# Roof underlay, membrane and ventilation: layers, when to use one and mistakes to avoid

> What each layer of a tiled roof does, when an underlay or membrane is needed, the difference between those that let vapour through and those that do not, where the air has to flow and what to ask the contractor.

- Page: https://telhadoseguro.pt/en/guides/roof-underlay-and-ventilation
- Language: English
- Updated: 2026-11-05

## Summary

- The tile does not guarantee watertightness on its own (APICER and CTCV). We found no Portuguese law that requires an underlay: the guide treats it as an extra watertightness layer.
- Air under the tile is essential: batten over counter-batten, ventilated eaves and ridge, little mortar.
- A flexible membrane must be clearly vapour-permeable, otherwise it creates a strong risk of condensation. Flat-roof bitumen sheet is not an underlay (EN 13707).
- Ask for the product, the declaration of performance (EN 13859-1, class W, Sd) and where the air comes in and goes out. We found no prices or minimum ventilation area.

Between the tiles and the attic there are layers that almost nobody sees: battens, underlay, air. When they fail, water gets in or vapour gets trapped and the timber rots. This guide explains what each layer does, when an underlay or membrane is needed, the difference between those that let vapour through and those that do not, and where the air has to flow.

*Updated on 4 November 2026. General information, not a substitute for a technician's visit. "Common practice" means general site knowledge with no specific official source.*

## The essentials

- **The tile does not work alone.** The APICER and CTCV guide says the tile's own water resistance does not guarantee watertightness: the slope and correct fitting count.
- **We found no Portuguese law that requires an underlay.** The guide treats it as an extra watertightness layer.
- **Air under the tile is essential** (APICER): batten over counter-batten, eaves and ridge that let air through.
- **A flexible membrane must let vapour through.** If it does not, it creates a strong risk of condensation (APICER).
- **Ask in writing for the product name and data sheet.** For prices, see the [cost guide](https://telhadoseguro.pt/en/guides/roof-cost-per-m2).

## The layers, from top to bottom

| Layer | What it does | Note |
|---|---|---|
| Tile | Carries the water to the eaves | Does not guarantee watertightness alone |
| Batten (ripa) | Supports the tile | Spacing depends on the tile model |
| Counter-batten (contra-ripa) | Piece running down the slope, under the batten. Opens an air channel under the tile | The guide prefers batten over counter-batten |
| Underlay or membrane | Second barrier to water that gets between tiles | See below |
| Insulation | Keeps heat in | The tile should not sit on it (see [insulation](https://telhadoseguro.pt/en/guides/roof-insulation)) |
| Rafters and lining | Structure that holds it all | |
| Vapour barrier | Stops vapour rising from the house | Only in very damp spaces, always on the warm side of the insulation |

These are the guide's layers for insulation along the slope. For an attic that is not used, the guide describes another solution: insulate the attic floor and lay the tile on battens, with no lining, with ventilated eaves and ridge (case 6).

## When an underlay or membrane is needed

- **Slope below the tile's minimum.** Cerâmica Torreense then requires extra watertightness elements (see [minimum slope](https://telhadoseguro.pt/en/guides/minimum-roof-slope)). Onduline speaks of complementary systems when the slope cannot be met.
- **Insulation applied along the slope**, in a lived-in attic: the guide's diagrams include the extra watertightness layer there.
- **Old roof with leaks.** Onduline recommends its underlay in rehabilitation. But it is the one that sells it.

We found no rule saying when an underlay can be left out. Ask the technician to justify the choice in writing.

## Two different products

**Corrugated underlay.** Fibre-bitumen sheets, light (about 3 kg/m²), flexible and non-structural. The Onduline manual (June 2026) asks for a slope never below 15% and, for canudo tiles, a continuous base. The corrugation lets air circulate above and below the sheet. At the eaves, the water runs onto the eaves tile.

**Flexible membrane (tela or manta).** The EN 13859-1 standard defines the characteristics of these sheets under the tile in discontinuous roofs (BSI, 2014 edition). A CE-marked product comes with a declaration of performance. The one for a Soprema example (5 July 2018) states watertightness class W1, tear resistance, reaction to fire and the **Sd** value. Sd measures resistance to vapour, in metres of equivalent air (common definition): the lower it is, the more vapour passes.

**Flat-roof bitumen sheet.** EN 13707 covers reinforced bitumen waterproofing sheets and expressly excludes those used as underlays in discontinuous roofs. The APICER guide says to avoid sheets not designed for roofs (guideline 16).

Underlays and sheets may have to prove their reaction to fire (APICER, section 4.2.2).

## Vapour-permeable or not?

The sources do not pick a winner. There are three ideas.

1. **Portugal, APICER (2018):** membranes that appear as an alternative to the underlay must be clearly vapour-permeable, otherwise they create a strong risk of condensation. Vapour comes from the house and rises: if it is stopped and there is no air, it condenses.
2. **Not every underlay is a breathable membrane.** The Onduline manual does not declare that property for the corrugated one: the manufacturer answers with air, through the corrugation and through ventilators. It also warns that a bitumen sheet over an insulating panel can act as a vapour barrier and cause strong condensation.
3. **Breathable does not replace air.** A leaflet from Hertfordshire Building Control (UK, March 2011) defines a low vapour resistance underlay (0.25 MNs/g or less) and says it is normal to ask for independent certification. It says very tight coverings, such as metal sheet or fibre cement, may need ventilation between the membrane and the covering; clay tile normally does not. It accepts reduced ventilation only with a well-sealed ceiling, with no gaps. It is a British rule from 2011 and does not apply here: it shows only the principle.

Our conclusion: the whole assembly decides (air, sealed ceiling, insulation in the right place, a product with a data sheet), not the underlay alone.

## Ventilation: where the air comes in and goes out

- **Eaves and ridge.** The guide asks for dry eaves with a ventilation batten or comb, which lets air through and keeps animals out, and a ventilated dry ridge (section 3.8.1). Traditional ventilated eaves give the timber and the tile more durability and reduce condensation in winter (case 3).
- **Counter-batten and ventilation tiles** work against condensation under the tile (case 11).
- **Underlay ventilators (Onduline).** Opening of about 240 cm², one per 30 m², in a "W" layout and under a ventilation tile.
- **Little mortar.** The guide says to avoid it in the field of the roof, at ridges and at hips. Onduline says less mortar lets the air circulate.
- **Birds.** Close the entries with parts that still ventilate: [gulls, pigeons and nests](https://telhadoseguro.pt/en/guides/gulls-pigeons-nests-on-roof).

## Common mistakes

- **Blocking eaves or ridge** with insulation, foam or mortar. Onduline lists lack of ventilation, sometimes from excess zeal, among the most common errors. See [attic condensation](https://telhadoseguro.pt/en/guides/attic-condensation-roof-void).
- **Tile sitting on the insulation** (APICER).
- **Plastic or sheet not made for roofs** (APICER, EN 13707).
- **Vapour barrier** missing or wrongly placed. The guide lists it among site defects.
- **Incomplete solution**, without flashings, tapes and accessories (Onduline).
- **Walking on the underlay without boards.** Onduline says to use boards to spread the weight.
- **Membrane left in the sun.** The British leaflet says most materials do not resist UV in the long term. Ask how long it can stay exposed.

## Signs of trouble

Drops on the underside of the tiles on cold mornings are, as a rule, condensation (APICER, case 11), not a faulty tile. A stain only after rain with wind suggests a leak (common practice). Tell them apart in the [leak guide](https://telhadoseguro.pt/en/guides/roof-leak-causes).

## What to ask the contractor

1. Does it have an underlay or membrane? Why on this roof?
2. Which **product**, from which manufacturer? Ask for the data sheet and the **declaration of performance** (EN 13859-1, class W, Sd, fire).
3. Does it let vapour through? And does the roof **ventilate**?
4. Is there a **counter-batten**? Where does the air come in and go out: eaves, ridge, ventilation tiles?
5. Where do the insulation and the vapour barrier go, and on which side?
6. How are eaves, chimneys and skylights finished?
7. Who guarantees the work? See the [warranty](https://telhadoseguro.pt/en/guides/roof-works-warranty) and how to [choose a contractor](https://telhadoseguro.pt/en/guides/choosing-a-roofing-contractor).

**See also:** On summer heat on the top floor, see [hot attics and cool roofs](https://telhadoseguro.pt/en/guides/summer-heat-attic-cool-roof).

## What we could not confirm

- **A Portuguese law or standard** that requires or waives the underlay. We found none.
- **LNEC documents** on underlays.
- **The edition of NP EN 13859-1 in force at the IPQ.** We only saw the 2014 EN.
- **The minimum Sd** of a "clearly permeable" membrane: the guide gives no number.
- **Minimum ventilation area** in Portugal.
- **An independent comparison** of the two product families.
- **Sandwich panel and metal sheet:** the sources cover ceramic tile.

## Sources (read on 4 November 2026)

- APICER, coordinated by CTCV, [Guia para Projeto e Aplicação de Telhas Cerâmicas](https://www.apicer.pt/apicer/media/5c06bd0e8d436.pdf), 2018 (sections 3.8.1, 4.2.2, 5.2.2, 5.2.5, 5.3; cases 3, 6 and 11).
- Onduline, [Reabilitação de Coberturas Inclinadas](https://pt.onduline.com/sites/onduline_pt/files/2026-06/manual_subtelha_ondutherm_2026_06.pdf), Porto, June 2026.
- Soprema, [Declaração de desempenho STRATEC II](https://lotus.soprema.fr/www/reftechsop.nsf/($AllByUNID)/6A593F6924D7B190C12583280035AD45/$File/DOP_PROFR0001.a.PT_STRATEC%20II.pdf), 5 July 2018.
- BSI, [BS EN 13859-1:2014](https://knowledge.bsigroup.com/products/flexible-sheets-for-waterproofing-definitions-and-characteristics-of-underlays-underlays-for-discontinuous-roofing) and [BS EN 13707:2013](https://knowledge.bsigroup.com/products/flexible-sheets-for-waterproofing-reinforced-bitumen-sheets-for-roof-waterproofing-definitions-and-characteristics) (catalogue pages only).
- Hertfordshire Building Control Technical Forum, [Technical Guidance Leaflet No. 13](https://www.hertsmere.gov.uk/Documents/09-Planning--Building-Control/Building-Control/13---Technical-Leaflet---Permeable-Underlays---March-2011.pdf), March 2011 (UK, as a reference only).

## FAQ

### Does a tiled roof have to have an underlay?

We found no Portuguese law that requires it. The APICER and CTCV guide treats the underlay as an extra watertightness layer, and Cerâmica Torreense requires extra elements when the slope is below the tile's minimum. Ask the technician to justify the choice in writing.

### Does a breathable membrane replace ventilation under the tile?

No. The APICER guide says ventilation under the tile is essential. A 2011 British leaflet even says very tight coverings, such as metal sheet or fibre cement, may need ventilation between the membrane and the covering. It is a UK rule: it shows only the principle.

### Can I use flat-roof bitumen sheet as an underlay?

It is not a good idea. EN 13707 excludes the sheets used as underlays in discontinuous roofs, which follow EN 13859-1. The APICER guide says to avoid sheets not designed for roofs, and Onduline warns that a bitumen sheet over insulation can act as a vapour barrier.

### Do I need a vapour barrier under the roof?

Only with judgement. The APICER guide says to use it with great care, always on the warm side of the insulation, and above all over very damp spaces. An underlay is not a vapour barrier: a membrane above the insulation must let vapour through.

### What should I ask the contractor about the underlay?

The product and manufacturer name, the data sheet and the declaration of performance (EN 13859-1, watertightness class W, Sd value, reaction to fire), the manufacturer's instructions, the eaves and ridge detail and an explanation of where the air comes in and goes out. Everything in writing, in the quote.

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Source: https://telhadoseguro.pt/en/guides/roof-underlay-and-ventilation. Independent information, not legal or tax advice.