Water on the underside of a roof does not always come from outside. Warm air from showers, cooking, drying clothes and breathing carries a surprising amount of moisture upstairs, and where it meets a cold surface it turns back into droplets. In a loft, the coldest surface is usually the underside of the roof covering, so the beads form there, run down the rafters and sometimes mark the ceiling below. The stain looks exactly like a leak. The cure is different.
The underlay is the layer that sits beneath the tiles or slates and catches whatever the covering lets through. It is also the part of the roof that decides how air and vapour can move. Some underlays let vapour pass through, some do not, and that single difference changes how much ventilation the roof needs.
This guide sets out the two main types, the eaves and ridge openings that ventilate a cold roof, the signs that point to condensation, and the order we work in on a Sussex house before recommending a repair, an upgrade or no work at all. It is general guidance, and the details of any one house are settled on site, in the loft.
What the underlay does, and why it matters for damp
Tiles and slates are not watertight on their own. Wind pushes fine rain and spray up the laps between them, and snow can blow in as powder. Beneath them, a flexible sheet is laid over the rafters and under the battens (the slim timber laths fixed crosswise to the rafters to carry the tiles). That sheet is the underlay. It collects what gets past the covering and guides it down into the gutter, and without it a loft would be wet after every gale.
On many older houses the underlay is bitumen felt, a black sheet that has been used for generations. On houses built or re-roofed more recently, you are more likely to find a plastic membrane that looks like a heavy fabric. Both do the job of shedding water. They behave very differently with moisture rising from inside the house, and that difference is the subject of the rest of this guide.
Damp with no rain behind it
If the underlay blocks vapour and the loft has little airflow, moisture has nowhere to go. It condenses on the cold felt and the timber, and over a winter it can soak insulation, spot the rafters and encourage mould. None of that needs a single missing tile. We see the pattern most often after loft insulation has been topped up and the old gaps at the eaves have been packed out, or after a bathroom has been added with no extractor.
So the air paths get our attention before the covering does. Our guide on how to tell condensation from a leak covers the tell-tale signs from inside the house, and this page covers what the roof itself can do about it.
High-resistance and low-resistance underlays
The two families are named after how hard it is for water vapour to pass through them. A high-resistance underlay, shortened to HR, is the traditional kind. Bitumen felt and similar sheets hold vapour back. A low-resistance underlay, or LR, is the breathable membrane, with a structure that lets vapour escape outwards while still stopping liquid water coming in.
The reason this matters is that a roof with an HR underlay needs a clear flow of air between the insulation and the underlay, because that moving air is the only way for the moisture to leave. A roof with an LR underlay can, in the right conditions, let some moisture out through the membrane itself, so the airflow demands are lighter.
Choosing between them
Neither suits every house, and the choice is made on the evidence in the loft. A breathable membrane makes sense where a loft has been converted, where the eaves are hard to open up, or where dormers, hips and valleys break up a straight run of air. A traditional felt can still be perfectly sound on a simple gable roof with properly ventilated eaves. What does not work is mixing the two without thinking: fitting an HR underlay and then covering the eaves with insulation, for example.
Breathable does not mean maintenance-free either. Membranes need laps, fixings and support at the eaves to be right, and a torn or sagging sheet will hold water however it is rated. We cover that failure in torn roof felt.
How a cold roof is ventilated: BS 5250 and the eaves gap
Many pitched houses have what the trade calls a cold roof: the insulation lies on the loft floor, so the space above it is outside the warm envelope and stays close to the outdoor temperature. Cold roofs stay dry by swapping air with the outside. The guidance is set out in BS 5250, the British Standard on managing moisture in buildings, and Marley summarises the figures for each underlay type.
With an HR underlay, Marley's reading of the standard is a 25mm slot all along the eaves on pitches of 15° or less, and 10mm on anything steeper. Many domestic pitches are steeper than 15°, so the working figure on many houses is 10mm all along the eaves. Note the word continuous: it is a gap that runs the full length of the eave, usually formed with an over-fascia vent strip or by vented soffit boards, and a few small holes do not match it.
Where it goes wrong
Those openings are easily blocked from inside. Insulation pushed hard into the eaves is the usual culprit, and so is a bird's nest, a stored mattress leant against the rafters or paint over a vent strip. From the loft we look along each eave for daylight, and for a baffle, a rigid plastic or cardboard sleeve that holds the insulation back and keeps a channel open above it. Where nobody has ever fitted one, the fix is often as simple as pulling the insulation back and slotting baffles in.
If you have an LR underlay on a well-sealed ceiling with an air-permeable covering, Marley says the ventilation that normal eaves construction provides, about 3mm, is enough, provided the certification requirements for the product are met. That is a narrower route than it sounds, and we would always check the product's own paperwork before relying on it.
High-level vents, ridge ventilation and roof tile vents
Air that enters at the eaves and has no way out at the top stalls. That is why steep roofs and wide roofs need ventilation at a high level as well. Marley's reading of BS 5250 adds a second, 5mm slot at the top when the pitch is over 35°, when the span is more than 10m, plus any lean-to or mono-pitch roof. A mono-pitch roof is a single slope with no ridge at all, so the opening has to be formed at its upper edge.
There are several ways to form that opening. A dry ridge system, where the ridge tiles are held with mechanical clips and a vented strip sits beneath them, gives continuous ventilation along the whole top of the roof. Individual tile vents, which replace a tile with a grille section, give a spread of openings across the slope. A vent at the top of a lean-to is usually a purpose-made strip detailed under the abutment.
Matching the vent to the roof
The point is to give moist air a route out, not to drill holes at random. A single small grille on a large slope does very little, and vents placed too low on a roof can short-circuit the airflow, drawing air in and out of the same area. We would rather work out where the air should enter and leave, then choose a ridge system or tile vents to suit. Our guide on roof tile vents covers the options on a tiled roof.
On the coast, whatever is chosen has to survive a Channel gale, so the fixings and end caps of a vented dry ridge matter as much as the vent. Our wind guide on tile fixing in coastal wind explains how ridges are held down.
Signs that the loft is a condensation problem
A torch and a cold morning are enough to learn plenty from the loft, provided you keep your weight on boards or joists and step on nothing else. The signs tend to cluster.
Droplets or a fine frost on the underside of the underlay or on the nail tips poking through the felt are the most obvious. Dark spotting on the rafters and battens, or a musty smell, points to repeated wetting. Insulation that is damp or compressed at the eaves, where the airflow is meant to pass, suggests the gap is blocked. A ceiling stain that appears in cold weather, with no link to rain, is another clue. A bathroom or kitchen directly below the stained area is worth noting too.
Leak or condensation: the pattern
A leak leaves a trail. There is a line of tide marks on the timbers running down from a point of entry, and it follows heavy or wind-driven rain. Condensation spreads across a wide area and is worst after long cold, still nights, and the wet is spread evenly instead of following one line. Often both exist together: a small leak at a flashing, plus damp that comes from poor airflow. Treating only one leaves the stain coming back, which is why we prefer to rule each cause in or out before pricing anything.
One thing we say clearly. Condensation can rot battens as surely as a leak can. If timber is already soft, ventilating the roof will not bring it back, and the affected lengths need replacing. Our guide to rotten roof battens shows what that involves.
Fixing it, and what we check on a Sussex roof
The first step is usually to reduce the moisture at its source. An extractor fan that vents outside, not into the loft, a bathroom door kept shut when showering, and a tumble dryer that is ducted out make a real difference. None of that costs much, and it takes load off the roof.
The second is to open up the airflow already built into the roof. Clear the eaves, fit baffles so the insulation sits back from the eaves line, add vent strips where soffits are solid, and look at the high-level openings. Often that is all the house needs.
When the underlay itself is the issue, the answer depends on its condition. Perished or torn felt over a roof that is otherwise sound can sometimes be dealt with in sections, but if the covering has to come off anyway, replacing the underlay with a breathable membrane at that point is the practical moment. A full recovering is a bigger decision, and we only recommend it when the repair does not properly solve the fault.
A word on timing and consent
Changing the underlay without stripping the covering is not usually possible, so any underlay replacement tends to come with a re-roof of that slope. The Planning Portal says a building control application is not normally required when less than 25 per cent of the building envelope and less than 50 per cent of the roof is affected. Above those limits, approval applies, and an upgrade to the insulation may follow. We explain which applies and handle the notification where it is needed.
Older village houses with clay peg-tile roofs often have their own airflow and need gentler handling. Our autumn roof check is the quickest way to catch a blocked eave before winter, and the full range of pitched roofing work is set out on its own page.
