The short answer
Yes, if it is a cold roof: insulation laid on the ceiling and a loft left cold above it. With a non-breathable HR underlay, Marley's guidance from BS 5250 is 25mm of opening along the eaves for pitches up to 15 degrees, and 10mm for anything steeper. A breathable LR underlay over a sealed ceiling can work on less.
Why a cold roof breathes
In a cold roof the warmth stays in the rooms below because the insulation lies along the ceiling. The loft above is left cold on purpose, close to the temperature outdoors. Moisture drifting up from the house lands on cold surfaces, so it needs a way out.
The way out is a draught of outside air: in at the eaves, across the roof space and away, with the vapour. Approved Document C, paragraph 6.11, treats ventilating cold deck roofs as meeting the condensation requirement. It then adds that ventilating a small roof, such as the one over a porch, may not always be needed.
The standard behind this is BS 5250, on the management of moisture in buildings. The current edition is BS 5250:2021, listed as under review. Approved Document C still cites the 2002 version, so you may see both quoted.
The practical lesson is that ventilation is part of the roof, not an optional extra, and it is cheaper to keep the openings clear than to cure the damp that follows when they are not. Our page on condensation in the loft shows what happens when it goes missing.
HR underlay and the eaves gap
How much ventilation is needed turns on the underlay under the tiles. The HR in HR underlay means high resistance, and the sheet does not breathe, so vapour cannot cross it and the whole load falls on air movement.
Marley, working from BS 5250, sets the openings for HR underlay at 25mm for roofs of 15 degrees or under, and 10mm above 15 degrees, measured along the eaves. A soffit vent or eaves strip has to deliver at least that, with no insulation pushed into the gap.
It follows that a shallow roof needs a bigger opening than a steep one. A steep pitch lets air rise through the space more readily, whereas a shallow one holds still air.
Check the vents are clear. Paint, debris, birds' nests and insulation shoved into the eaves can close a slot that looks open from outside. If you are fitting a comb to keep birds out, it needs to let air in as well, as our page on rotten eaves felt also notes for the eaves line.
A roofer checks the opening both from the soffit side and from the roof space with a torch.
LR underlay and the sealed ceiling
An LR underlay, short for low resistance, is the breathable kind, so vapour can pass through the membrane itself. The demand on the eaves drops accordingly. Marley says such a roof, with a well-sealed ceiling under it, can rest on the 3mm of fortuitous air movement in normal eaves construction, if the product's certification says so.
The condition that matters is the sealed ceiling. If warm, moist air leaks up into the loft through an unsealed hatch, gaps around pipes or a poorly closed light fitting, a breathable membrane will struggle to shed it all. AD C paragraph 6.12 asks for a proper draught seal on loft hatches because of this.
Do not assume a breathable underlay is always the answer. It relies on the product's certification and on the ceiling being sound. If either is doubtful, a vented roof with clear eaves is the safer arrangement.
Our page on breathable roof membranes covers how they differ from older felts. HR underlay is not breathable, and that is the most common mistake.
Check the datasheet of any product before relying on it, since each maker states its own conditions.
Checking your own loft
A torch and a quiet look tell you plenty. On a cold, damp morning, look for beads of water on the underside of the roof, dark spotting on the timbers, damp insulation and a musty smell. These point to ventilation that is not working.
Look at the eaves from inside. If the insulation lies on top of the opening, or stuffed into it, air has nowhere to enter. Look from outside at the soffit or eaves detail for a vent strip, slotted soffit or open gap.
Not every roof is a cold roof. A converted loft, a warm roof with insulation above the rafters and a flat roof each have different rules, and ventilation requirements differ between them. The first question is always which type you have, and a photograph of the loft usually answers it.
The survey identifies the roof type, photographs the eaves and the underside of the covering, and measures the openings that matter. Each finding is explained in plain words, with what it means for the roof.
If a re-roof is planned, the underlay and ventilation get decided together. Our quote is one total. The pitched roofing hub lists the related work we do in Farnborough, Camberley and Fleet.
