In construction, problems rarely start from flat, open surfaces. Problems always begin where two different materials meet. On a balcony, the tile meets the wall, concrete meets the railing steel, and large slabs meet each other.
If these junction points are sealed the wrong way (or ignored entirely), your balcony will become a "colander" from the very first winter. Let's look at the two biggest culprits.
Your balcony tiles bake in summer at 60°C and freeze in winter. Heat makes them expand and cold makes them contract.
If the tiler sticks all tiles tightly together without leaving any gaps, using rigid cementitious grout between every piece, in summer the tiles will have absolutely no room to expand. Result? They'll pop off, burst into the air or crack the grout. Rainwater then passes through those cracks completely unhindered and soaks into the screed layer below.
Every 9 to 12 m² (and always around the perimeter where tile meets wall), an empty joint about 1 cm wide must be left: the Expansion Joint.
This gap is NEVER filled with ordinary grout! A foam polyethylene cord (backer rod) goes in the bottom and Polyurethane Sealant fills the top. The sealant becomes an incredibly strong rubber. When tiles move, the rubber stretches and compresses, keeping the joint 100% watertight for decades!
Balcony water flows (thanks to the slope) toward the edge. There it meets the "parapet" (a low wall) or metallic railing bases bolted or embedded into the concrete.
If waterproofing is applied at a sharp 90° corner (where the horizontal floor meets the vertical parapet wall), the material usually cracks at the slightest vibration or thermal movement. The proven solution is a "Chamfer". Before waterproofing, the installer uses repair mortar to "round off" the corner, creating a smooth, curved transition instead of a sharp angle. This stops water from pooling in the angle and lets the waterproof sealant hug the joint smoothly without developing stress cracks.
If railings are bolted to the floor, water will try to seep around the bolts or posts. If it gets through, the steel will rust inside the concrete, swell and blow off the tiles! The fix: Every point where metal "pierces" the floor must be wrapped with polyurethane sealant before any adhesive or tile is laid.
Our balcony is large (20 m²) and we've just applied the waterproofing. The tiler is about to start.
The tiler lays tiles flush against the wall and parapets, filling every gap with rigid acrylic grout. No flexible joints at all. In August the tiles expand. The hard grout around the parapet cracks, opening a 2 mm fissure. Come winter, rainwater streams through that crack under the tiles. Water freezes, and by spring we have 4 "hollow" tiles that have detached.
The tiler leaves an 8 mm gap around the perimeter (between tile and wall/parapet) and cuts the balcony in half with another joint. After tiling, he cleans these joints perfectly, inserts backer rod and fills the gap with polyurethane sealant colour-matched to the tile. The tiles can now move fractions of a millimetre in summer without anything cracking, and water is blocked 100%.
The Final Conclusion: A balcony almost never leaks through the middle of a tile. The battle against water is won or lost exclusively at the joints, parapets and junctions where different materials meet each other. A single cartridge of good polyurethane sealant costs less than €10, but it saves your balcony and your roof from damage that could cost thousands of euros to repair!
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