# Chimneys, flashing and roof joins: where water gets in and what to ask for in the quote

> Chimneys, walls, valleys, pipes and rooflights: where water gets in, which materials are used for flashing (lead, zinc, copper, aluminium, mortar, self-adhesive strips), signs of failure seen from the ground, repair or redo, and what to ask for in writing.

- Page: https://telhadoseguro.pt/en/guides/chimney-flashing-roof-joins
- Language: English
- Updated: 2026-11-03

## Summary

- Joins (chimney, wall, valley, pipes, rooflights) are, according to the APICER and CTCV guide, among the most vulnerable points of a roof.
- Flashing should form a cover or channel between the tile and the wall. The guide says to avoid rigid mortar joins and membranes not made for roofs.
- Filler or silicone over an old join is not what the guide describes: a repair means opening up, cleaning and fitting a transition element that can take the movement.
- In the quote, ask in writing for each point to be treated, the material and fixing of the flashing, room for expansion and the warranty.

Many leaks do not start in the middle of the tiles. They start where the roof meets something else: a chimney, a taller wall, a rooflight, a pipe that goes through it. That is where water looks for a weakness. This guide shows those places, what seals them, how to spot a failure from the ground and what to ask for in the quote. We do not name firms or prices.

*Updated on 3 November 2026. General information, not a substitute for a professional's visit. Where we write "common practice", we mean general building knowledge with no specific official source.*

## The essentials

- **Joins are weak points.** The technical guide of APICER and CTCV puts them among the most vulnerable points and asks for a qualified technical approach.
- **Flashing (rufo) is the sheet or strip that joins the last tile to the wall.** It should form a cover or a channel, not just a bead of filler.
- **Rigid mortar does not last.** The same guide says to avoid rigid joins to neighbouring buildings and membranes not made for roofs.
- **Filler on top of filler is not a repair.** A repair means opening up, cleaning and fitting a new element that can take the movement.
- **Ask in writing** for the material, the length and the fixing of each flashing.

## Where water gets in

**Chimney.** According to the guide, the join needs special care on the upper side, where the water arrives, and a path for the water all around the perimeter. A British lead supplier describes chimney flashing as several pieces: front apron, side pieces, stepped covers, pieces under the tiles and a gutter behind the chimney. The ones underneath cannot be seen, so the visible flashing can hide the failure. The same page says that open joints and porous brick in the chimney itself also let water through: replacing only the flashing may not be enough.

**Taller wall or side wall.** The guide asks for flashings that form a cover or channel between the tile and the wall. The join to the wall is fixed by screwing and sealing, by building it into the wall, or by slipping it under the cladding of the neighbouring wall. If the wall closes the roof at the top, parallel to the eaves, the flashing must overlap the cover and the channel of the tile.

**Valley (the corner where two slopes meet).** The guide asks for suitable metal flashings, extended and protected against water splashing over.

**Vent pipes, aerials and solar panels.** The guide counts pipes through the roof and fixed equipment among the vulnerable points. Badly done, they fail early.

**Rooflights and roof windows.** The guide lists them as singular points. VELUX Portugal says to clean the flashing around the window once a year, so water can drain freely.

**Gutters.** The guide prefers external gutters and downpipes. Internal ones depend on the material, the section, the slope and the join under the tile, and need regular maintenance. See [gutter cleaning](https://telhadoseguro.pt/en/guides/gutter-cleaning).

## Materials: what we found

| Material | What the source says | Cautions |
|---|---|---|
| **Lead** | British supplier: rolled sheet sold in thickness "codes" (standard BS EN 12588); code 3 for hidden pieces, code 4 for ordinary flashings | Commercial, British source, not Portuguese. The page links cracks in lead to thermal movement |
| **Zinc** | RHEINZINK (edition 11/24): expands 2.2 mm per metre per 100 K (our sum: 4.4 mm on a 2 m piece) | Leave room to move. Wash off mortar splashes at once (RHEINZINK); acetic silicone is not suitable (VMZINC) |
| **Copper** | No data of our own. In contact with zinc it is a problem when it sits above | Water running from copper onto zinc corrodes it (VMZINC, RHEINZINK) |
| **Aluminium** | Compatible with zinc (VMZINC, RHEINZINK). Onduline uses an aluminium strip at the eaves | Durability in Portugal not confirmed |
| **Mortar** | APICER guide: avoid rigid joins to neighbours | Cracks with movement between materials |
| **Self-adhesive strip** | The guide mentions flexible, mouldable, adhesive strips as an alternative to metal flashing | No durability data |

**Self-adhesive strips.** Onduline (June 2026) says its bituminous strip bonds cold to a dry, clean surface, and that below 10 °C or on non-bituminous surfaces adhesion may be lower and a primer is used. That is for its product: ask for the data sheet of the strip they will fit.

## Signs of failure, seen from the ground

Common practice, no climbing:

1. A stain on the ceiling or wall **near the chimney**, with rain and wind.
2. With binoculars or your phone's zoom: flashing lifted or open, a cracked mortar bead, rusted or split sheet, filler smeared over a join.
3. Open joints or crumbling brick in the chimney.
4. A stain **lower down** than the entry point: water runs along the timber. See the [guide to leak causes](https://telhadoseguro.pt/en/guides/roof-leak-causes).
5. A stain on cold mornings, without rain: it may be condensation, not flashing.

Photograph with the date and do not climb up ([work at height](https://telhadoseguro.pt/en/guides/work-at-height-roof)).

## Repair or redo?

For a badly made join, the guide describes a usually local job: lift the tiles around it, clean the area thoroughly and fit transition elements that can take the movement between materials. If the join also carries water, the section, the height of the side upstands and the slope are checked.

Our conclusions, based on the guide:

- **Redo the flashing** when the old one is only filler or mortar, or when several points are failing. Sealing over it is not what the guide describes.
- **Spot repair** when the fault is a loose piece or one joint and the rest is sound.
- **Look at the timber.** If water has been getting in for years, the guide says the structure may be rotten and the job bigger. See [repair or replace](https://telhadoseguro.pt/en/guides/roof-repair-or-replace).

## What to ask for in the quote

1. **Each point to be treated**, by name: chimney, gable, valley, pipe, rooflight.
2. **Flashing material**, thickness or code, and the brand of strip, if any.
3. **How it is fixed to the wall:** built in, screwed and sealed, or under the cladding.
4. **Room for expansion** in long pieces.
5. **Which fillers are used**, and the warning that it is not just covering up.
6. **Condition of the chimney:** who deals with the joints.
7. **Broken tiles:** who pays for them.
8. **Photographs** before and after.
9. **Warranty** in writing ([warranty guide](https://telhadoseguro.pt/en/guides/roof-works-warranty)).

Compare with the [quote checker](https://telhadoseguro.pt/en/roof-quote-checker), choose with the [contractor guide](https://telhadoseguro.pt/en/guides/choosing-a-roofing-contractor) and [check the firm](https://telhadoseguro.pt/en/roofing-contractor-check). Prices: [roof cost per m²](https://telhadoseguro.pt/en/guides/roof-cost-per-m2). More in the [repair hub](https://telhadoseguro.pt/en/roof-repair).

If the roof is old fibre-cement, do not drill or walk on it: see the [asbestos guide](https://telhadoseguro.pt/en/guides/asbestos-roof-law-removal).

## What we could not confirm

- **A Portuguese standard** for flashing. We did not find the text.
- **Flashing instructions** from Portuguese tile makers. We did not find them.
- **Durability** of lead, zinc, copper, aluminium and strips in the Portuguese climate.
- **Prices** for flashing: we do not publish them without an official source.
- **Rules for handling lead.**
- **Your house.** Only someone who sees the join can say.

## Sources (read on 3 November 2026)

- APICER, coordinated by CTCV, [Guia para Projeto e Aplicação de Telhas Cerâmicas](https://www.apicer.pt/apicer/media/5c06bd0e8d436.pdf), 2018 (singular points, joins to adjoining elements, case 7 and execution rules).
- Onduline, [Reabilitação de Coberturas Inclinadas, under-tile manual](https://pt.onduline.com/sites/onduline_pt/files/2026-06/manual_subtelha_ondutherm_2026_06.pdf), Porto, June 2026 (flashing strips).
- RHEINZINK, [Information on Material and Processing](https://www.rheinzink.com/fileadmin/redaktion/RHEINZINK_GLOBAL/Downloads/Technical-Documents/rheinzink-information-material-and-processing-107760-RZ-INT-002-11-24_K05.pdf), edition 11/24, © 2025 (expansion, metals, mortar).
- VMZINC, [Compatibility with Other Materials](https://www.vmzinc.com/en-in/technical-support/basic-principles/compatibility), undated page.
- The Lead Lads (lead supplier and contractor, United Kingdom), [Leaks around chimneys](https://theleadlads.co.uk/leadwork-failures/roof-leaks-around-chimneys/) and [BS EN 12588](https://theleadlads.co.uk/?p=1938), undated pages. Commercial source.
- VELUX Portugal, [Cuidados e manutenção](https://www.velux.pt/recursos/cuidados-manutencao), undated page (©2023-2026).

## FAQ

### Lead, zinc, copper or aluminium: which is the best flashing?

We found no Portuguese source that says. We know that zinc expands about 2.2 mm per metre per 100 K (RHEINZINK, edition 11/24), that water running from copper onto zinc corrodes it (VMZINC) and that lead and aluminium are compatible with zinc. Ask the contractor for the material, the thickness and the data sheet.

### Can I fix a chimney leak with silicone or filler?

The APICER and CTCV guide describes something else: lifting the tiles around it, cleaning the area and fitting transition elements that can take movement between materials. A British lead supplier says sealants and patches can hide the problem and let the leak return. VMZINC says acetic silicone is not compatible with zinc.

### What can I check without climbing onto the roof?

From the ground, with binoculars or your phone's zoom, look for lifted or open flashing, cracked mortar beads, rusted sheet and open joints in the chimney. This is common practice, with no official source. VELUX Portugal says to clean the flashing of its roof windows once a year.

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Source: https://telhadoseguro.pt/en/guides/chimney-flashing-roof-joins. Independent information, not legal or tax advice.