The short answer
A new roof needs a route for moist air to leave the loft, set by the underlay and the pitch. With a non-breathable HR underlay, Marley's guidance under BS 5250 asks for a continuous eaves gap: 25mm on pitches of 15 degrees or less, 10mm above that. A breathable membrane above a tightly sealed ceiling can get by on the gap that ordinary eaves construction gives.
Moisture from cooking, washing and breathing rises into the loft. A new roof has to let it out, and the underlay beneath your tiles decides how.
Why a loft needs air moving through it
Damp, warm air leaks up from the rooms beneath through gaps around the hatch, light fittings and pipes. In a cold loft, where the insulation lies on the ceiling and the space above is left unheated, that air reaches cold timber and underlay and condenses. You end up with damp rafters, dripping nail points, mould beneath the covering, and insulation that stops doing its job. Bathrooms and kitchens without working extractor fans add to the load.
What is a cold roof?
It is the standard form of pitched roof: insulation at ceiling level, a ventilated void above, and the covering on top. BS 5250 is the code of practice for moisture in buildings, listed by BSI as current though under review. It does not lay down one rule for every roof. It sets out what each roof construction needs.
When we strip a roof, the loft timbers are in plain view, and the survey photos include the underside of the covering, so you can see for yourself whether the old ventilation was doing its work. Our roof replacement page explains the wider job.
HR underlay and eaves gaps
Underlay is the sheet fixed above the rafters and beneath the battens. The traditional type is called HR, for high resistance, and it is non-breathable. Moist air cannot pass through it, so the ventilation has to be built into the structure.
How big should the eaves gap be?
Marley's guidance, which follows BS 5250, calls for an HR-underlay cold roof to have a ventilation gap running the full eaves length: 25mm where the pitch is 15° or less, 10mm where it is steeper. The eaves are the point where the roof's lower edge projects past the wall. The gap is formed with eaves vent trays or ventilated soffit boards, the soffit being the board under that overhang.
Some roofs add ridge or high-level ventilation as well, usually in the form of a vent tile or a ridge system, so air can flow through the void from bottom to top. Whether yours needs it is a decision for the survey, since roof shape, loft layout and the behaviour of the old roof all count. A hip or valley can interrupt the air path, and a loft divided by a chimney stack needs thought.
Breather membranes: the LR option
The alternative is an LR underlay, for low resistance, which is a breathable membrane that lets water vapour pass through. With a breather membrane the vapour can leave through the underlay itself, so the demand on vents changes.
Does a breathable membrane mean no vents at all?
Not quite. Marley's guidance accepts a 3mm allowance from ordinary eaves construction, which it calls fortuitous ventilation, when an LR underlay is used over an airtight ceiling. That depends on third-party certification, meaning the membrane's maker has had the system independently assessed for that use. The sealing of the ceiling below matters just as much, and ask for the product's certificate if a breather membrane is proposed, since the allowance rests on it. If air leaks up through the ceiling, a breathable membrane will not rescue the roof.
Two warnings follow. Never assume an underlay is breathable because it looks modern: HR underlay is non-breathable, and confusing the two causes damp. And never block the eaves when you top up loft insulation. Insulation stuffed into the eaves blocks the air path the roof relies on.
What to ask when you get a quote
Ventilation is the sort of detail that vanishes beneath the covering and is only noticed when it goes wrong, so ask about it before the job starts.
Which questions are worth asking your roofer?
Ask which underlay will be used and whether it is HR or LR. Ask what eaves ventilation the roof will have and how it is formed, and whether any ridge or tile vents are planned. Ask how the ceiling below is sealed at hatches and fittings, and whether your loft insulation will be kept clear of the eaves.
On a conversion or an extension the Building Regulations also have a say, and your building control body can say what applies. Somerset Building Control is on 0300 303 7790. If the roof you are replacing suffered from condensation, tell us at the survey, because that points to a ventilation cause which has to be solved first. Our guide to rotten roof timbers shows what damp does over time, and the re-roofing process shows where the underlay fits in the order of work. The price we give afterwards is one total in writing.
Vent questions
Can I keep my old vents if I fit a breathable membrane?
Old eaves vents seldom do harm, but whether they are needed depends on the membrane's certification and how well the ceiling below is sealed. Do not close them off to see what happens. At the survey we read the product data, inspect the loft and say what the new roof needs.
Do roof vents make the house colder?
Ventilation moves air through the void above the insulation, so it does not draw heat from the rooms when the insulation and ceiling are sound. The real cause of a cold loft hatch or a draughty ceiling is gaps, not vents. Sealing those gaps does more for comfort than closing vents ever will.
The full picture: Roof replacement and new roofs in Taunton · More short answers