Condensation in the loft is not a roof leak

You have been in the loft and it is wet. There is moisture on the underside of the roof, maybe drips coming off the felt, possibly black mould on the timbers near the eaves. The insulation feels damp. Your first thought is that the roof is leaking.

Quite often it is not. Especially if all of the following are true:

• It is winter
• It was dry up there in summer
• The moisture is spread over a wide area rather than in one spot
• There is no obvious damage to the roof from outside
• The worst of it is at the edges, near the eaves, or on the north-facing slope

That pattern is condensation in loft spaces, and no roof repair fixes it, because there is nothing wrong with the roof.

Why it happens

Your house produces a surprising amount of water vapour. Showering, cooking, washing and drying clothes, breathing. A family of four puts something like ten to fifteen litres of moisture into the air every day.

Warm air holds more water than cold air. That warm, moist air rises, and it gets into the loft through gaps: the loft hatch, downlights, pipe and cable penetrations, gaps around the ceiling perimeter.

The loft is cold, because your insulation is doing its job and keeping the heat below it. When the warm moist air meets the cold underside of the roof, it cannot hold the water any more, and the water comes out as condensation on the coldest surface available.

That surface is usually the roofing felt or membrane, which is why the drips appear to be coming through the roof. They are not coming through it. They are forming on the inside of it.

How to tell it apart from a real leak

The definitive test costs nothing. Go up during a cold dry spell. No rain for several days, cold outside. If it is wet up there, it is condensation. Water cannot get through a roof that nothing is falling on.

The other test: leaks leave marks. Water running the same route repeatedly stains timber and leaves tide lines. Condensation forms evenly and tends to leave general darkening and mould rather than a track.

The four things that actually fix it

Condensation is a balance between how much moisture goes up there and how much air moves through. You fix it by reducing the first, increasing the second, or both.

1. Ventilate the loft

This is the main one, and on older houses it is usually the missing piece.

Air needs to enter low, at the eaves, and leave high, at the ridge. Modern roofs are built with that in mind. Older Leeds housing frequently is not, and it got away with it for a century because those roofs were draughty and had no insulation to speak of. Then somebody added 300mm of loft insulation, which is a good thing, and the roof space got properly cold for the first time, and the condensation started.

Practical fixes: soffit vents or a continuous vent strip at the eaves, over-fascia vents where there is no soffit to work with, tile or slate vents up near the ridge, or a ridge vent system if the ridge is being redone anyway. On a roof with no felt at all, which is common on unmodernised Victorian property, the roof breathes through the slates and condensation is rare. It is the ones with old bitumen felt that suffer most, because that felt is a vapour barrier with no way through it.

2. Stop the moist air getting up there

• Insulate and draughtproof the loft hatch. It is often the single biggest hole in your ceiling and it is a twenty pound fix.
• Seal around pipes, cables and downlights. Recessed spotlights in an upstairs ceiling are effectively small chimneys venting warm damp air into your loft.
• Extract at source. Bathroom and kitchen extractor fans, running long enough, vented outside. Not into the loft, which is a surprisingly common and genuinely damaging piece of DIY.
• Do not dry washing on radiators if you can avoid it, and if you must, open a window in that room.

3. Check the insulation is not blocking the eaves

This is the one I find most often, and it is free to fix.

Somebody has laid loft insulation right out to the edges, and stuffed it hard into the eaves. That blocks the exact gap the air was supposed to come in through. Pull it back, or fit eaves trays, which are plastic channels that hold the insulation clear and keep the airway open. Ten minutes and a few pounds.

4. Do not board over a damp loft

Boarding the loft for storage, laid straight onto the joists, compresses the insulation and blocks air movement across the loft floor. If you are boarding, use raised loft legs so there is a gap.

Where this goes wrong for people

The expensive version of this story goes: damp loft, ring a roofer, roofer replaces some slates, next winter the loft is damp again, ring a different roofer, gets told the roof needs replacing, spends several thousand pounds, and the following winter the loft is damp again. Because it was never the roof.

I would rather tell you that on the phone. It costs me a job and it is the reason most of my work comes from somebody passing my number on.

When it genuinely is the roof

To be fair to the other possibility, do call somebody if:

• The wet appears during or shortly after rain, every time
• It is in one identifiable place
• There is staining running down a rafter or a wall
• You can see daylight through the roof
• It is near the chimney, a valley, or where a roof meets a wall, which are all flashing failures rather than roof failures
• It is happening in summer

And there is a hybrid case worth knowing about. A roof with a genuine ventilation problem can also have a small leak, and the condensation masks it. If you have improved the ventilation, sealed the hatch, and it is still wet after rain specifically, get somebody up there.

Leeds specifics

The houses that suffer most round here are the ones that have been part-modernised: an old roof with 1960s or 1970s bitumen felt under the slates, then modern loft insulation added at some point in the last twenty years, and no ventilation added with it. That describes a very large number of terraces and semis through Headingley, Burley, Meanwood, Kirkstall and Chapel Allerton.

Loft conversions are the other one. Converting a loft means insulating between and under the rafters, and if that is done without a ventilation gap above the insulation, you get condensation in the roof structure itself, which is harder to spot and does more damage because it is happening inside a closed build-up.

The short version

If your loft is wet in winter and dry in summer, it is condensation, and ventilation plus stopping the moist air at source will fix it for a fraction of what a roof repair costs.

If it is wet whenever it rains, it is a leak, and that is worth a phone call.

Test it in a cold dry spell before you spend anything. That one visit up the ladder answers the question.