Warm, damp air rises from a house into the loft, from cooking, washing and breathing. If it meets cold roof timbers and has nowhere to go, it condenses on them. Over time that means damp timber, wet insulation and, sometimes, the staining that shows up on ceilings. The usual cure is a steady flow of outside air through the roof void, drawn in at the lowest edge of the roof and let out at the top or through the covering itself.
The openings at the eaves are the soffit vents and the over-fascia vents. They are small parts of the roofline, easy to ignore, and they do real work. The figures in this guide come from Marley's published guidance on BS 5250, the moisture-management code the industry works to. Treat them as typical guidance and never a promise for one particular roof; the underlay and the covering on your house change what is needed. For the staining side, see our answer on mouldy soffits. The wider service is on our gutters, soffits and fascias page.
Why the eaves are where the air comes in
A cold roof is one where the insulation sits on the loft floor, between and over the ceiling joists, and the space above it is left unheated. The roof timbers and the underlay stay close to outside temperature. Any moist air that leaks up from the rooms below needs somewhere to escape, or it condenses on the coldest surface it finds, usually the underside of the underlay.
Air moves best when it has a way in and a way out. Cool air enters at the eaves, which are low on the roof, and moves up the slope and out again, either at the ridge or through the covering. Without a gap at the eaves the system has no inlet, and the loft sits damp and still.
Where the eaves gap actually is
On a roof with a boxed overhang, the underside is closed with a soffit, the flat board under the projecting rafters. The easiest way to let air in is to put a vent in that board. On a roof with little or no overhang, the vent goes above the fascia instead, in the gap between the top of the fascia board and the first course of tiles or slates. That second type is the over-fascia vent.
Soffit vents, over-fascia vents and the others
There are several ways to give the eaves an opening. They do the same job in different places.
| Type | Where it sits | Where it suits |
|---|---|---|
| Soffit vent, disc or grille | A round or rectangular hole in the soffit board, fitted with a mesh grille | Houses with a boxed overhang where there is room to cut openings at intervals |
| Continuous soffit strip | A long slot along the soffit, or a soffit board with the opening built in | New soffits, where a vented board can be chosen from the start |
| Over-fascia vent | Above the fascia, below the first course of the covering | Houses with a shallow or flush eaves, or where the soffit cannot be altered |
| Eaves vent tray or rafter vent | Inside the roof, at the foot of the rafters | Used with any of the above to keep the opening clear of insulation |
What the grille is for
Every vent carries a mesh or a slot small enough to keep out wasps, birds and rodents while letting air through. A mesh that has clogged with paint or dust is worth knowing about, because it reduces the opening you thought you had.
Fitted to the board, not bolted on
Vents are fixed in the same way as the boards around them, so a plastic soffit takes plastic vents and a timber soffit takes a vent cut cleanly through the board and finished to match. A ragged hole or an unpainted edge lets the board rot around the opening, which defeats the purpose of fitting it. Spacing matters as well: a few vents bunched at one end of an elevation leave the rest of the eaves dead.
Choosing one for the house
The roof dictates the choice. A vented soffit board is simple where the soffit is being renewed anyway. An over-fascia vent suits a roof where the soffit is shallow, on timber or plastic. On a listed or period house, a visible vent may not be acceptable, and then the options narrow. That is the subject of our guide to gutters and fascias in conservation areas.
How much opening the guidance asks for
The guidance in BS 5250 is set out as openings along the length of the eaves, and the figure depends on the underlay and the pitch. Marley's page on ventilating a cold roof gives these typical values.
With a non-breathable underlay
Where the underlay is the traditional type, known as HR, vapour does not pass through it and the air has to carry the moisture out. Marley's guidance is for a continuous opening along the eaves of 25 mm for pitches of 15° or less, and 10 mm for pitches above 15°. On steeper roofs over 35°, wide spans over 10 m and lean-to or mono-pitch roofs, the guidance adds a further continuous 5 mm opening up at high level.
With a breathable underlay
A breather underlay, known as LR, lets some vapour through. Under that guidance, when the ceiling below is well sealed and the covering lets air pass through its laps, the small gap built into ordinary eaves construction (3 mm) is described as enough, or 5 mm along the ridge. A normal ceiling changes the answer: Marley then advises air into the roof void at the eaves, plus the batten zone beneath an air-tight covering.
Why we read the product guidance
These are manufacturer figures reflecting a code, and the product on your roof may set its own requirements. If your underlay or covering carries its own ventilation instructions, we follow those and check them against what is already in the eaves.
What blocks the airflow
A vent is only as good as the gap behind it, and a grille can look perfect while nothing moves. The commonest blockage we look for is insulation. Laid into the loft, quilt insulation is pushed out to the edge to stop heat loss, and it ends up sitting hard against the eaves, closing the opening the soffit vent was fitted to feed. The vent looks fine from the ground and passes no air.
Keeping the gap clear
The fix is a barrier at the foot of the rafters that holds the insulation back while leaving a channel above it for air. This is the job of an eaves vent tray (sometimes called a rafter tray or baffle), a thin formed piece that fits between the rafters and the underlay. It sits flush with the eaves, protects the air path and lets the insulation carry on over the wall plate. It is a simple part that takes little time to fit, and it is often missing.
Paint, dust and nests
Paint over a mesh grille, a build-up of dust, or a nest in the eaves each restricts air in a way that is not visible from the ground. Birds will use any gap that is big enough, and a nest in a soffit is a practical block. Where birds are nesting, work waits; nesting birds are protected, so we plan around the season.
Insulation upgrades
Anyone adding loft insulation should remember the vents. The thicker the layer, the more likely it is to reach the eaves. If you are adding insulation while the zero rate for VAT on home roof and loft insulation runs (until 31 March 2027), ask whoever fits it to protect the eaves gap. Your quote shows VAT.
Fitting vents when the soffits are replaced
The least costly moment to improve eaves ventilation is when the soffits are coming off for another reason. The boards are already down, the scaffold is up and the rafter ends are visible.
MyBuilder's 2026 price guide puts soffit replacement at £100 to £180 per metre and fascia at £80 to £150 per metre, with scaffold at £20 to £40 per m². Those are national ranges for the boards, not our prices, and a vented soffit board or a run of over-fascia vent adds to the cost only by the cost of the part. The written quote we give after the survey covers the whole job. Our roofline cost guide goes through what pushes the figures up or down.
What we check before choosing a vent
Our first look is from inside the loft. We want to see whether there is an eaves tray, how far the insulation reaches, which underlay is in place, and whether anything inside has stained or darkened. Photographs of the loft edge are part of the survey, so you see what we see.
Old boards with no vents
Many older soffits are plain boards with no opening at all, because the roof was ventilated through gaps in the old construction. If the loft has since been insulated, sealed or re-roofed with a tighter underlay, those gaps may have gone. Adding vents is then a straightforward improvement, as long as the house allows it.
When a ventilated eaves is the wrong answer
Not every roof is a cold roof. A warm roof, where the insulation follows the rafters and the loft space is part of the heated house, is built differently and may need no eaves openings at all. A roof converted to a bedroom, a flat roof and a lean-to under a membrane all have their own detailing.
Cutting vents into the wrong roof can do harm. It can let cold air into a heated space and undo the point of the insulation. That is why we establish how the roof is built before suggesting a vent, and not the other way round.
Ventilation is not the same as a leak path
There is also a link to rainwater. An eaves that is regularly flooded by an overflowing gutter will soak the soffit and the vents with it. Our guide to gutters in heavy rain, leaf fall and ice covers why that happens.
If you suspect poor ventilation, the signs include condensation or mould in the loft, damp insulation, dark patches on ceilings, or a musty smell on the landing. Send photographs and we can say whether it points to the eaves. The survey photographs the loft and the roofline of your own house, our quote follows in writing, and the work carries a 10-year workmanship guarantee, in writing.