The short answer
Condensation forms when moist air escaping from the rooms below meets cold roof timbers or underlay and turns to water. The usual causes are blocked eaves ventilation, a loft hatch that lets house air up, and a roof lined with non-breathable underlay that has no airflow beneath it. It can look like a leak, so check the pattern of the damp before blaming the tiles.
Where the moisture comes from
A household puts water vapour into its air every day, through kettles, showers, washing lines indoors and breath. Warm air holds more of that vapour than cold air, and it rises. Once it finds a path into the loft, it reaches surfaces that are cold, namely the underlay and the rafters, and the vapour condenses into drops.
Under Approved Document C, a roof has to be built so that moisture condensing inside it, called interstitial condensation, does no damage to the roof's structure or warmth. In paragraph 6.12 it calls for a proper draught seal around a loft hatch, since a loose hatch lets house air climb straight into the roof space.
Gaps around pipes, light fittings cut into the ceiling and the edges of the ceiling board are others. Each one lets a little warm air through, and in a cold spell the result is visible: frost or beads of water on the nails that come through the felt, and damp patches on the rafters.
Extra insulation without extra ventilation can make matters worse, because deeper insulation leaves the loft colder and condensation settles more readily on its timbers.
Blocked eaves and the wrong underlay
A cold roof is meant to breathe. Air enters at the eaves, passes through the roof space and carries moisture away. Marley's guidance on cold roof ventilation, based on BS 5250, says that with a non-breathable HR underlay the eaves need ventilation of 25mm along their length for pitches of 15 degrees or less, and 10mm for steeper pitches.
Those gaps get blocked more easily than people think. Quilt insulation rammed into the eaves, bird-proofing fitted without a vent, and paint or debris in the soffit slot all choke the airflow. Without air moving, moisture sits.
HR stands for high resistance. An HR underlay is not breathable, so it traps vapour beneath it, which is why the eaves ventilation matters so much. A low resistance, or LR, underlay is breathable and lets vapour out through the membrane. Marley adds that a breathable LR underlay over a tightly sealed ceiling may depend on the 3mm of incidental air movement at normal eaves, if the product's own third-party certification allows it.
Our page on whether a loft needs ventilation sets out the arrangements in more detail, and the one on breathable roof membranes covers the underlay side.
Condensation or a leak?
The two look alike at first, but their habits differ.
- Condensation is widespread. It covers the underside of the roof, the nail tips and several rafters at once, and tends to appear in cold, damp weather whether or not it has rained.
- A leak is local. It marks a single patch, often below a damaged tile, a valley or a flashing, and appears after rain.
- Condensation drips or runs from many points. A leak usually follows one path.
- Condensation often comes with mould or dark spotting on the timber and a musty smell. A leak may leave a clear tide-mark.
It is possible to have both. A roof with a cracked tile and poor ventilation can show a local wet patch against a general damp background.
Photograph the loft in the morning on a cold day, and again after rain. Note where the damp is and how it changes. That tells us a great deal before we climb a ladder. If you suspect a leak, our page on rotten eaves felt covers one cause that can look like either.
Do not seal the loft airtight in an effort to stop the damp. That makes condensation worse, not better.
Fixes that work
The cure follows the cause, and often costs little.
- Fit a draught seal to the loft hatch and keep it shut, as AD C paragraph 6.12 recommends.
- Seal gaps where pipes, cables and light fittings pass through the ceiling.
- Clear the eaves, so insulation stops short of the vent path and nothing blocks the soffit slot. Eaves vents or trays can hold insulation back.
- Run extractor fans in kitchens and bathrooms to the outside, never into the loft.
- Add ventilation at the eaves, and at the ridge if the roof design calls for it, where the underlay is HR.
- Choose a breathable underlay in place of a failing HR one if the roof is being stripped and re-covered anyway.
Our survey checks the loft, the eaves, the underlay and the ceiling gaps, and photographs each one. You see the findings, then hear which fix is proportionate. A repair or a vent is fine where that solves it, and a new underlay is raised only when the roof is being stripped.
The quote is one total. The pitched roofing hub lists the other repairs we carry out around Farnborough, Camberley and Fleet.
