Why is there a dark stripe along the joists on my ceiling?

Ghosting is not mould and not a leak. It is dust and combustion particles depositing preferentially on the coldest lines of a surface, which happen to be exactly where the timber or the fixings sit.

What you'll need

• A torch
• A cloth and mild detergent, for a test patch
• A hygrometer
• An infrared thermometer, if you have one

Step by step

1. Check the pattern.Regular parallel stripes at 400 or 450mm centres, sometimes with dots between them marking the fixings. That regularity is the signature and rules out most other causes.
2. Rule out mould.Mould is patchy, often greenish or black with a speckled edge, and grows where it is damp. Ghosting is a uniform grey-brown deposit following straight lines.
3. Rule out a leak.A water stain has a defined tide mark and is usually yellowish or brown at the edge. Ghosting has no tide mark.
4. Work out where the cold is coming from.A ceiling below an uninsulated or poorly insulated loft, an external wall with cold bridging at the studs, or a room over an unheated garage or passageway.
5. Look at what is in the air.Candles, especially scented and paraffin ones, are by far the biggest domestic source. Open fires, oil lamps, cooking, smoking and a poorly maintained boiler all contribute.
6. Measure the humidity.Ghosting is much worse in a humid house, because damp particles are stickier and cold surfaces get a microscopic film of moisture. Above 60 percent in winter is a problem.
7. Address the cold bridge.Topping up loft insulation, and specifically making sure the insulation continues over the joists rather than only between them, is the durable fix for a ceiling.
8. Reduce the particle source.Cutting back on candles alone often stops it recurring. It is remarkable how directly the two correlate.
9. Improve ventilation.Extraction at source in kitchens and bathrooms, background ventilation elsewhere. Lower humidity means less deposition.
10. Clean carefully.A dry sponge or a mild detergent on a test patch first. Some emulsions mark easily and you can end up with a clean stripe instead of a dirty one.
11. Expect to redecorate.Once the cause is addressed, a stain-blocking primer and repaint deals with the marks. Painting without fixing the cause brings them back in a year or two.

Tips

• Ask yourself whether you burn candles. In practice it is the answer far more often than anything structural, and it is the cheapest thing to change.
• An infrared thermometer shows the cold lines clearly on a winter evening and confirms the diagnosis in seconds.
• Ghosting on a wall rather than a ceiling usually maps the studs of a poorly insulated timber frame or the mortar joints of a cold solid wall.

Common mistakes

• Treating it as mould with a fungicidal wash — It is a dust deposit, not a biological growth, so a mould treatment does nothing to the marks and nothing to the cause. Meanwhile the cold bridge and the particle source both continue.
• Painting over it without addressing the cold bridge or the candles — The deposition mechanism is unchanged, so the stripes reappear on the fresh paint. It usually takes a year or two, which is just long enough for people to blame the paint.

If it doesn't work

Questions people ask