Question answered
Is it condensation or a roof leak?
The short answer
A roof leak is tied to rain: the damp appears or worsens during or after a shower and sits beneath a specific fault. Condensation follows cold, still weather and steamy rooms, shows as beads of water, black spotting and damp over a wide area, and often runs along the underside of the roof. Leaks are mended on the roof. Condensation is cured by ventilation and by keeping moist air out of the loft.
Damp in a loft or on a ceiling has two common causes that look alike and need different cures. The pattern of the damp, and when it appears, usually settles which one you have.
Read the timing first
Timing is the quickest clue. A leak answers to rain. It drips during a downpour, or a day later as water works through the insulation, and it eases soon after the weather clears. Condensation keeps a different calendar. It builds on cold mornings, in still damp weather and in the evenings after cooking or showers, and it can be at its worst when the sky is dry.
Keep a simple diary for a fortnight. Record the weather, the wind, whether the heating was on and where the damp appeared. Wet patches that match rainfall point to the roof; patches that match cold nights and steamy rooms point to the air inside the house. A mixture of both is common, which is why the cause is confirmed on site and not guessed from the landing. Take the diary with you to any survey; it saves time and avoids assumptions.
Read the pattern of the damp
A leak is local. It marks one patch or one line beneath a flashing, valley, gap or cracked tile, and the timbers around it carry a dark tide mark that stops where the water dried. If the same patch returns after a repair, a roof still leaking after repair explains the next step.
Condensation spreads. Look for droplets hanging from the felt or boards, rusty nail tips, black spotting on rafters and damp across a whole slope, often the colder one. The insulation may be wet on top and dry beneath. Mould in the corner of a bedroom ceiling, away from any wall that meets the roof, usually means moist air in the house and not water from outside. A stain can be both at once, so note where the marks sit in relation to the likely way in before you decide.
Why a loft gets damp from inside
Every household makes moisture. Cooking, washing, drying clothes indoors and even breathing put water vapour into the air, and warm air rises through light fittings, hatches and gaps around pipes into the loft. There it meets cold felt and timber, cools and drops its water. Insulating the loft floor makes the roof space colder, which strengthens the effect when that damp air is trapped.
The cure combines two things: stopping vapour reaching the loft, and letting the roof space breathe. Draught-proof the hatch, send extractor fans out through the wall or roof and not into the loft, and keep insulation from being packed tight at the eaves, where it chokes the air path. If the insulation is already soaked, wet loft insulation covers what to do about it.
What loft ventilation should provide
Marley, summarising BS 5250 for a cold roof, gives ventilation figures that depend on the underlay. With a high-resistance underlay it describes a 25 mm continuous opening at the eaves where the pitch is 15° or less, and 10 mm where it is steeper, plus a further 5 mm near the ridge when the pitch exceeds 35°, the span exceeds 10 m, or the roof leans against a wall or has a single slope.
For a low-resistance underlay laid over a well-sealed ceiling, beneath a covering that lets air through, Marley says the 3 mm gap left by normal eaves construction can be enough, provided the third-party certification conditions are satisfied. Which case applies depends on what is actually in your roof, so it is checked on site and never assumed. Eaves vents, tile vents and ridge vents are the usual tools. The upkeep side is set out in how to make a roof last longer.
How we tell on site
Our first stop is the loft, where a torch and a damp meter show what the timbers are doing, and then we examine the outside for likely points of entry. If the tide marks sit under a lifted flashing, a slipped tile or a split valley and the wood is wettest there, it is a leak. If the felt is beaded across the whole slope, the nails are rusting and the bathroom extractor ends in the loft, it is condensation, and a patch will not cure it.
You are shown pictures of what is found on your own roof, with a written price to follow the visit. If both problems are present, the leak is closed first and the ventilation second, so you do not dry out a loft that is still taking water. The wider range of faults we trace is on our roof repairs page.
Damp in the loft: two more points
Can poor ventilation after loft insulation cause condensation?
Yes. Extra insulation keeps the roof space colder, and if it blocks the eaves or the house is not sealed from the loft, moist air condenses on the cold felt and timber. Keep insulation clear of the eaves so air can pass, and have ventilation checked if damp appears after the work.
Is black mould on a bedroom ceiling a roof leak?
Not necessarily. Mould in a corner or along an outside wall is often condensation from moist indoor air meeting a cold surface. A roof leak tends to leave a brown, defined stain that grows after rain. Check the loft above the room for tide marks before deciding which one it is.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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