The short answer
It depends on the roof. A traditional cold roof under non-breathable felt needs air at the eaves, and soffit vents are the usual way to provide it. A roof with a breathable underlay and a well-sealed ceiling can depend on very small gaps in the normal eaves construction instead. Pitch and underlay set the figures, so check which you have before you fit or block anything.
A soffit vent lets air drift through the roof space. Whether yours needs one comes down to the roof's pitch and the underlay beneath the covering.
What soffit vents do
Beneath the overhang of a pitched roof, a flat panel fills the space between wall and fascia. That panel is the soffit. A vent in it, either a row of small holes or a continuous slotted strip, lets outside air into the loft.
Why does a loft need air moving through it?
Steam from cooking, showers and drying clothes rises into the loft. With no way out, the moisture settles as condensation on cold surfaces, including the underside of the underlay and the rafters. That leads to damp insulation, mould on the timber and, over the years, rot. BS 5250, the standard that governs moisture control in buildings, covers how to deal with it, and the 2021 edition is currently listed by BSI as under review.
The air entering at the eaves is one half of the answer. In some designs it leaves at the ridge or through high-level vents. The aim is a gentle, steady flow of air, not a draught, and loft insulation should never be pushed into the eaves to block it. A baffle, a thin rigid board that holds the insulation back, keeps the gap clear for the air.
Underlay type sets the rule
Underlay is the membrane sheet laid beneath the battens, the timber laths that carry the tiles or slates. There are two broad families, and Marley's guidance on cold roofs treats them differently under BS 5250.
Why do HR and LR underlay get different rules?
HR underlay does not breathe, so vapour cannot pass through it. With HR underlay, Marley gives a 25mm opening along the whole eaves on roofs pitched at up to 15 degrees, and 10mm on steeper ones. LR underlay is the breather type, which lets vapour out. With a well-sealed ceiling, Marley accepts incidental ventilation, the 3mm leakage found in ordinary eaves construction, but only where the underlay carries third-party certification.
So an older roof with felt may need proper vents all along the soffit, while a newer roof with a certified breather underlay can manage with very little. The product data sheet for your underlay is the document to check, and guesswork is unwise, because a vent fitted where none is needed can let in wind-blown rain. Our answer on eaves felt rot shows how damp at the eaves looks when the detail goes wrong.
Signs your roof space is short of air
The loft itself tells you a good deal. Look for dark spots on the timbers, soggy insulation, rusty nail tips showing through the underlay and a musty smell. On cold mornings, frost on the roof covering's underside melts later in the day and drips.
Can I see whether my soffits are vented?
From the ground, look up at the soffit. Small round holes or a long slotted strip mean vents are fitted. A plain, solid panel may mean there are none, though the roof can still be ventilated in a different way. Inside the loft, daylight at the eaves and a clear gap above the insulation is a good sign. A torch and a few minutes are all it takes to see which applies.
Green growth on soffits is a separate matter, caused by damp air and shade rather than missing vents, and our answer on green algae on soffits explains how to treat it. If the soffit is old and you think it could be asbestos insulating board, do not drill or cut it. Our answer on asbestos soffits covers what to do first.
Fitting vents when the soffits are replaced
New soffits are the easiest moment to correct ventilation. Vented boards or continuous strips can go in as the old panels come off, and the roof space can be checked at the same time, which saves a separate visit, with the insulation pulled back where it has crept over the eaves.
Will new uPVC soffits need vents?
If the roof is a cold roof with HR underlay, yes, the new boards should supply the opening the standard sets, and nothing should block it. If the roof has a certified breather underlay, the detail may differ. We find out which you have at the survey, look in the loft where it is safe and accessible, and tell you what we see, with photos for the file.
The cost turns on the run of the eaves, the access, the board type and whether the fascia is changing too. Ask that VAT and access are stated in any quote, since scaffold for a long eaves run may make up a big part of the cost. We write one total after the survey. Read more on the gutters, soffits and fascias page.
More on eaves ventilation
Is it a problem if my soffit has no vents?
Not always. A solid soffit is fine where the roof uses a certified breather underlay or is ventilated another way. A cold roof with older non-breathable felt and no air path is the worry. Look in the loft for damp, mould or rusty nails, and have a roofer identify the underlay.
Can insulation block soffit vents?
Yes, and it often does when loft insulation is added without care. Insulation stuffed into the eaves closes the air gap. A baffle fitted at the eaves holds the insulation back and keeps the route open, so the vents continue to work after the loft has been topped up.
The full picture: Gutters, soffits and fascias in Taunton ยท More short answers