A loft is a cold, enclosed space sitting on top of a warm house. Every shower, pan of pasta and drying shirt adds moisture to the air indoors, and some of it finds its way up through ceiling gaps and light fittings. Unless that damp air can leave the roof space, it settles on the coldest surface it meets: the underside of the roof covering, the rafters and the felt. Over a winter that becomes condensation, then spotting, and in the end mould and rot.
Ventilation at the eaves is the usual answer. The soffit is the panel under the roof overhang, and small vents in it, or a continuous gap behind it, let fresh air drift in and across the loft. Standards set the size of that opening, and the figure changes with the pitch of the roof and the type of underlay.
Not every roof needs soffit vents, and some need more than a vent. Here we explain the sizes, the roof types that matter, how to spot a problem from inside the loft and what happens when old soffits are replaced. It belongs to our wider gutters, soffits and fascias work.
What eaves ventilation actually does
The idea is cross-flow. Air enters low at the eaves, rises along the slope beneath the covering and escapes by way of the ridge, or a vent placed high up on the roof. Moving air carries moisture out before it can condense. It also keeps the timbers closer to the temperature of the air outside, which matters because wood that stays damp for long periods is where fungal decay begins.
A ventilated loft does not need to be draughty. The aim is a steady trickle, not a gale. That is why the openings are measured in a few millimetres along the whole length of the eaves instead of in large holes at either end. Spread along the full run, a narrow gap gives even airflow across every rafter bay, while a few big vents leave dead pockets between them.
Cold roofs and why they dominate
A cold roof is common in British houses: the insulation lies along the loft floor, which leaves the space above it cold and ventilated, and the roof covering is the only barrier between loft and weather. That is the construction the standard has in mind when it sets eaves gaps. A warm roof is different. The insulation follows the slope, there is no cold loft to ventilate, and the design depends on a continuous air and vapour layer. Putting soffit vents into a warm roof can undo the thermal performance, so we always find out which one we have before drawing any conclusion.
Beneath either type sit the ceilings. A sealed ceiling, with the loft hatch draught-proofed and light fittings and pipe holes filled, lets far less damp air upward in the first place. That helps more than any vent. A badly fitted extractor fan that vents into the loft, not outdoors, is a surprisingly common source of the moisture that ventilation is then asked to cope with.
The gap sizes in BS 5250
BS 5250 is the code of practice for managing moisture in buildings, and its eaves figures are summarised in Marley's guide to ventilating a cold roof. They depend on the type of underlay, so start with that. A high-resistance underlay is one that does not let moisture vapour pass easily, as with the older bitumen felt on many roofs. A low-resistance underlay is breathable and lets vapour through.
With a high-resistance underlay, Marley sets out these openings:
| Roof situation | Eaves ventilation along the length |
|---|---|
| Pitch of 15° or less | 25mm |
| Pitch of more than 15° | 10mm |
| Pitch over 35°, span over 10m, or lean-to and mono-pitch roofs | Add a continuous 5mm opening at high level |
Those figures are for the ventilation opening along the whole length of the eaves, not for one vent. A single disc vent has a small free area, so a run of them is spaced to deliver the total.
When less is enough
Marley says that where a low-resistance underlay sits over a well-sealed ceiling and below a covering that lets air through, the leakage through ordinary eaves construction, around 3mm, is enough, provided the requirements of third-party certification are fully met. In plain terms, a breathable membrane on a properly sealed ceiling can sometimes manage without separate vents, but only where the product certificate says so.
This is why a flat answer of yes or no is rarely useful. A roof relaid with breathable felt years ago, under a ceiling full of recessed lights, may behave very differently from a neighbour's identical house with bitumen felt and a sealed ceiling. We check the underlay type from the loft, measure the pitch, look at the existing eaves and then say what the roof needs.
Vent types and how they are fitted
There are four common ways to get air in at the eaves, and each suits a different situation.
Disc vents are round plugs pressed into holes cut in the soffit board, spaced along it. They are neat and cheap to add to existing boards, but each has a small opening, so the spacing has to be close to reach the total gap. Strip vents are long slim grilles that fit into a slot cut along the soffit, giving a more continuous opening. Ventilated soffit boards are made with built-in slots or perforations, and they are the usual choice when a whole run of new boards goes up. Over-fascia vents are fitted above the fascia board, beneath the lowest row of tiles, and suit where the soffit is too narrow for any of the others.
Keeping the path clear inside
The vent is only half of it. Insulation pushed hard into the corner of the loft at the eaves blocks the very gap the vent feeds. A rigid eaves tray, sometimes called a baffle, holds the insulation back and leaves a channel from the vent up under the underlay. Without one, a house can have perfect vents and still have a stuffy loft. We look for this from the inside, and on a roof with suspected condensation we look at it early.
Every vent has an insect mesh or a fine grille so wasps, flies and birds do not use it as a way in. That mesh can clog with paint, with salt and dust, or with cobwebs. The seaside makes this worse: gutter overflow and salt deposits settle on soffits, and rain can never wash them clean because the board sits under the overhang. Wipe the grilles with clean water when you wash the soffit, and keep them free of paint.
Reading the signs from inside the loft
You can learn a lot about a roof's airflow with a torch and ten minutes. Look first at the underside of the felt or the covering. Beads of water, dark patches or black spotting suggest air that is not moving. On older boarded roofs, rafters with a dark fur of mould near the eaves tell the same story. Insulation that is damp, flattened or stained is another warning. So is a smell like a damp cellar when you lift the hatch on a cold morning.
Winter is the best time to look. Frost or fine ice on the tips of roofing nails, where they poke through the felt, means moisture is condensing in the cold. It melts when the sun comes up and drips on the insulation, which is why a ceiling can show a stain after a frosty night with no rain at all. Our guide to telling condensation from a leak explains how to separate the two.
What the eaves themselves tell you
From outside, look at the soffits. Staining, flaking paint or green mould on the board suggests a damp roofline, though that can come from a leaking joint above as much as from poor airflow. A soffit that has never had a vent and sits under a loft with condensation is a likely candidate. Equally, a soffit with vents that are plugged with paint is no better than one with none.
Do not stuff anything into the eaves to stop draughts, and do not block vents because the loft feels cold. A cold loft is meant to be cold. If you want to cut heat loss, improve the insulation on the floor and seal the ceiling, but leave the airflow gap alone. If you are unsure what you are looking at, send us pictures and we will say what we see before suggesting any work.
Older roofs and what changes with new work
A lot of older Worthing houses never had soffit vents and did not seem to need them. The reason is that traditional roofs leaked air. Gaps between slates, loose battens and open joints at the eaves let air through by accident, and old lath-and-plaster ceilings were not airtight either. The roof ventilated itself.
That changes when the house is improved. A new, tightly sealed felt, thicker loft insulation, a plasterboard ceiling or double-glazing and draught-proofing all cut the accidental airflow and raise the humidity inside. A roof that never gave trouble can start to show condensation within a winter or two of an upgrade. If you are planning work like this, ask whether the eaves ventilation is still adequate afterwards.
The moment to add vents
The easiest time to deal with ventilation is when the soffits are being renewed or the roof is being recovered, because the boards are off or the felt is exposed and nothing needs to be cut into sound material. Vented boards are made for exactly this, and an over-fascia vent can be fitted when the fascia is replaced. The page on how long roofline replacement takes shows how the work fits together on site.
On a re-roof, the right underlay and its eaves details are chosen together, because the product's certificate sets the conditions. Replacing the felt without checking the eaves is a missed chance.
One caution before anything is cut. Boards on a building from before 2000 may be asbestos insulating board, and cutting vent holes into it is the very last thing to attempt. Our guide to asbestos soffit boards explains why we stop and ask for identification first.
Replacing soffits with ventilation in mind
When new soffits go up, the vents should be planned into the job and not added afterwards as an afterthought. We start with the survey photographs from the loft and the eaves, which show the underlay, the state of the insulation at the edges and whether any old vents are present. That tells us whether we are adding airflow, restoring it or leaving it alone.
As a guide to what drives cost, MyBuilder (updated 21/08/2026) quotes a typical UK range of £100 to £180 per metre for soffit replacement, and £80 to £150 per metre for fascia. The vents themselves are a small part of that. The bigger swings come from scaffold access, how long the roofline is and what the survey finds behind the old boards.
What to ask for
Ask any contractor which underlay type is on the roof, what eaves opening the standard calls for on your pitch, and how the opening will be delivered. A good answer names the product and the spacing. A poor answer is that vents are not needed or that they will be fitted anyway, without looking at the loft.
If birds or bats are using your eaves, mention it before work starts, since both are protected and the plan may change.
On a full soffit and fascia renewal the waste leaves with us, and the vents are checked for airflow before the last board goes up. A written quote with one total arrives once we have surveyed. If the survey points to the ceiling or an extractor fan and not the roofline, you hear that from us, along with the lightest fix that will hold.
