On a tiled roof, water runs off. On a terrace or flat roof, water has to be carried away and a thin skin keeps it out. If that skin fails, or if water stands on top of it, a stain appears on the ceiling. This guide explains why flat roofs leak, which systems exist, what to ask in a quote and when the leak is below the skin. We do not name firms.
Updated on 3 November 2026. General information, it does not replace a technician’s visit.
The essentials
- Standing water is the enemy. The General Regulation of Urban Buildings (RGEU, 1951) asks for fast and complete drainage and a slope of not less than 1 per cent on terraces (article 43, § 2).
- Many failures are at the edges and details: door thresholds, parapets, drains, downpipes and joints.
- There are several systems: bituminous membranes, PVC, TPO and liquid membranes. None works without a dry substrate, the right slope and good edge details.
- Maintenance: a manufacturer’s data sheet asks for a check at least twice a year, in spring and autumn.
- The leak can be far from the stain, and water can be trapped under the membrane. Ask the technician to explain how the entry point was found.
Why flat roofs leak
The RGEU says terrace roofs must use materials and methods that ensure waterproofing (article 43) and that rainwater must drain from any part of the building (article 91). It is a 1951 law, read here in a compiled text.
A dissertation from NOVA University of Lisbon (2014), supervised by an LNEC researcher, reviews the technical literature. It cites that about 80% of defects could be avoided with better design and workmanship, and that damp usually shows up 1 to 2 years after the work. These are figures from old studies. Causes it describes, besides those in the guide on the causes of leaks:
- Standing water. It comes from a weak slope, a badly made screed, blocked drains or a substrate that has sagged. It attracts dust and seeds, and roots damage the membrane.
- Door thresholds. They sit only a little above the floor, so the upstand must continue under the threshold.
- Parapets. Without a capping with a drip edge and a slope towards the inside, rain gets in through the top of the wall.
- Expansion joints. Failures come mostly from the joint not being treated as a break in the layers.
- Overlap joints. They come unstuck with wind, too little adhesive, poor welding or a narrow overlap.
- Rigid paving without joints. It moves with the heat and can tear the membrane.
- Punctures. Aerials, tanks, air conditioning units or clotheslines resting on the membrane can puncture it.
An example: in a 2026 tender specification for the roof of an LNEC building, the plan was to correct slopes to remove ponding, with a screed of at least 2%. It is one project, not a rule.
Which kinds of waterproofing exist
- Polymer-modified bituminous membranes (APP or SBS). They are welded with a flame. On accessible roofs a double layer is advised, one of them reinforced with polyester felt.
- PVC. About 1.2 to 1.5 mm thick, with joints welded by hot air. It should not touch expanded polystyrene or polyurethane, which draw out its plasticisers and make it less flexible.
- Polyolefins (TPO/FPO). Similar thickness and welding to PVC. The dissertation mentions resistance to UV rays.
- EPDM. The dissertation advises slopes of not less than 1.5%, above the RGEU’s 1%.
- Liquid membrane (polyurethane). A manufacturer’s data sheet (Sika, November 2026) describes a cold-applied product, reinforced, about 1.5 mm dry thickness. It asks for a dry substrate, “without standing water”, and experienced professionals.
We do not say which is best: it depends on the substrate, access and the design. LNEC tests bituminous and PVC membranes (static and dynamic puncture, wind uplift, watertightness) and issues Application Documents. Ask for the data sheet and, if there is one, the approval document.
Accessible or non-accessible terrace
The dissertation describes four classes: non-accessible, accessible to people, accessible to vehicles and special (for example, green roofs). Two of them:
- Non-accessible: walked on only for maintenance or repair, along a walkway that protects the membrane.
- Accessible to people: the membrane has protection on top. If it is screed or mortar, LNEC (2012, cited in the dissertation) asks for joints that divide it into panels of up to 4 m × 4 m.
Inspection and maintenance
A manufacturer’s data sheet (Danosa, in Portuguese, from 2022) recommends, at least twice a year and preferably in spring and autumn:
- Check the membrane and the details at low walls, chimneys, rooflights and gutters.
- Check and clean downpipes, gutters and drains.
- Remove moss, weeds, leaves and sediment.
- Check membrane overlaps.
The sheet asks for more frequent checks on roofs with no slope. Whoever cleans must have fall protection: see work at height. The guide on gutter cleaning has the rest.
When the leak is below the waterproofing
Water can be trapped between the membrane and the substrate. The same dissertation was born from the need to detect water under the waterproofing in an LNEC test cell, and it studied infrared thermography. It also describes blisters, caused by air or vapour under the membrane, for example by internal condensation. Our conclusion, with no specific source: the stain on the ceiling can be far from the entry point.
So patching only what you can see does not always solve it. Before you pay, check that the stain really is a leak (damp at home).
Who pays in a condominium, VAT and warranty
The Civil Code (article 1421) says the roof or the roof terraces are common parts of the building, even if they serve only one flat. Who pays: see roof in a condominium. We also have guides on VAT and the works warranty.
What to ask for in the quote
- System: type of membrane, number of layers, data sheet.
- Substrate: slope correction, removal of old layers, drying.
- Edge details: height (the dissertation gives 0.15 m as the general value), thresholds, parapets, flashings.
- Drains and downpipes: connection piece, protective strainer, unblocking.
- Expansion joints included.
- Written warranty and a maintenance plan.
- Safety on site.
See the quote checker and how to choose the contractor.
Prices
There is no official table. The Habitissimo portal (updated 10 June 2026) gives an average of 52.5 €/m², between 45 and 60 €/m², and on another line 40 €/m². We did not check these figures against an official source.
What we could not confirm
- Whether the RGEU, from 1951, applies to your terrace, and which more recent technical standards exist: we did not read NP or EN standards.
- The dissertation’s figures (80%, 1 to 2 years, 0.15 m, 4 m × 4 m, 1.5%) come from authors it cites, some from 1994 and 2004.
- The date of the LNEC leaflet does not appear in the document.
- Manufacturer data sheets: they are examples from one brand each, not general rules.
- Prices: only private portal data, with figures that contradict each other.
Sources (read on 3 November 2026)
- Decreto-Lei n.º 38 382 of 7 August 1951 (RGEU), articles 43 and 91, compiled text, PGDL. In Portuguese.
- Código Civil, article 1421, compiled text, PGDL. In Portuguese.
- Ana Cláudia Melrinho, Anomalias em impermeabilizações de coberturas em terraço, master’s dissertation, FCT/NOVA University of Lisbon, June 2014 (chapters 2 and 3). In Portuguese.
- LNEC, Memória descritiva e justificativa, waterproofing of part of the roof of an LNEC building, tender specification, file dated July 2026. In Portuguese.
- LNEC, Unidade de Revestimentos de Coberturas, leaflet.
- Sika Portugal, Product data sheet Sikalastic-625 N, November 2026, version 05.02. In Portuguese.
- Danosa, Banda de reforço E 30 P Elast, data sheet, file from 2022, hosted on a third-party documentation site. In Portuguese.
- Habitissimo, Impermeabilização de terraços, updated 10 June 2026 (private portal). In Portuguese.