Roof terraces and flat roofs cause more leaks into apartments than anything else we inspect. Water sits on them, the edges move with the sun, and a lot of the waterproofing was done cheaply or covered over later. Here is where they leak, how to prove it, and what the fix cost on our own reports.
Buying a flat with a terrace? Our pre-purchase inspection traces leaks and puts a cost range on every finding, from €490 + IVA. Holding a quote? Quote Check, €220 + IVA.
Where a terrace or flat roof lets water in
Hardly ever through the middle. The leak is nearly always at an edge or an opening: the base of the upstand where the floor meets the wall or parapet, the outlets, the door threshold, or the capping along the top of the parapet. On a 1990s apartment block in Porto, the last repair had been a thin liquid membrane rolled over the old covering. The mesh was showing through and it was lifting at the upstands, the one place it had to hold.
The other faults we keep finding are outlets blocked with leaves or, on one Porto terrace, plant roots. Terraces that fall towards the door. Parapet cappings with open joints and no drip. And new tiles laid over old ones, with the failed membrane still underneath.
Prove the source before anything gets stripped
A stain under a terrace doesn’t prove the terrace is leaking. It can just as easily be the parapet, a window frame or a pipe in the slab. A flood test settles it: block the outlets, flood the terrace to a marked depth, and watch the level and the ceiling below. On our reports it’s €200–380, the cheapest money on the whole job.
Inside repairs wait for the result. On one flat we priced cutting back, drying out and replastering under a terrace at €2,500–7,000, once the flood test had proved the source.
What it costs to fix a leaking terrace or flat roof
From our inspection reports in 2025 and 2026, ex IVA:
- Clearing roots and blocked outlets, apartment terrace, Porto: €900–1,600. A 10 m strip drain along the door: €3,500–6,000.
- Balcony of about 5 m², strip and waterproof: €1,100–1,900.
- Terrace of about 25 m², strip and waterproof: €2,800–4,500.
- Terrace of about 80 m², full strip, waterproof and flood test: €6,000–10,000.
- Veranda capping joints and outlets, 2000s house, Gaia: €3,500–6,500, plus €1,800–4,500 for the door frame and floor the water had already ruined.
- Flat roof on an apartment block, Porto: liquid membrane redone at the manufacturer’s full thickness, or a reinforced bituminous system, €60–120/m² through the condominium.
The spec worth paying for
Strip back to the slab and check the falls run to the outlets. Take the membrane well up the walls and parapets, finish it under a flashing or capping, and bond new outlets into it. Flood test before the tiles go back down, because after that nobody sees the membrane again. A liquid membrane can do the job if it goes on at the specified thickness with the mesh fully bedded. Rolled over a failed covering, it’s how the Porto block ended up back on our list.
Looking at a flat with a terrace or under a flat roof? Our pre-purchase inspection finds where the water gets in and prices the fix before you sign. From €490 + IVA, across Porto, the Minho and central Portugal.
Buying a flat with a terrace
Look at the ceilings in the rooms underneath, and in the flat below if you can. Fresh paint under a terrace, new tiles on a tired one, or new grout round the outlets all raise the same question: what was done to the waterproofing?
If the terrace or flat roof covers other flats, the fix may be a condominium matter as well as yours. Read the minutes and ask the administrator whether works have been voted and who pays. On one Porto building the roof works came to €55,000, and the flat we inspected carried about €1,687 of it on a 30.68‰ share.
More reading: buying a top-floor flat under a shared roof, the condominium reserve fund, where damp in Porto comes from and what we find when we inspect a house for sale.
Figures are from our own inspection reports in 2025 and 2026, ex IVA, with names and addresses left out. Every building is different, so treat them as a guide to scale, not a quote.
Buying a flat with a terrace or under a flat roof?
We inspect it before you sign: the terrace, outlets, parapets and the ceilings below, plus the roof, structure, electrics and plumbing. If the source isn’t clear we tell you, and say what test will prove it. Every fix gets a cost range so you can take it into the negotiation. Already own it? We check the builder’s quote before you sign it.
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