In a typical house the insulation is laid across the loft floor, leaving the void beneath the tiles cold. Roofers call this a cold roof. Warm, moist air from kitchens, bathrooms and living rooms drifts upward through every small gap in the ceiling. Once it reaches the cold underside of the covering it cools, and the water it carries condenses. The remedy is airflow: a path for damp air to leave, and a steady supply of drier air to replace it.
That path is built from ventilation at the eaves (where the roof overhangs the wall), sometimes at the ridge or high on the slope, and from the underlay. This guide covers the whole system. If you only want to know whether a particular roof needs tile vents, see tile vents on your roof. Condensation is also sometimes mistaken for a leak, which we cover in why tiled roofs leak. For the wider service, see pitched roofing.
Where loft moisture comes from
Everyday life puts water vapour into the air: cooking, washing, drying clothes, breathing. Warm air holds more moisture than cold air, and it rises. If the ceiling below the loft has gaps around the hatch, downlighters, pipes or cables, that vapour reaches the loft in a steady trickle. Once it meets a cold surface it turns to liquid, first as a faint dew on nails and then as droplets on the underlay and rafters.
This is why a loft can be wet while the roof outside is sound, and why the problem tends to show in winter. It is also why the cause is often sealed, not roofed: gaps in the ceiling are as much a part of the system as the vents at the eaves.
Why insulation changed things
Adding loft insulation keeps heat in the house and leaves the roof space colder than before. A roof that coped with an uninsulated loft can start to show condensation once a thick layer goes down. Insulation that has been pushed hard against the eaves also blocks the airflow route there, which turns a ventilated roof into a closed one. A baffle, a rigid or semi-rigid board that holds the insulation back from the eaves, keeps the gap open.
Warm roofs are different
Some roofs have insulation between or above the rafters and no ventilated void. They are designed to a different method and need their own checks. This guide covers cold roofs only, the arrangement found in most tiled lofts.
What BS 5250 asks for at the eaves
BS 5250 is the standard guiding how moisture is handled in buildings. We read its roof figures through a manufacturer's published summary, Marley's guidance on ventilating a cold roof, so treat the numbers as a guide and check the product data sheet for your tiles.
The first split is the underlay. With an HR (non-breathable) underlay, which covers traditional bitumen felt, the guidance asks for a 25 mm opening the full length of the eaves at pitches up to 15 degrees, and 10 mm above that. Our underlay guide explains the types.
| Condition | Eaves ventilation | High-level ventilation |
|---|---|---|
| HR underlay, pitch 15 degrees or less | 25 mm along the eaves | 5 mm if over 10 m span or lean-to |
| HR underlay, pitch above 15 degrees | 10 mm along the eaves | 5 mm above 35 degrees, over 10 m span, or lean-to |
| LR underlay, sealed ceiling, air-permeable covering | About 3 mm from normal eaves construction | Or 5 mm at ridge or high level |
What the gap means in practice
Ten millimetres along the eaves is a continuous slot, not a few holes. It is delivered by a soffit vent, a fascia vent strip or an over-fascia ventilator. A tile roof with perfectly good ventilation in the soffit can still be blocked inside by insulation, a dropped felt or birds' nests, so the survey checks the path as well as the product.
High-level ventilation and why pitch changes the picture
Air moves through a roof because of two things: wind pressure and the stack effect, where warm air rises and escapes at the top. A slot at the eaves alone can leave stagnant pockets near the ridge. So the guidance adds a continuous 5 mm opening at high level once the pitch passes 35 degrees, the span passes 10 m, or the roof is a lean-to with a single slope. In these cases the air has farther to go, or nowhere to rise to, and needs an exit.
High-level ventilation can be a ridge vent, a set of ventilating tiles near the ridge, or dry-fix ridge components with built-in gaps. The dry-fix systems tested to BS 8612 can combine ventilation with a mechanically fixed ridge. Marley's guidance calls that standard the test the dry verge, hip and ridge products must pass.
Lean-tos and flat-ish roofs
A lean-to extension against a house wall has no ridge to vent. High-level ventilation is then provided at the top edge where it meets the wall, often with a purpose-made vent under the abutment flashing. These roofs are among the places where ventilation is most often forgotten or covered up during building work.
Low pitches
Roofs at 15 degrees or less need the largest eaves gap in the HR guidance because air moves more slowly at shallow angles. If the roof is also covered in tiles with their own minimum pitch, the combination matters; see tiles on a low-pitched roof.
The symptoms of poor ventilation
Signs vary in severity. Early on there may be only a faint dampness on the underside of the underlay or a cold, clammy feel in the loft on winter mornings. Later come the clearer signs: droplets hanging from nails, staining on rafters, black or grey mould on timber and on the underside of the covering, and a damp smell that comes down through the hatch.
Inside the house, look at the ceilings of the top-floor rooms. Mould in corners, patches that come and go with the weather and peeling paint where the loft sits above are all worth noting. Stored boxes and clothes in a loft can also absorb moisture and go musty.
Condensation or leak
A useful rule of thumb separates the two. A leak tends to mark one spot and gets worse after rain, often with a stain that has a tide line. Condensation tends to be spread over a larger area and varies with how cold the weather is and how much moisture the house is producing, not with whether it has rained. The two can occur together, which is why we inspect the roof as well as the loft.
Left alone, repeated wetting rots the battens and the ends of rafters, rusts nail heads and can reduce the effect of the insulation. None of that happens overnight, but the damage is cumulative, and it is hard to see once the loft is boarded out.
The fixes, in the order we try them
The least costly fixes come first. Stopping moisture reaching the loft costs less than removing it afterwards, so we start with the ceiling and the house itself.
Seal and extract
Seal gaps around the loft hatch, pipe penetrations, cable runs and light fittings, and fit a draught-strip and a proper catch to the hatch. In kitchens and bathrooms, ducted extractor fans that vent outside, not into the loft, take moisture away at the source. Tumble dryers need an outside vent as well.
Clear the path
Pull loft insulation back from the eaves and fit baffles so air can enter. Check the soffit vents are clear of paint and debris. Where the original roof relied on gaps at the eaves that have since been boxed in, we add a vent strip or a fascia vent.
Add ventilation
If the eaves cannot be improved enough, high-level ventilation is added with ridge or tile vents, sized to the BS 5250 figures for the underlay type and pitch. A roof with a felt underlay that has no ventilation at all is the most obvious case.
Renew the underlay
Sometimes the underlay is the cause: old felt that traps vapour, or a membrane that has been damaged. Renewing it means stripping the tiles. Where a roof is being re-covered anyway, that is the moment to choose a breathable membrane and review the ventilation. Our underlay guide explains why.
In every case, we photograph what we find in your own loft and explain which fix we think is justified and which is optional.
Conservation roofs and spray foam
Traditional roofs ventilated themselves. Torching, the lime mortar fillets pointed under the tiles from inside, and gaps between tiles let air move through constantly. A building that has breathed in that way for generations is easy to upset. Sealing the underside tightly, adding a plastic membrane or insulating hard against the tiles can trap moisture against old timbers.
On a listed building or one in a conservation area, changes to the structure or covering of the roof may need consent. Our guide to tiled roofs on listed and conservation-area homes covers what the council expects. Check with Stroud District Council's planning team before you change a roof like this.
Spray foam
Foam sprayed onto the underside of the roof deck turns a cold roof into a sealed one. The RICS consumer guide of March 2023 warns that poor installation can lead to dampness, such as uncontrolled condensation, and says that thermal calculations and condensation risk checks should be carried out by a qualified professional first. It also advises checking with your mortgage provider and your insurer. In November 2024 Today's Conveyancer reported a BBC investigation in which some lenders said they would decline such homes and others would look at each case. We take no position beyond that: if you already have spray foam, tell us before any work starts so we can plan around it.
Cost, consent and what to expect
Ventilation work is usually one line within a larger job. Eaves vents and baffles are modest items when a roof is already open; adding them on their own needs access and some time. MyBuilder's 2026 repair guide puts roof repairs generally at £150 to £1,500, and a single scaffold tower or access is part of that picture. Treat them as national guide figures only.
If ventilation is changed as part of a re-cover, vents, baffles and underlay are covered in the quote that follows the survey.
Regulations
Ventilation is part of how a roof meets the Building Regulations, and a re-roof that goes beyond the Planning Portal thresholds of 25 per cent of the building envelope and 50 per cent of the roof needs approval. Insulation is a related question: where the whole roof is renovated, the Part L rules can require a thermal upgrade, and the loft insulation must not block the eaves. VAT on loft insulation installed in a home is charged at the zero rate until 31 March 2027, whereas ordinary roofing work carries standard-rate VAT, shown on your quote.
For notifiable work the paperwork is ours to do, and any consent is explained to you. Waste leaves with us, and the 10-year workmanship guarantee is in writing.