How to waterproof a walkable terrace: materials and calculation

A leak on a terrace almost never appears where the real fault is: the water gets in at one point and shows up metres away, on the ceiling of the flat below. That's why waterproofing a walkable terrace can't be done "by eye" or by leaving out details such as expansion joints.

Two different systems depending on use

• Liquid polyurethane membrane: applied by brush, roller or airless and forms a continuous layer with no joints, ideal on roofs of complex geometry where a prefabricated sheet would be hard to adapt. Approximate coverage: 1 litre per m² per coat, always applied in 2–3 crossed coats (each coat in a direction perpendicular to the previous one) to avoid voids and ensure a uniform thickness.
• Waterproofing mortar for walkable terraces: applied under the final paving (tile or porcelain stoneware), with mesh reinforcement at junctions and joints. Approximate consumption of 1.5 kg/m² per mm of thickness, with usual thicknesses of 2–5 mm.

Worked example with waterproofing mortar

For a 20 m² terrace with a 3 mm layer:

1. Area to treat: 20 m²
2. Estimated consumption: 20 × 3 × 1.5 = 90 kg
3. 25 kg bags needed: 4 bags

This is the same calculation the PresupuestoYa calculator applies automatically for the Waterproofer trade.

What gets forgotten and causes most leaks

• The waterproofing's own expansion joints: it isn't enough for the paving on top to have them. If the waterproofing is left continuous without its own joints on a large terrace, it can crack from thermal movement even though the final floor does have them.
• Protective geotextile: on inverted flat roofs it is laid over the waterproofing before the gravel or paving fill. It's not an optional extra — it's the layer that stops a simple footstep with a stone underneath from puncturing the membrane, both during installation and in future maintenance.
• Prior bituminous primer: improves the bond of the sheet or membrane on concrete, with an approximate coverage of 5 m²/litre. It must be left to dry completely to the touch before laying the next layer — applying onto it wet doesn't improve the bond, it only means the next layer doesn't sit properly.

Large-area flat roof: sheet instead of liquid membrane

For large warehouses or communal terraces, PVC sheet welded with hot air (instead of bonded, like EPDM) gives a very reliable joint between panels, with good UV resistance and easy to repair locally if one spot is damaged. It carries a 10% margin for welded overlaps.

Frequently asked questions

How many coats of liquid membrane are needed as a minimum? 2 crossed coats as a minimum; on more demanding surfaces 3 are recommended, always applied in directions perpendicular to each other.

Why does it still leak if I've already put waterproofing over the whole terrace? Most often the waterproofing system's own expansion joints are missing, not just the paving's, or the point of failure is at a junction (parapet, drain outlet) rather than in the general area.

Is geotextile needed if the terrace has no gravel? It's essential above all on inverted flat roofs with a gravel finish; on terraces with paving directly over waterproofing mortar, the protection is provided by the system itself with its reinforcing mesh.

Conclusion

Choosing the system well —liquid membrane or mortar under paving— and not skipping the expansion joints or the protective geotextile is what separates waterproofing that lasts decades from one that fails within a few years. Automating the calculation of material by thickness and coats gives a tight quote and avoids running short halfway through application.