In short: what makes a roof reliable
Reliable roof waterproofing rests on three things together: a fall that carries water to the outlets, a continuous membrane with properly executed junctions, and a water test before the membrane is covered. If any one of the three is missing, a leak is only a matter of time.
A mistake at this stage is invisible on handover day and a month later. It shows up after the first heavy autumn downpour, when the ceiling is already painted and the roof is covered in tiles or gravel. The fix then starts by removing everything on top.
Layer order on a flat roof
A flat roof on a house typically has: the structural slab, a screed laid to falls towards the outlets, a vapour control layer where needed, insulation, the waterproofing membrane, a separating layer and a protective or finish layer, whether gravel, pedestal-mounted pavers or a green roof. In an inverted roof the insulation sits above the membrane and shields it from sun and temperature swings.
Which build-up to use is decided in the project under the CTE, together with insulation and whether the roof will be walkable. For the owner what matters is this: each layer covers the one before, and once the next is laid the one below can no longer be inspected. So checks happen between layers, not at the end.
Falls and outlets
Water left standing on a roof after rain will eventually find a weak spot in the membrane. The fall to the outlets is formed before waterproofing, and checking it is simple: after rain or a hose-down there should be no puddles. If a puddle lasts a whole day, the fall is wrong, and it gets corrected now, before the membrane goes down.
There should be at least two outlets, or one plus an emergency overflow through the parapet. A single outlet blocked by leaves on a roof with parapets on all sides turns it into a pool. On houses near the sea or in the Garraf with pine trees nearby, outlet guards are fitted and cleaning them becomes part of maintenance.
Junctions: parapets, penetrations, solar panels
Most leaks do not start in the middle of the roof but where the membrane meets something vertical: a parapet, the wall of a stair enclosure, a chimney, a rooflight, a vent pipe, an air-conditioning stand. The membrane is turned up the vertical to the height set in the project, fixed and capped, and penetrations are formed with purpose-made collars, not a ring of sealant.
Solar panels are a topic of their own. On our house with a pool, the panels were in the design from the start, and that is how it should be: fixings drilled through the membrane after it is laid create dozens of new risk points. Panel supports are either installed before waterproofing and detailed as penetrations, or a ballasted system is used that never pierces the membrane. The cable exit from the roof is decided at the same time.
Water test and photos before covering
Once the membrane is down, the roof is tested with water: outlets are plugged, the roof is flooded to a few centimetres and held for the time set by the site supervision team, then the ceilings are inspected from below. On pitched roofs the alternative is prolonged spraying. Only after a clean result does the protective layer go on.
Every layer is photographed before it is covered: falls, membrane, every junction and every penetration, with an overall shot and a close-up. If a stain appears on a ceiling years later, those photos show where to look without lifting the whole roof. How to keep that archive is covered in our article on construction photographs.
Coastal climate and walkable roofs
In Castelldefels, the Garraf or the Maresme a roof faces three things: strong summer sun, short but very intense autumn downpours and salt in the air. A membrane left exposed to the sun ages faster, so the protective layer here is not a formality. Metal fixings and flashings are chosen with sea air in mind.
If the roof will carry a terrace, furniture, planters or a hot tub, decide that before design: it changes the load, the protective layer and the detail at the roof access door. That door threshold must sit above the level water would reach if an outlet blocked. Planning the roof terrace itself is covered separately.