Let's start by debunking a massive misconception that costs thousands of euros in renovations: Tiles are NOT waterproofing. The tile surface itself may not absorb water, but your balcony isn't a single tile. It's made of dozens of pieces joined together with grout. And standard (cementitious) grout is porous. It absorbs water like a sponge! When it rains or when you hose down the balcony, some water seeps through the joints, passes through the adhesive and reaches the bare concrete slab.
If the contractor hasn't applied waterproofing under the tiles, the countdown to your balcony's destruction has already begun.
What happens when water sneaks under the tiles of an unprotected balcony?
The water can't evaporate easily because the tiles block it from above. It stays trapped inside the adhesive layer and the screed (the cement slope layer), unable to escape. Over time, this permanent entrapment saturates the concrete and starts attacking the internal structure of the balcony.
The water dissolves cement salts. When it finally finds an exit (usually from the balcony edge or through joints in summer), the salts emerge on the surface creating those ugly, white stains.
The water advances deep into the slab's concrete until it reaches the steel reinforcement. The steel rusts. When it rusts, it expands (up to 7× its volume). This expansion is so powerful it cracks the concrete from inside out ! Result? The neighbour below sees the balcony soffit "exploding" as chunks of plaster and concrete fall, revealing the rusted steel bars.
To avoid this nightmare, there is one strict rule during construction or renovation: Waterproofing goes ABOVE the slopes and BELOW the tile adhesive.
The base on which the entire system is built.
Provides gradient toward the drain so water flows.
Applied in 2-3 cross coats. Creates a flexible, completely waterproof membrane that bonds perfectly with the cement substrate beneath it. This membrane is the critical barrier between water and your concrete slab.
Must be flexible (S1) to follow thermal expansion. Applied directly onto the dried waterproofing.
Ideally epoxy or water-repellent grout for maximum protection.
This way, whatever water manages to pass through the joints will stop at the waterproofing membrane and (thanks to the slope) will be guided slowly toward the drain through the adhesive, without ever touching the concrete slab!
We've removed the old mosaic tiles and we're down to bare cement. We've bought expensive granite porcelain tiles.
The tiler says "Come on, you don't need waterproofing, we'll use good grout". He sticks the tiles directly onto the cement. We save €200 on waterproofing materials. After 3 years, the slab's steel corrodes, the slab swells and 5 tiles "pop off" and shatter. To fix it, we must rip ALL of them out, costing over €1,500.
We insist the tiler applies 2 coats of flexible cementitious waterproofing (with fibreglass mesh at corners) before tiling. We pay €250 extra for waterproofing materials and labour. It pours with rain, we hose the balcony daily. The water stops at the membrane. Our balcony remains "immortal" and the neighbour below sleeps soundly!
The Final Conclusion: Trying to save money by skipping waterproofing under tiles is a recipe for total destruction. Never let the tiler start until you see the grey/blue waterproof membrane covering the entire balcony floor.
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