Drops under the tiles on a cold morning, dark timber in the loft, damp insulation. Many people conclude that the roof lets water in. Sometimes it does. Other times the water comes from inside the house, rises as vapour and condenses in the cold of the roof void. This guide explains how it happens, what the sources say about ventilation and insulation, how to tell the two situations apart from the loft floor and what to ask a technician.
Updated on 3 November 2026. General information, not a substitute for a technician’s visit. Where we write “common practice”, we mean general building knowledge with no specific official source.
The essentials
- Condensation is water that comes from inside. The APICER and CTCV guide says it depends on the make-up of the roof and on the differences in temperature, humidity and air movement between inside and outside.
- Blocking ventilation makes it worse. Onduline lists an air-tight roof as a common mistake and says intense condensation is mistaken for a leak.
- The source is often down below: extractor fans that discharge into the loft and openings in the ceiling let humid air rise (Building America Solution Center, USA).
- You can test from the loft floor, without climbing onto the roof.
How it forms
Household air carries water vapour: showers, cooking, drying laundry, breathing. That air rises through gaps, hatches and cable holes (common practice). When it meets a cold surface (the underside of a tile, timber, a nail), the vapour turns back into liquid. A FEUP summary (2003) says indoor humidity is the balance between the vapour produced and the ventilation, and that weak or uneven ventilation leads to condensation.
A University of Aveiro report (2020) cites a classification in which a well-ventilated home that is not overcrowded has medium hygrometry, and a poorly ventilated one has strong.
The WHO (2009) links persistent damp and mould to more respiratory symptoms, allergies and asthma.
Ventilating the roof void: what the sources say
The APICER and CTCV guide (2018) says, for traditional roofs:
- Roof void not in use: insulate the loft floor (the ceiling of the top storey) and ventilate strongly, with tiles on battens, no lining, ventilated eaves and ridge and, on large slopes, ventilation tiles.
- Ventilating the underside of the tile is a condition for no condensation: counter-battens, ventilation tiles, ventilated ridges and eaves.
- Ventilated eaves on old houses lower the condensation risk in winter.
- Tiles should not rest directly on the insulation.
Onduline (June 2026) adds that ventilation matters a lot on old roofs with timber structure and ceilings, because internal condensation can be as serious as a leak. We did not find an official Portuguese rule for the opening area per square metre of roof void.
Where the insulation goes
See the roof insulation guide. For condensation, three points:
- Do not block the eaves or ridge with insulation. Our conclusion, from the rule that air has to circulate.
- Vapour barrier: only with care and on the warm side of the insulation, that is, the house side. The guide calls for it where there are very humid spaces below.
- Membranes and felts above the insulation must let vapour through. Onduline warns that an asphalt felt over an insulating panel of its system can act as a vapour barrier and cause strong internal condensation.
Condensation or leak?
Common practice, helped by the sources:
| Sign in the loft | Points to |
|---|---|
| Drops and damp timber in the morning, on cold, dry days | Condensation |
| Dries by midday and returns on the next cold day; spread over many tiles | Condensation |
| A trickle following a batten or rafter, only after rain | Leak |
| A stain near a chimney, a wall or a rooflight | Leak at the flashing. See chimneys and flashing |
| Grows without rain or cold | Plumbing. See damp at home |
Both can happen at once. Causes of leaks: leak guide.
How to test from the loft floor, without climbing onto the roof
The APICER guide warns that light in the roof void is usually poor and that dirt can lead to wrong diagnoses.
Before going in. The Building America Solution Center advises spreading a tarp under the hatch, wearing gloves, an N95 mask and eye protection, and bringing a head torch. Common practice: bring another person, step only on the joists, never on the ceiling lining. If you see old fibre-cement, do not touch it (asbestos guide). If in doubt, do not go in, and do not climb onto the roof (work at height).
What to do (common practice):
- Choose the day. A cold morning after dry nights shows condensation; a visit during rain shows leaks.
- Look at: the underside of the tiles, nails, battens, the top of the insulation, timber near the eaves.
- See where the fan duct ends. If it ends inside the loft, it is a source of vapour (Building America Solution Center).
- See whether insulation blocks the eaves, and where air enters and leaves: eaves, ridge, ventilation tiles.
- Photograph with the date and note the weather. The damage file helps.
- Repeat in two weeks. A cheap hygrometer inside and outside gives figures, but we did not find an official value for risk.
Do not: block openings, fill the eaves with insulation, put foam or filler on the tiles, paint over mould.
What you can do at home
Reduce vapour: use the extractor fan, close the door when showering, ventilate when drying laundry indoors. If the fan discharges into the loft, the Building America Solution Center says to take the duct outside and seal the housing and the ceiling holes.
What to ask the technician
- What measurements did you take, and at what time?
- Is it condensation, a leak or both? Where does the vapour come from?
- Do the eaves, ridge and ventilation tiles let air through?
- Where is the insulation, and is there a vapour barrier? On which side?
- Does the underlay or felt let vapour through?
- Does the quote separate ventilation, ceiling sealing and insulation?
Ask for the diagnosis in writing and compare with the quote checker. See also the contractor guide.
What we could not confirm
- LNEC or ADENE documents on a ventilated roof void. We did not find them.
- The current official rule for air change in dwellings.
- Minimum ventilation area of the roof void in Portugal.
- Relative humidity above which there is risk in the loft.
- Sandwich panel and metal sheet: the APICER guide covers ceramic tile.
- Climate: the US guidance comes from a different one.
- Your house. Only measurement on site confirms.
Sources (read on 3 November 2026)
- APICER, coordinated by CTCV, Guia para Projeto e Aplicação de Telhas Cerâmicas, 2018 (insulation, cases 3, 6 and 11, roof void inspection).
- Onduline, Reabilitação de Coberturas Inclinadas, June 2026 (common mistakes, ventilation, panel).
- M. Pinto and V. P. de Freitas, Importância da ventilação nas condensações superficiais, PATORREB 2003, FEUP (abstract).
- R. J. B. Pinto, Acompanhamento de execução de coberturas de edifícios, University of Aveiro, 2020 (chapter 3).
- Building America Solution Center (US Department of Energy, PNNL), Air sealing bathroom and kitchen exhaust fans, updated 16 September 2022, and Look at exhaust fans for venting to outside, updated 25 July 2025.
- WHO, Dampness and mould, 2009 (summary page).