Balcony and Terrace Waterproofing

Practical rules for balconies and terraces: where the water goes, falls, the waterproofing layer, edge profiles, door thresholds and tile joints.

An open balcony or terrace is one of the parts of a building that works in the harshest conditions. The sun heats the surface and it cools at night; rain and watering soak it; people walk on it and drag furniture across it. If the waterproofing is weak under that load, water shows up on the ceiling of the floor below, as rust stains and spalling concrete on the underside edge of the balcony slab, or on the room floor next to the door.

Where leaks start

The most common causes on balconies and terraces are similar:

the surface has no fall, or falls towards the wall, so water runs to the door threshold;
tiles were laid directly on a screed with no waterproofing, so water passes through the joints into the structure;
there is no drip profile at the edge, so water runs onto the underside of the slab and breaks down the concrete;
junctions around walls and balustrade posts were not detailed;
thermal expansion was ignored, so tiles debonded and joints cracked.

This list also shows the components of a correct system.

Falls: water away from the house

Everything starts with the fall. The balcony surface is built with a slight fall away from the building so water runs away from the wall and door towards the edge or a gutter. On large terraces water is led to drains or channel drains.

On a balcony the slope is formed in the screed first, and only then is the membrane laid over it. Trying to create the fall with the waterproofing itself, or "correcting" the surface with the thickness of tile adhesive, is wrong: water then collects within the adhesive bed.

The height difference between the door threshold and the balcony surface matters too. When the levels are equal — especially where a step-free exit is wanted — a channel drain in front of the door or another extra drainage measure should be considered. Otherwise water enters the room in heavy rain or wind.

The waterproofing layer

Elastic waterproofing is chosen for balconies and terraces because the surface moves with temperature changes. Under tiles, elastic polymer-cement slurries or liquid membranes are usually used, and on large terraces sheet membranes. For exposed, trafficable finishes with no tiles, UV-resistant polyurethane systems are available.

During application:

the substrate must be sound, dry and dust-free; old tiles and weak screed must be removed completely;
elastic tape is set into wall-floor corners and the waterproofing is turned up the wall;
balustrade posts, drains and pipe penetrations are detailed with collars;
the membrane is applied in as many coats, and left to cure for as long, as its data sheet requires;
application rules for sunny, very hot weather are checked separately, since a coat can dry too fast and crack.

Edge profile and drip

The balcony edge needs special attention. The waterproofing is taken to the edge and dressed over a metal or aluminium edge profile. The lower part of the profile has a drip: water falls from this point instead of running along the underside of the slab. Without a drip, water gets onto the concrete soffit, corrodes the reinforcement and in time the concrete spalls.

If an edge gutter is installed, the waterproofing is directed into the gutter and the junction is sealed separately.

Tiles, joints and expansion

For an open balcony, frost-resistant, low-absorption tiles are chosen: porcelain or ceramics rated for outdoor use. Adhesive and grout must also be flexible and suitable for exterior conditions. Tiles should be laid with full adhesive coverage; water collects in voids under a tile and, when it freezes, pushes the tile off.

On large terraces movement joints are essential. They allow the surface, divided into bays, to move, and are filled with an elastic sealant. A flexible joint is also left between the tiles and the wall along the perimeter.

In some cases setting tiles loose on adjustable pedestals is the more reliable solution: water passes between the slabs and runs freely over the waterproofing, while the slabs are freed from freeze and expansion stresses.

Existing leaks and maintenance

Laying new tiles over old ones or applying a clear sealer usually gives only a temporary result. First the water's path must be established: is the leak at the edge, under the door, or across the whole surface? In most cases the lasting fix is to strip the old layers, restore the fall and rebuild the waterproofing.

Ongoing maintenance is simple but must be regular:

Drains and gutters are cleaned regularly.
Sealant in wall and movement joints is checked from time to time.
Protective pads are placed under heavy planters and furniture.
In winter, avoid breaking up snow and ice with metal tools.

In villa and apartment renovations Zaferoğlu İnşaat plans balconies and terraces together with the interior, taking the direction of water and waterproofing details into account.

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