshold, wall junctions on two or three sides, an outer edge, a drainage point, and fixings for the balustrade. On a larger flat roof that same number of details is spread across a much bigger area of straightforward field. On a balcony there is barely any field at all.

The recurring causes, in the order we meet them:

The door threshold. The most common single entry point on a London balcony. The finished terrace level needs to sit low enough relative to the internal floor for the waterproofing to be turned up behind the door, and the pressure to keep the step down for level access works directly against that. Where the upstand is too shallow, driving rain and standing water get behind the threshold and into the floor structure.

The junction with the building. The waterproofing has to turn up the wall and be terminated so water cannot get behind it. On older balconies this is often a mortar fillet that has cracked, or an upstand that has pulled away.

Balustrade fixings. A balustrade has to be held by something, and on a terrace over a room the honest answer is that the structure should carry it without perforating the waterproof layer. Where posts have been bolted straight through a membrane, each bolt is a permanent risk, and the movement of the balustrade in wind works on those holes continuously.

A previous deck laid over a failed surface. Very common. Somebody put boards over a balcony that was already leaking, and the leak carried on with the evidence hidden. Water now enters at the original defect, travels under a deck nobody can see through, and appears at a point that has nothing to do with where it got in.

Movement. Balconies are exposed on more sides than any other part of the building and they heat and cool faster than the structure they are attached to. A detail that cannot accommodate that movement opens up.

For a balcony on a block above 11 metres there is a further constraint worth raising early: building regulations in England restrict the use of combustible materials on balconies attached to relevant buildings of that height, which directly affects what the deck can be made of. That is a rule about the building, and it needs checking before a specification is agreed rather than after boards are ordered.

Why a certified liquid system suits terraces particularly well

Because a terrace is almost nothing but awkward geometry, and a seamless layer has no joints to put in the awkward places.

Consider what the waterproofing on a terrace has to do. Run across the field. Turn up at every wall. Go correctly into the threshold. Wrap an outlet. Terminate at an edge. Go around a balustrade base. Every one of those is a place where a sheet material has to be cut, lapped or welded, and a lap is a joint, and joints are what fail.

A cold-applied liquid waterproofing system is applied wet and cures into one continuous layer, so the field, the upstands, the threshold and the outlet are the same unbroken surface with nothing to lap. It is also applied cold, with no hot works, which matters on a balcony attached to an occupied building. K & M Roofing Solutions is a certified applicator of HYDRONYLON, a liquid-applied waterproofing system certified to ETA-23/0735.

That is not the only answer and it is not always the right one. Where the deck below has already failed, no covering fixes that, and the honest advice is structural work first. What decides it is the survey rather than the preference of whoever is quoting.

What to do if you already have a terrace and a damp ceiling

Establish where the water is entering before anyone lifts anything, because on a terrace the entry point and the stain are usually in different places.

Worth checking first, and none of it requires taking the deck up:

• Does it follow driving rain from one direction rather than any rain? That points at a threshold or a wall junction rather than at the field.
• Does it follow heavy rain rather than light rain? That points at drainage: an outlet that copes until it does not.
• Is the stain against the external wall, or out in the middle of the ceiling? Against the wall points at the perimeter.
• Can you see the outlet at all, and is anything standing on the deck after rain?
• Has anything changed above? A new planter, a hot tub, a barbecue, furniture with hard feet, or trades working on the roof.

And one thing that is not a leak at all: on a top floor room under a terrace, condensation can produce staining that looks exactly like water ingress. Our guide to flat roof leak diagnosis sets out how to tell them apart, and it is worth reading before you pay anyone to open up a terrace.

If it does turn out to be the waterproofing, the good news is that a terrace is one of the cases where an overlay onto a sound deck is often possible, so the boards come up but the structure does not. That decision is set out in flat roof repair and waterproofing.

At K & M Roofing Solutions we survey terraces and balconies as part of our flat roofing work across South and West London, and the survey is free. We would rather tell you the deck below is dry and the outlet is blocked than sell you a new terrace you did not need.