Misted glass or leaking flashing: telling them apart

Two different faults that get confused

A sealed unit is made of panes held apart by a spacer, with a bonded edge seal that keeps the air between them dry. When that seal gives up, damp air gets in, and the gap fogs. Nothing is wrong with the roof. The window has simply lost the airtight sandwich it was built as.

A leak is a different thing. Rain, melted snow or wind-driven spray finds its way through a joint where the rooflight meets the roof. The flashing is the metal or lead kit that makes that joint. The fault can also be in the frame, the seals round the sash or the roof covering beside the window. The glass may be perfectly clear, and still the ceiling stains.

Because the two can occur together, it helps to look at where the water is and when it appears, not just what you can see through the pane.

Reading misting

Misting has a recognisable look. The cloudiness or droplets sit between the panes, so wiping the glass on either side changes nothing. It often starts as a faint haze at one corner or along the bottom edge, and it comes and goes with the weather: clearer on a dry, warm afternoon and heavier on a damp morning. In time it can leave a white mineral film on the inner glass faces.

Other signs:

• The marks are inside the unit, not on a surface you can reach.
• The ceiling and walls near the window are dry, with no stains or peeling.
• The frame feels solid and the window opens and closes normally.
• It shows at the glass edge first, where the seal is stressed by sun and frost.

The cause, the common patterns by age and how to tell it from ordinary condensation are in our answer on why a rooflight goes misty between the panes. The cure is a new sealed unit, or a new rooflight if the frame is tired, and that is what we price.

Reading a leak

A leak announces itself differently. The signs are in the room around the window, and they follow the rain.

A stain on the plaster. A brown ring, a bubbling patch or a hairline of damp that deepens after a storm. The shape often points to the side of the window where water enters.

Water that runs after heavy rain. If it appears during or just after downpours and stops in dry spells, rainwater is the source. If it appears on cold mornings with no rain, we are looking at something else.

Drips at the corners of the frame. The corners of the sash and the bottom of the lining are where flashing failures tend to show.

Rot or softness in the timber. Wood that gives under a fingernail, or paint that lifts in blisters, points to long exposure to water.

Damp in the loft or the roof void. Wet insulation or dark stains on the rafters beside the window mean water has been tracking down the roof structure.

Not every leak is a flashing fault. A cracked slate or tile beside the window, a failed seal at the top of a flat rooflight kerb or a blocked gutter above a lantern can all send water into the same place. So the survey takes in the roof around the window too, and the loft side where we can reach it.

The one that fools people: condensation

A third thing produces damp near a skylight and is neither misting nor a leak. Warm, moist room air meets cold glass or a cold frame, and water forms on the room side. A bathroom, a kitchen or a loft bedroom full of people breathing overnight can all do it.

The clue is where the water sits. Condensation collects on the surface you can wipe, on the lower edge of the glass and on the frame. It leaves the sealed gap clear. It tends to appear on cold mornings with no rain in sight, and it clears as the room warms. If you wipe it away and it returns the next cold night, it is condensation.

Repeated condensation can darken the timber lining and lift paint, which looks like a leak. We go through the causes, and the roles of insulation round the frame and ventilation in the room, in our answer on why a roof window gets condensation.

A way to tell them apart in ten minutes

You do not need tools for a first read. A torch, a phone camera and a dry cloth are enough to sort most cases into misting, condensation or a leak before anyone visits.

To do your own check, pick a dry morning and then a wet afternoon. Look at the glass from inside with a torch held at an angle. Photograph any marks with the date, and note the weather. Those few notes tell us more than a general description does. If you can safely look into the loft beside the window, check the insulation and the timber for damp, and tell us what you find. Never climb onto the roof to look; the survey does that safely.

When it is both

A failed unit and a leak can sit in the same window, and sometimes one causes the other. Water running down the glass can attack the edge seal. A leak at the flashing can keep the frame damp, which stresses the rebate where the unit sits. A window that has had years of both is rarely a good candidate for a glass-only swap.

When we find both, the question becomes whether the frame and flashing are worth keeping. A sound frame with a failed seal is a candidate for a new unit, as in our page on swapping the glass alone. A frame with soft timber and a lifted flashing needs a new rooflight, fitted with a new flashing kit and a proper junction to the roof covering.

We say which at the survey, and the quote sets out both the narrow job and the full one, so you can see what each buys.

What we do and what the quote includes

If the survey finds a failed sealed unit in a sound frame, we quote a new unit. If it finds a frame past saving, or a flashing that has failed with the window, we quote a replacement rooflight with its own flashing kit, bedded and lapped to the roof covering. A new window brings a new, matched flashing, which is why a leaking unit that is also old tends to be better replaced whole.

Every quote is fixed, in writing and carries a life plan. For a replacement it names the unit, names each part that will need replacing and the route for doing it, including whether the roof is disturbed. On a pitched roof window whose sash lifts out, a future glass swap can be done from inside, which is a point in that model's favour.

The workmanship under our 10-year guarantee shows in the joint at the roof. The flashing laps the covering the right way, the sealant is tooled neatly, and the gaps are closed to the manufacturer's detail. You can see all of that from a ladder, and we are happy for you to look at it on completion.

A new window is controlled work, so the Building Control notification is ours, sent to the council or a registered building control approver. Hold on to the record: a later buyer's solicitor may ask for it, and we file it with your quote.

Why a quick fix to the wrong thing costs more

It is tempting to treat what you can see. A mist on the glass gets a new unit, and the ceiling stain is painted over. If the stain is a leak, the new unit sits in the same wet frame and the mist returns in time. The reverse also happens: a flashing is redone while the sealed unit goes on fogging.

Get the order right and the job is smaller. Find the water first, then decide on the glass. At the survey we check the flashing, the sash seals, the frame timber and the condition of the covering round the window, and we look at the loft side where we can reach it. After that the quote names one fault or the other, or both, and it says what we propose for each.

Because skylights round Sheffield sit on stone, slate and tile roofs of many different ages, we also look at how the old flashing was fitted. A window that was put in with a flashing kit meant for another covering is a leak waiting to happen, and a replacement is the way to correct that.