Most leaks on terraces and balconies come not from poor materials but from a wrongly built layer sequence. A correctly formed terrace section drains water off the surface and off the top of the waterproofing without loading it. This article covers what job each layer does, in what order it comes, and the details most often skipped on site.
The order of layers in a terrace section
The sequence below is the general arrangement for a terrace that will be covered. Whether the thermal insulation sits below or above the waterproofing depends on the system type.
1. Structural slab and surface preparation
The concrete surface must be clean, sound and free of dust. Honeycombing, projections and formwork residue are removed. Depressions where water would stand are made good with levelling mortar.
2. Screed to falls
The layer that makes water run to the outlet. The fall is formed below the waterproofing, not above it, because water standing on top of the waterproofing eventually finds a weak point. A continuous fall towards the outlet must be formed with no reverse falls and no level areas.
3. Primer
Allows the waterproofing to adhere to the substrate. If it is skipped, or applied to a dusty surface, the membrane does not bond, water travels beneath it and the source of a leak becomes impossible to trace.
4. Waterproofing layer
With bituminous membrane, laps of at least ten centimetres are torch welded fully to the substrate. Where two layers are used the laps are staggered so that upper laps do not sit over lower ones. With liquid systems, the number of coats needed to reach the dry film thickness in the manufacturer data sheet is applied, favouring thin coats over one thick coat.
5. Separating layer
Prevents contact between chemically incompatible materials. Direct contact between PVC membranes and bitumen, or between certain liquid systems and screed mortar, causes problems. Geotextile fleece performs this role and provides mechanical protection at the same time.
6. Thermal insulation
In an inverted roof the insulation sits above the waterproofing. In that case a water resistant, non absorbent board such as extruded polystyrene must be used. In a conventional build up the insulation is below the waterproofing and the vapour balance must be calculated separately.
7. Protection and load distribution layer
A screed or protective concrete distributes the load of the finish without damaging the waterproofing. Movement joints are formed in this layer and aligned with the joints in the finish.
8. Finish
Tiles, pavers or slabs. A two stage outlet is needed to drain water collecting beneath the finish, covered below.
The seven details leaks come from
Outlet connection
The most frequent source of terrace leaks. The outlet must be joined to the waterproofing with a flange and the membrane taken down inside it. A separate drainage opening under the finish is needed so water at waterproofing level can escape. A single stage outlet traps water beneath the tiles.
Wall junction
Waterproofing cannot be terminated on the horizontal surface. It must run at least twenty centimetres up the wall and its top edge mechanically fixed into a reglet or termination profile. Leaving the top edge loose behind the render means rain gets behind it.
Door threshold
Where insufficient level difference is left between terrace and interior, rain enters over the threshold. The waterproofing must be carried under the threshold into the room and a water check detail formed.
Corners
At internal and external corners the membrane cannot take a sharp turn, it stretches and tears over time. A forty five degree fillet is formed in mortar at internal corners and corners are reinforced with corner tape.
Pipe and duct penetrations
Every point where the waterproofing is pierced is a risk. Penetrations are sleeved and sealed around with detail tape. Applying mastic alone is not a lasting solution.
Parapet top
The top of the parapet is directly exposed to rain. The coping must fall towards the terrace and carry a drip edge. A flat coping lets water run down the wall face and eventually enter at the base of the parapet.
Expansion joints
Structural movement joints must continue through the waterproofing layer. Waterproofing run straight over a joint tears with the first seasonal movement. Purpose made joint tapes or bellows profiles are used at joints.
What changes on balconies
A balcony is smaller than a terrace but denser in details. The length of edge per square metre is greater, which increases the margin for error.
On balconies water is usually drained not through an outlet but over the outer edge via a drip profile. A continuous drip channel must run beneath the profile. Without that channel water wraps around under the profile, travels to the soffit and stains the ceiling of the flat below.
Balustrade fixings are the weakest point of a balcony. Where fixings pass vertically through the waterproofing into the slab, every fixing is a potential leak. Where possible the balustrade should be fixed to the side face at the nose. Where vertical fixing is unavoidable, each hole must be individually sealed.
Checks during application
Flood test. Before the finish is laid the outlets are plugged, the surface filled with water and left at least twenty four hours. If this test is skipped, problems only surface after everything is complete.
Weather conditions. Liquid systems must not be applied immediately before rain, below dew point or onto a very hot surface. Bituminous membrane becomes brittle at low temperature, so for winter work rolls should be kept in a heated store on site.
Thickness control. With liquid systems the consumption per square metre must be recorded. If total usage divided by the applied area does not match the data sheet figure, the thickness is inadequate.
Lap control. A thin, continuous bead of bitumen should appear at the edge of every lap in bituminous membranes. No bead means the weld is incomplete.
Summary
Success in terrace and balcony waterproofing depends far less on the brand of the product than on building the section correctly and resolving seven critical details. The fall must be below the waterproofing, the waterproofing must run up the wall, the outlet must work in two stages and a flood test must be carried out before the finish. Get those four right and most leaks are prevented already.
If your terrace is leaking, the point of entry should be found before the finish is removed. An on site inspection usually narrows the source and avoids unnecessary demolition.