Terrace Waterproofing: A Practical Guide

Where terraces actually leak

In order of how often we find them:

• The parapet junction. Where the horizontal slab meets the vertical parapet. It moves, it is rarely coved, and membranes are often stopped short of it.
• Around rainwater outlets. Water concentrates here by design. If the membrane is not dressed into the outlet, water goes around it and straight into the slab.
• Construction and expansion joints. These move. Coating over them is not treatment.
• Service penetrations. Pipes, conduit, dish and antenna mountings, lift machine room walls.
• Ponding areas. Any dip where water stands for days after rain will find its way through eventually.

Falls come before anything else

A waterproofing system is designed to shed water, not to hold it indefinitely. If your terrace ponds, the system is being asked to do something it was not designed for. Before any membrane, we flood-test and check that water reaches the outlets. Where it does not, a screed to fall is laid first. It adds cost and it is the difference between eight years and three.

The systems, and what each is for

• APP / SBS torch-applied membrane. A thick bituminous sheet, torch-bonded. Very robust, excellent over large open areas, best where the terrace will be covered with a protective screed or tiles. Detailing at junctions requires skill.
• PU liquid-applied membrane. Seamless, elastic, UV-stable, and it bridges hairline cracks. Excellent on complex terraces with many penetrations, because there are no laps to detail. Higher material cost.
• Acrylic elastomeric coating. Economical, reflective, easy to recoat. Suited to sound slabs with surface porosity rather than active leaks. Lower film build than PU.
• Cementitious slurry. Two-component, breathable, bonds directly to concrete. Ideal for wet areas, tanks and substrates that will be tiled over. Not elastic enough on its own for a slab with movement.
• Crystalline. Reacts with moisture in the concrete to grow crystals in the pores. Excellent for below-grade and water-retaining structures. Not a surface membrane.

Preparation, which is most of the job

Whatever system is specified, the sequence is broadly the same and the preparation is where honest and dishonest contractors separate:

• Remove all loose material, old coating, laitance and vegetation growth.
• Test moisture. Applying over a wet slab is the most common cause of blistering.
• Rout out and fill cracks; treat live cracks differently from static ones.
• Cove all wall-to-slab junctions in polymer-modified mortar. A sharp internal corner is where a membrane tears.
• Prime, then apply the system to the specified number of coats and dry film thickness.
• Dress into outlets and up the parapet to at least 150–300 mm above finished level.
• Flood-test for 48 hours before any protective screed goes down.

Questions worth asking before you sign

• What is the dry film thickness, and how many coats? "Two coats" without a thickness means nothing.
• Are the parapet junctions being coved, and how high does the membrane go up the parapet?
• How are the outlets being detailed?
• Is a flood test included, and for how long?
• Does the quotation include a screed to fall if ponding is found?
• What exactly does the warranty cover, and what voids it?

A contractor who cannot answer these has not surveyed your terrace properly.