Terrace and balcony waterproofing is the system that stops rain and snowmelt on open living areas from reaching the floor below. In these areas leaks usually come not from the material but from a wrongly built layer sequence and skipped details.
Signs of a leak
Staining and blistering paint on the ceiling below. Drips and lime marks on the soffit of the balcony. Darkening and moss in the joints of the finish. Tiles debonding and sounding hollow. Rust runs around balustrade fixings.
Layer sequence
The correct build up runs as follows: preparation of the structural slab, screed to falls towards the outlet, primer, waterproofing layer, separating layer, thermal insulation, protective screed and finish.
The fall is formed below the waterproofing, not above it. Standing water on top of the waterproofing eventually finds a weak point. No reverse falls or level areas should be left.
Critical details
Outlet. Must be joined to the waterproofing with a flange and the membrane taken down inside it. A second stage drain is needed to release water collecting under the finish. A single stage outlet traps water beneath the tiles.
Wall junction. The waterproofing must run at least twenty centimetres up the wall and be mechanically terminated into a reglet or profile.
Door threshold. Sufficient level difference must be kept between the terrace and the interior, with the waterproofing carried under the threshold and into the room.
Corners. Internal corners receive a forty five degree fillet in mortar and are reinforced with corner tape.
Balustrade fixings. The weakest point of a balcony. Where vertical fixings pierce the waterproofing, each hole must be individually sealed. Where possible the balustrade is fixed to the side face.
Drip edge. A continuous drip channel must run along the balcony nose. Without it, water wraps under the profile and travels to the ceiling below.
Application and checking
A flood test is carried out before the finish is laid. Outlets are plugged, the surface is filled with water and left at least twenty four hours while the ceiling below is observed. Skip this test and the problem appears only after everything is finished, at multiplied cost.
With liquid systems the total material used is recorded and divided by the area to compare against the manufacturer figure. Thickness cannot be judged by eye.
Frequently asked
Can it be done without removing the tiles
In some cases systems that can be applied over the existing finish are available, but where water is trapped underneath this is not a durable solution. The decision follows an on site inspection.
Is thermal insulation also needed on a balcony
A balcony slab forms a thermal bridge. Where there is mould on the ceiling below, thermal insulation should be considered together with waterproofing.
Can work be done in winter
Liquid systems do not cure below dew point and bituminous membrane becomes brittle at low temperature. A suitable weather window is needed.
If your terrace or balcony is leaking, identifying where the water enters before stripping the finish prevents unnecessary demolition.