When do old terrace tiles or old roof membranes have to be stripped off completely before new waterproofing?

While many roofs and balconies can be waterproofedWaterproofingThe work and materials that stop water getting through on flat roofs, balconies, bathrooms, basements, planters and tanks. over the existing surface, there are cases where the old layer has debonded so badly that anything applied on top of it would simply lift away with the old material.

On this terraceFlat roof (roof terrace)The flat concrete roof on top of a building. It is the most common kind of roof in Greece and the part that takes the most sun and rain., the old tiles had crackedCrackA split or gap in render, concrete, a wall, a coating or wood. Fine cracks are usually only on the surface; ones that grow, keep reopening or run diagonally need attention. and underneath sat a wooden expansion jointGrout jointThe gap between tiles, slabs or stones, and the material that fills it. Grouting means filling these joints, which locks the surface together and keeps out water and dirt. strip that had rottedWood rotDecay of wood caused by fungi when it stays damp for a long time. Rotten wood goes soft, gives when pressed or crumbles, and can't be saved with paint. through. The tiles and rotten timber were stripped out, the joints were cleaned and sealed along their full length, and a new cementitious waterproof coatingElastomeric waterproof coatingA thick, rubbery waterproof paint for flat roofs, parapets and roofs. It stretches without cracking as the building expands and bridges hairline cracks. was applied directly onto the sound substrate.

The same applies when an old bitumen membraneBitumen membraneWaterproofing that comes in rolls of reinforced bitumen. It is torched onto the roof in sheets that overlap at the seams. has wrinkled, split and come loose from the slab: it is pulled up sheet by sheet and cleared off the roof so the new liquid membraneLiquid waterproofing membraneA waterproofing product applied as a liquid by roller, brush or spray. Once dry it forms one continuous, flexible membrane with no seams. or polyurethane foamSpray polyurethane foamA foam sprayed on as a liquid that expands on the spot and sets into one seamless layer. It insulates against heat and cold and closes cracks and joints. bonds directly to the clean slab.