The soffit is the board under the overhang at the eaves, the strip you see when you look up at the edge of a roof. On many houses it is plain and does nothing but bridge the space between brickwork and fascia. On others it carries vents, and those vents are how a cold roof gets fresh air into the loft. Warm, damp air from a house rises into the roof space. With no exit, that vapour turns to water on cold timber and felt, and repeated winters bring mould, wet insulation and rot. Ventilation at the eaves, with a way out at the ridge or high level, keeps the air moving. This guide explains how soffit vents work, the gap sizes quoted for common roof types, what blocks them, and how to decide whether yours are doing their job. The wider roofline work sits under gutters, soffits and fascias.
What a soffit vent does
Insulation laid along the loft floor makes a pitched roof a cold roof. The roof space above the insulation stays close to the outside temperature, and moisture that leaks up from the house, from cooking, bathing, drying clothes and breathing, ends up there. The job of ventilation is to replace that damp air with outside air before it condenses.
At the eaves the air enters through a gap or a vent in the soffit. It travels up the underside of the roof covering, picks up moisture, and leaves at the ridge, through tile vents or through high-level openings. Even a gentle breeze or a small temperature difference gets it moving, because warm air rises and draws cooler air in below. For the system to work, air has to be able to cross the roof space from one side to the other, which means nothing can block either the inlet at the eaves or the path inside.
The vent itself is a simple part. It might be a row of circular discs pushed into holes in the soffit board, a continuous strip of perforated plastic, a soffit board with slots built in, or a vent fitted over the top of the fascia. All do the same thing: let air in, keep insects and birds out.
You will not see air moving. The evidence shows when air is missing: dark mould on the boards beneath the covering, rusting nail points showing through the sarking or underlay, and insulation that feels damp. Those are signs of condensation, and they point to a ventilation problem, a source of too much moisture in the house or both.
Cold roofs and warm roofs
Everything here applies to a cold roof. A warm roof, where the insulation follows the pitch of the rafters, may be built to different rules, and some are designed with no ventilation gap. Do not add vents to a roof without finding out which kind you have.
How much opening is needed
The quantities come from BS 5250, the British Standard on moisture in buildings, and they are published by manufacturers in their own guidance. Marley's guide to ventilating a cold roof gives the following figures, which are worth treating as typical values and not as a spec for your house. Ask the maker of your underlay and your roofer what applies to your exact roof.
Where the underlay is the older, non-breathable type, called HR, and the insulation sits at ceiling level, the guide gives continuous eaves ventilation of 25 mm for roofs at a pitch of 15 degrees or less, and 10 mm above 15 degrees. A further 5 mm of continuous ventilation at the ridge or high level is added where the pitch is over 35 degrees, where the span is more than 10 m, or where the roof is a single slope leaning against a wall.
A breathable underlay, called LR, changes the sums. What is required turns on the airtightness of the ceiling and on whether the covering itself passes air. Marley's example is a well-sealed ceiling with an air-permeable covering, which can need an eaves gap of just 3 mm where third-party certification supports it, or a 5 mm opening up at the ridge. The point is that the answer is not one number for every house.
These are widths of continuous opening, not the diameter of a hole. Individual disc vents need to be spaced to give an equivalent free area along the eaves, and a vent that looks substantial on the face of a soffit can have little open area behind its grille. The free area is printed on the product data sheet.
A practical check is to look at what exists. A soffit with a thin slot of a few millimetres, or a handful of discs on a long eaves run, may be well below what the underlay supplier expects, particularly if the insulation has been topped up since the roof was built.
What blocks soffit vents
The commonest culprit is insulation. When loft insulation is extended to the edge of the eaves, it fills the gap between the rafters and shuts the air path. This often happens in a top-up job when someone pushes new roll insulation into the corner. The remedy is a baffle, a small plastic channel pushed up against the roof boards so the insulation stops short and a gap runs from the soffit into the roof space. Each rafter bay needs one.
Paint comes next. Perforated soffits and disc vents get painted over in a hurry, and a clogged slot lets nothing through. So does a build-up of dirt and cobwebs in a soffit that has not been cleaned. Spiders and wasps like the sheltered space behind the board, and a nest can seal a vent.
A third problem is a replacement soffit that has no ventilation at all. When timber soffits are replaced in plastic boards, the old vents are sometimes left out. The boards look neat and the roof loses its air supply. Anyone renewing the soffits should confirm what ventilation the roof had before, and match or improve on it. Our page on soffits and fascias sets out the replacement work and what to specify.
Debris can fall inside too. Sarking felt that has drooped into the eaves, or underlay laid so that it sags into the gap, can close the path from inside the roof. Fixing the underlay properly or adding an eaves support tray holds it clear.
Last, look at any extractor fan that ends in the loft. Bathroom and kitchen fans ducted into the roof space put a large amount of water vapour exactly where condensation is worst. They should discharge outdoors, and a roofer cannot fix that by adding more soffit vents.
Spray foam, boarded eaves and other reasons vents fail
Some roofs have had their ventilation deliberately removed. Foam sprayed onto the rafters from inside seals them, and it can fill the soffit and eaves gaps as well. The RICS consumer guide on spray foam warns of condensation that is not controlled, and suggests checking lender policy with your mortgage provider. A BBC investigation reported on 20 November 2024 that some lenders would refuse a loan on a home with it. Where we find foam, we look at the timbers behind it first. The point for this page is that soffit vents do nothing for a roof that has been sealed from the inside.
Eaves that have been boxed in during a past alteration cause a quieter version of the same problem. Where a conversion or extension has taken the ceiling up to the roof slope, or where a dormer has been added, the air path may now run through a space that was not designed for it.
Roof windows and loft conversions also shift the picture. A converted loft with insulation between the rafters needs its own ventilation arrangement, and it is a design matter for the person doing the conversion. If you suspect the work was not done to a standard, a building control body can look at the records.
Birds and bats are the other consideration. Roosting bats and active bird nests are protected, so stop and take advice if you meet either in the eaves, before anything is sealed. Vents with fine mesh keep birds and insects out, and a roofer can fit them without closing off an established roost.
None of this means soffit vents are always the answer. A roof with a breathable underlay, a sealed ceiling and a well-vented covering may need very little. A roof with condensation may have a moisture problem at its source. Knowing which, from a proper look in the loft, matters more than adding vents on spec.
Reading the evidence outside and in the loft
You can learn a lot without going on the roof. Stand outside and look up at the soffit. Is it a plain board or does it have a grille, a row of discs or a slot? Is there paint in the slot? Is there staining or algae along the board, which suggests damp has been escaping nearby?
Then look inside, if the loft is safe to enter and the joists are boarded or you can step on them. Torch the eaves. You should see daylight or at least an open gap where the insulation stops short of the wall. If insulation touches the roof boards at the eaves, the air path is closed. Check the boards under the covering, and the rafters. Dark patches, white fur, rusty nail tips or a damp smell are the signs of condensation.
Notice what happens with the seasons. Condensation shows after cold nights, in winter and when the house is full of moisture. A loft that is dry in summer and wet in January points to condensation. A stain that appears only after heavy rain points to a leak from the covering instead. Our guide on rotten fascia covers the damage that follows when water gets behind the roofline from outside, which looks similar but has a different cause.
Check the extraction. Does the bathroom fan discharge outside? Is the tumble dryer vented to the outside? Does the landing hatch fit snugly, so warm air does not escape into the loft? Sealing the paths of moisture from the house into the roof, and then ventilating the roof, is a better order of work than ventilating alone.
If you find signs but are unsure what they mean, a survey will say. We photograph your own roof from outside and, where access allows, from the loft, and report in ordinary language what is blocked and what is missing.
Adding or restoring vents, and what it costs
Where soffits are in good condition and simply unvented, vents can be cut in. Disc vents are the quickest method, and a hole saw makes a clean opening in a timber or plastic soffit. Strip vents are fitted along the board for continuous air flow. In a full soffit replacement, vented boards or over-fascia vents are specified at the start so the roof gets what it needs.
The work usually goes with other eaves jobs. If the fascia is being replaced, the soffit and gutter are normally done at the same time, because they are all fixed to the same timbers. The gutter cost guide at guttering cost shows the typical ranges for the whole roofline. For the soffit alone, MyBuilder's gutter, fascia and soffit guide (2026) reports typical UK costs of £100 to £180 per metre for soffit, with scaffold adding £20 to £40 per square metre. Adding vents to existing boards is a small part of that, and the fixed price comes after we have looked.
Access shapes the cost. Soffits on a second storey need scaffold or a tower, and the work cannot be done safely from a ladder alone. That is another reason it makes sense to deal with ventilation when the eaves are open for other work.
Where baffles and tidy-up fit in
Fitting eaves baffles is a loft job and not a roofline job. It costs little in materials but takes time, since each rafter bay is done by hand in a cramped space. If you have topped up insulation yourself, check the eaves before you close the hatch. Loft insulation that is cut back to leave a clear path of airflow loses very little of its performance.
After any work, check again within the first winter. A roof that was damp before will take time to dry out, but the signs should be easing. If they are not, the source of the moisture is somewhere else, and that needs tracing.