# Roof leaks: common causes and how to find where the water gets in

> Broken tiles, mortared ridges, flashings, gutters, flat roofs and pipes: how to trace a ceiling stain without climbing onto the roof, and how to tell a leak from condensation.

- Page: https://telhadoseguro.pt/en/guides/roof-leak-causes
- Language: English
- Updated: 2026-11-02

## Summary

- The leak is rarely right above the stain: water runs along the timber and drips further down.
- The causes most often cited are broken tiles, mortar on ridges, flashings at chimneys and walls, gutters and poor ventilation.
- Drops under a new tile with no defect are usually condensation.
- A stain that grows without rain may be plumbing: the water meter helps to confirm it.

A stain on the ceiling after rain is almost never right under the spot where the water gets in. Water passes a tile, runs along a batten or a timber and only drips further down. Before you pay for a repair, it is worth finding out where the leak comes from. This guide brings together the causes described by a technical guide from the ceramics sector, shows how to look for the source without climbing onto the roof, and helps you tell a leak apart from condensation or a burst pipe.

*Updated on 2 November 2026. General information, not a substitute for an inspection by a professional. Where we write "common practice", we mean general building knowledge, with no specific official source.*

## The essentials

- **The leak is rarely right above the stain.** Look further up, towards the ridge.
- **The causes most often cited** by the APICER and CTCV technical guide: broken or displaced tiles, too much mortar on the ridges, poor flashings at chimneys and walls, blocked gutters and a lack of ventilation under the tiles.
- **According to the same guide, most defects come from a lack of maintenance.** It recommends inspection and cleaning **once a year** and maintenance of the drainage **twice a year**.
- **Drops under a new tile with no defect** are usually **condensation**.
- **A stain that grows without rain** may be plumbing. The water meter helps to confirm it.
- **Do not climb onto the roof.** Look from the ground, from the loft or from a higher point.

## Where the water gets in

The *Guia para Projeto e Aplicação de Telhas Cerâmicas* (Guide to the Design and Laying of Ceramic Roof Tiles; APICER, coordinated by CTCV, 2018) groups defects into design errors, workmanship errors, improper use and poor materials. Among the ways they show up, it highlights **leaks**, tiles torn off by the wind and slipping.

**Broken or displaced tiles.** The guide's table of defects includes cracked tiles, wrong interlocking, wrong overlap and slipped tiles. In a storm, the wind can lift above all **the ridges and the flashings**. These are the first places to check after strong wind.

**Ridges and hips laid in mortar.** According to the guide, too much mortar causes **many cases of early decay and leaks**. Mortar and ceramic expand differently with heat and moisture, and the joint cracks. Mortars with a lot of cement make it worse. If the joint already lets water through, the guide says the repair must be **immediate**.

**Flashings at chimneys, walls and dormers.** Joints with chimneys, neighbouring walls, pipes through the roof and aerials or panels are, for the guide, points of **great vulnerability**. Flashings (*rufos*, the strips that bridge the tiles and the wall) must cope with the movement of the materials. At a chimney, the critical point is the upper side, where the water arrives.

**Gutters and downpipes.** A blocked or undersized gutter overflows and soaks the top of the wall. The guide links poor drainage to decay of the outer render, the masonry and the **roof timbers next to the façade**. It also advises against downpipes that discharge water from a higher roof onto the tiles of another.

**Underlay, membranes and ventilation.** Under the tiles there may be an underlay (*subtelha*) or a membrane. The guide warns that these membranes must let water vapour through freely, **or they create serious condensation risks**. Ventilation under the tiles is, according to the guide, essential for watertightness and for the tiles to dry.

**Flat roofs and terraces.** Here a waterproofing membrane keeps the water out. A master's dissertation from Universidade NOVA de Lisboa (2014), supervised by a researcher from LNEC (the national civil engineering laboratory), describes the most frequent defects:

- **upstands that are too low** at walls and door sills (the value generally adopted, quoted from an LNEC publication, is **0.15 m**);
- **upstands and seams coming unstuck**, from poor workmanship or bad welding;
- **standing water**, from too little fall or blocked outlets;
- **leaks around the downpipe outlets**;
- **vegetation** and **blistering** (bubbles under the membrane).

**Plumbing.** A pipe, a cistern or a water heater on the floor above can leave identical stains. The Câmara Municipal de Belmonte (Belmonte town council) publishes a simple test: turn off taps and appliances, write down the meter reading and see if the flow indicator moves. If it moves with no use, a leak in the house's pipework is very likely.

## How to trace the source without climbing up

The APICER guide sets out three stages: gather the history of the building, look at the roof **from outside** (from a neighbouring building, a high point or with a drone) and **from inside**, in the roof space. It warns that the light in the roof space is usually poor and that dirt can lead to wrong diagnoses.

**From the ground (common practice):**

1. With binoculars or your phone's zoom, look for broken tiles or rows out of line.
2. Check the ridge and the flashings around the chimney.
3. While it rains, see whether the gutter overflows.
4. Look at the roof line: according to the guide, a **sag visible to the naked eye** calls for an inspection of the structure.

**In the loft (common practice):**

1. Go in only if there is somewhere safe to step. Never step on the ceiling.
2. Take a strong torch and go during rain or just after.
3. Look for dark or wet timber, run marks and daylight between the tiles.
4. Follow the trail **upwards**: water runs along the timber down the slope.
5. Take photos and mark the spot to show the professional.

The guide points out that inspections require safety for whoever does them, with guard rails or lifelines to prevent fatal falls. If the roof is old fibre cement, do not walk on it or drill it: it may contain asbestos. See the [guide on asbestos roofs](https://telhadoseguro.pt/en/guides/asbestos-roof-law-removal).

## From symptom to likely cause

The table combines the sources with common practice. It is a pointer for the inspection, not a diagnosis.

| What you see | Likely cause | Where to look |
|---|---|---|
| Stain only after wind-driven rain | Displaced tile, lifted flashing | Exposed slope, ridge |
| Stain in line with the ridge | Cracked mortar | Ridge and hips |
| Stain next to the chimney or a tall wall | Poor flashing | Upper side of the chimney |
| Top of the outside wall stained | Blocked or small gutter | Gutter, downpipes |
| Drops under new tiles on cold days | Condensation | Ventilation under the tiles |
| Stain that grows without rain | Plumbing | Water meter |
| Terrace: damp along walls or sills | Low or loose upstand | Membrane upstands |
| Terrace: stain under an outlet | Downpipe outlet | Outlet |
| Sagging roof | Bent battens, rafters or structure | Roof space |

## Leak or condensation?

The APICER guide devotes a case to this mix-up. Lots of drops on the underside of a **new tile**, with no defect to explain them, are usually condensation, a phenomenon "entirely unrelated to the characteristics of the tile". It depends on the differences in temperature, humidity and air movement between inside and outside.

Typical signs (common practice): it appears on cold days, even without rain; the damp is spread out, not in one spot; it gets worse with laundry drying indoors or a kitchen without an extractor.

The fix is not to change the tiles. The guide points to **ventilation under the tiles** (counter-battens, vent tiles, ventilated ridges and eaves) and, over very humid rooms, a **vapour barrier always on the warm side**.

## Repair or replace?

For most defects, the guide describes local repairs: replacing tiles, redoing a flashing, fixing gutters, replacing bent battens. It points to bigger works when:

- the **structural timber** is rotten at the bearings, often from old leaks;
- the geometry is so complex that watertightness is at risk: lift the tiles, add a secondary waterproofing layer and relay them;
- the main structure has **collapsed**: a new structure, with a design.

The guide adds that hiding a defect should never be the only strategy. Before the works, see the [guide to permits for roof works](https://telhadoseguro.pt/en/guides/roof-works-permit-2026) and the [guide to VAT on roof works](https://telhadoseguro.pt/en/guides/roof-works-vat).

## Temporary measures and who to call

Common practice: move furniture and electrical appliances away from the wet area; if the ceiling is bulging with water, **switch off the electricity to that room**; photograph the damage with the date. A tarpaulin on the roof should only be fitted by someone with safety equipment.

- **Detached house:** a roofing contractor. Ask for the quote to state the cause found.
- **Flat in a building:** the condominium administrator. The roof is a common part (article 1421 of the Código Civil, the Civil Code).
- **Storm damage:** tell your insurer quickly. See the [guide to home insurance after a storm](https://telhadoseguro.pt/en/guides/home-insurance-storm-roof).
- **Plumbing:** close the main valve if needed and call a qualified professional, as the Belmonte guide advises.

## What we could not confirm

- **NP EN standards on tiles and roofs.** We did not read the texts, so we do not cite them.
- **LNEC publications on roofs.** We only cite them through the 2014 dissertation.
- **Repair prices.** We do not publish figures without an official source.
- **Sandwich panels and metal sheet.** The guide we read covers ceramic tiles. We did not confirm specific rules.
- **Your roof.** Only a professional, on site, can confirm the cause.

## Sources (read on 2 November 2026)

- APICER, coordinated by CTCV, with technical and scientific support from Instituto Pedro Nunes (University of Coimbra), [Guia para Projeto e Aplicação de Telhas Cerâmicas](https://www.apicer.pt/apicer/media/5c06bd0e8d436.pdf), PDF of October 2018 (chapters 3.8, 4.2.3, 4.3.4, 5.2.5, 5.3, 6.2 and 7, including tables 1 and 2 and the commented cases).
- Ana Cláudia Melrinho, [Anomalias em impermeabilizações de coberturas em terraço](https://run.unl.pt/server/api/core/bitstreams/fdc67fba-4de9-43a6-9477-c5fa6fe2dac2/content) (Defects in flat-roof waterproofing), master's dissertation, FCT/Universidade NOVA de Lisboa, June 2014 (chapter 3).
- Câmara Municipal de Belmonte, [Suspeita de uma fuga de água em casa?](https://www.cm-belmonte.pt/wp-content/uploads/2026/05/Suspeita-de-uma-fuga-de-agua-em-casa-1.pdf) (Suspect a water leak at home?), leaflet of May 2026.
- Código Civil, [article 1421](https://www.pgdlisboa.pt/leis/lei_mostra_articulado.php?artigo_id=775A1420&nid=775&tabela=leis&pagina=1&ficha=1&so_miolo=&nversao=), consolidated text in the database of the Procuradoria-Geral Regional de Lisboa (Lisbon regional public prosecutor's office).

## FAQ

### Why is the ceiling stain not under the broken tile?

Water gets in at one point, runs along the batten or timber down the slope and only drips further down. Always look higher up, towards the ridge.

### How do I know if it is a leak or condensation?

Condensation appears on cold days, even without rain, and the damp is spread out. The APICER guide says drops under a new tile with no defect are usually condensation, fixed with ventilation under the tiles.

### Can I climb onto the roof to see where the water comes from?

It is not advisable. Look from the ground with binoculars and from the loft. Roof inspections need safety equipment, and old fibre cement may contain asbestos.

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Source: https://telhadoseguro.pt/en/guides/roof-leak-causes. Independent information, not legal or tax advice.