Most people picture roof damp as water coming in from outside. A good share of the damp we find in lofts around Basingstoke arrives from the inside instead. Showers, cooking, drying clothes and breathing all put water vapour into the warm air of a house, and that air rises. If it reaches a cold loft with nowhere to go, the vapour condenses on the first cold surface it meets: the underside of the felt, the rafters, the ridge board. A vented ridge is one of the ways to give that moist air an exit. This guide covers how it works, what the options look like, and why venting a ridge is only part of the answer.
How condensation builds up in a roof space
Warm air holds more water vapour than cold air. When warm, humid air from the rooms below leaks up into a loft, it cools quickly, and the water it was carrying has to go somewhere. On a cold night it settles as droplets on the coldest surfaces: nails that poke through the felt, the felt itself, and the timber nearest the ridge. In the morning the loft can feel clammy even though no rain has fallen.
A little condensation that dries out each day does no harm. The problem is a roof that stays damp for weeks. Timber that is wet for long enough can grow mould, and in time it can rot. Insulation that has soaked up moisture loses most of its value. You may see black spotting on the rafters, a musty smell, water staining around the loft hatch, or drips onto the insulation on a cold morning. Those are the signs to take seriously.
It helps to separate three sources of loft damp, because the fix is different for each. Condensation comes from moist indoor air. A leak comes from rain getting through a gap, a slipped tile or failing lead. Rising moisture around a chimney stack comes from a defective flashing or a saturated brick. Our survey page on investigating damp and condensation in roofs shows how we tell them apart, and the homeowner page on condensation in the loft covers the everyday causes.
What a ridge vent does
Air moves through a roof space by two forces. Warm air rises, so air leaving at the top draws replacement air in at the bottom. Wind adds to that effect: moving air across a roof creates suction over the ridge and pressure at the eaves. A well-ventilated pitched roof lets cool air enter at the eaves, pass through the loft and leave at or near the ridge, carrying moisture with it.
A ridge vent is the outlet at the top of that path. It can take three forms.
Vented ridge tiles
These are ridge tiles with a built-in grille or channel, made by tile manufacturers to match their ranges. They sit on a mortar bed or a dry-fix batten like any other ridge tile, with an opening beneath that links the roof space to the outside. They keep the look of a traditional ridge and suit most conventional pitched roofs.
Vented dry ridge systems
Many dry ridge kits include a ventilating strip along the whole length of the ridge. A continuous batten is fixed, and a profiled, breathable roll or strip is laid so that air can pass out while rain and insects cannot get in. Because the opening runs the full length, the airflow is spread evenly along the roof. How a dry system goes together is in our page on how a dry ridge system is fitted, and the standard it is tested to is covered in our BS 5534 and BS 8612 guide.
Ridge vents on a ventilated cold roof
On some modern builds and roofs that have been re-laid, a ventilation gap is formed at the ridge as part of the whole design. This is a specified detail, designed together with eaves ventilation and the insulation, and not an afterthought. It works only if the rest of the roof lets air travel from one end to the other.
Where a ridge vent will not help
A vent at the ridge is not a cure on its own, and there are a few cases where it does little or the wrong thing.
The first is a loft with blocked eaves. If the air cannot get in at the bottom, an outlet at the top has nothing to draw. Loft insulation pushed hard into the eaves, a soffit with no vents, or blocked insulation baffles all choke the flow. We often look first at the eaves when a ridge is being asked to do all the work.
The second is a roof with a breathable underlay. Modern breathable membranes let vapour through while keeping water out, and a roof built with one may need less ventilation than a roof with old bituminous felt. The details differ between manufacturers, so the roof's specification matters, and we do not add vents on guesswork.
The third is a roof whose insulation has been sprayed directly onto the underside of the tiles or felt. In that design the loft becomes part of the warm envelope and the roof is no longer a vented cold roof at all. Spray foam can reduce roof-space ventilation and, according to RICS guidance, can hide damp timber behind the foam. A ridge vent bolted onto that kind of roof can make matters worse, because the vent opens into foam. If your loft has been foamed, see our spray foam insulation page, and read the RICS consumer guide before you decide what to do.
The fourth is a moisture source that is simply too big. A bathroom fan that vents into the loft, a tumble dryer with no outside duct, or a wet room with no extraction will out-pace any ridge vent. The better fix is at the source: ducting extractor fans to the outside and sealing gaps around the loft hatch and light fittings.
Condensation under dry ridge and tile systems
Owners who have had a dry ridge fitted sometimes ask whether it made their loft damper or drier. The answer depends on the product and on what was there before. A mortar-bedded ridge is not airtight: small gaps between tile and bed already leak air, and a few rooms in older houses depend on that accidental ventilation. Replace the mortar with a sealed dry system that has no vent, and you may reduce the airflow. Replace it with a vented dry system, and you can improve it.
That is why we check how the old ridge breathed before we change it. A pitched roof of the type built in many Basingstoke estates in the 1960s and 1970s usually has plain eaves, bituminous felt and an unventilated ridge, so condensation tends to show along the north slope in January. Adding a vented ridge, opening the eaves and keeping the insulation back from the soffit often turns the loft round without any other work.
| Symptom in the loft | Likely cause | What usually helps |
|---|---|---|
| Droplets on felt in cold weather | Moist air with no outlet | Vented ridge plus clear eaves |
| Black spotting on rafters near the ridge | Long-term condensation | Treat timber, improve airflow, stop moist air entering |
| Wet insulation at the eaves only | Blocked eaves or a leak | Clear the vents, check the tiles and felt |
| Damp only after rain, in one spot | A leak, not condensation | Trace and repair the entry point |
| Clammy loft after foam insulation | Loss of roof-space ventilation | Specialist assessment of the foam and timbers |
Fitting or changing a ridge vent
If a vented system is the right answer, the work is usually done at the same time as a ridge repair, because the tiles are already coming off. We lift the old ridge, check the ridge board and the felt underneath, and fit vented ridge tiles or a vented dry system in their place. The opening has to be positioned so wind-driven rain cannot be blown into the loft, and the ends of each run must be capped or closed so the air is drawn through the length of the roof and not around the ends.
The condition of the timber under the ridge matters. If we find a ridge board that has stayed damp for years, we say so on the survey and photograph it. Light surface mould can be treated and the cause removed. Timber that has lost its strength needs replacing before anything is fixed back on top. A vent over rotten wood is not a repair.
Fitting a vented ridge in a new covering sits within the usual rules for re-roofing. Small repairs and re-coverings that affect less than a quarter of the building envelope and less than half of the roof do not usually call for building control approval, according to the Planning Portal. Larger jobs do, and the roof's ventilation is then looked at alongside its insulation. Where a job is notifiable, we deal with the paperwork.
What we check on your roof
A free survey covers more than the ridge itself. We look at the loft from the hatch and, where we can safely, along the length of the roof, noting the colour of the timber, the state of the felt, the insulation and whether it is blocking the eaves. We look at the eaves from outside, at the ridge from the roof, and at the bathroom and kitchen extraction below. Photographs go into your report so you can see what we saw.
If a vented ridge is the right step, the fixed written quote sets it out as a line of its own. If the real fix is clearing the eaves or ducting a fan, we say that, because it is cheaper and works better. The same photographs can support a conversation with your building control body or a surveyor. To see how this fits into our wider work on ridges, hips and verges, visit ridge tile repairs in Basingstoke. The survey is free, and booking one takes a phone call to 01256 213956 or a WhatsApp message.
A note on older lofts and cold-weather mornings
If you only notice the problem in January, that is itself a clue. Condensation is worst when the outside air is cold and the house is warm and full of vapour, so a loft that is dry in summer and dripping in winter points to ventilation and moisture control, not to a hole in the roof. Take a torch up on a cold, still morning and look along the felt: droplets hanging from nail points are a sure sign of moist air reaching cold surfaces. Take a photo and send it to us, and it will help us decide what to look for on the day.
