The water has nowhere to go
If the terrace does not slope toward the outlets, rainwater ponds. Standing water finds every hairline crack and every weak joint. This is the most common cause we see, and no coating will last on a roof that holds water.
A proper fix starts with a sloped screed that drains water off the roof.
Cracks and failed joints
Concrete slabs expand and contract with heat. Over the years this opens hairline cracks, and the joints around parapet walls, pipes and drains are the first places to fail.
Cracks in the slab should be sealed (grouted) before anything is applied over them. Painting or coating over an open crack only hides it for a season.
An old or missing waterproofing layer
Many terraces were never waterproofed properly, or the original layer has worn out. Bitumen and thin coatings dry out in the sun and split.
A flexible membrane, such as a polyurethane (PU) coating, stretches with small movements and handles sun and rain far better.
Weak spots at walls and drains
Where the floor meets the parapet wall, and around drain outlets, water collects and attacks the edge of the waterproofing. The membrane should turn up the wall and wrap the drain.
How a lasting repair is done
A lasting repair treats the whole stack: cracks grouted, slope corrected, a seamless membrane laid and turned up the walls, then a flood test. We hold water on the terrace for 48 hours, and hand over only when the ceiling below stays dry.
If you are not sure where the water enters, a thermal scan can show moisture inside the slab so we open up only the right place.