Question 1

Why is my skylight leaking?

Accepted Answer

Most 'leaking' roof windows are condensing rather than leaking — glass is the coldest surface in the room and humid air runs off it onto the sill. Where water genuinely is getting in, it is nearly always the flashing kit rather than the window, and it typically shows at the top corners. The first thing to establish is whether water is coming in from outside or forming on the inside, because they look identical once there is a wet sill and a stained reveal. The tells for condensation are reliable. It appears on cold nights whether or not it is raining. It is worst in bathrooms, kitchens and bedrooms with the door shut. It forms on the glass itself and runs down, rather than arriving at the frame. And it improves noticeably the moment you start using the background ventilation flap. A genuine leak follows rain and typically shows at a corner or along the head. Where it is a real leak, the flashing is the culprit far more often than the unit. Manufacturers make covering-specific flashing kits — one for slate and thin flat coverings, another for profiled tiles, another for deep profiles — and using the wrong one, or dressing lead over the top because the right one was not ordered, is the commonest cause of a leaking roof window. The top corners fail first, because that is where the kit is taking water from the whole slope above and diverting it round the unit. The second commonest cause is the underlay. The roofing membrane has to be dressed and sealed around the opening to form a gutter above the window that takes water arriving from higher up and diverts it either side. Where that was cut and left, water running down under the covering goes straight into the opening — and no amount of work to the external flashing will stop it. Failed seals are the third, and they belong to older units. Gaskets harden with age and stop sealing, which lets driven rain past when the wind is in the right direction. On a unit of any real age that is usually the point where replacement makes more sense than repair — and a straight swap into the existing opening is far less work than people expect, because the major manufacturers have kept frame sizes stable for decades. One last check: make sure it is the window at all. Water travels along battens and rafters before it drops, so a stain near a roof window can easily originate from a valley or a flashing several metres away, with the window simply the nearest thing to blame. That is why the diagnosis starts in the loft rather than at the frame.

Question 2

Velux leaking at the top

Accepted Answer

Most 'leaking' roof windows are condensing rather than leaking — glass is the coldest surface in the room and humid air runs off it onto the sill. Where water genuinely is getting in, it is nearly always the flashing kit rather than the window, and it typically shows at the top corners. The first thing to establish is whether water is coming in from outside or forming on the inside, because they look identical once there is a wet sill and a stained reveal. The tells for condensation are reliable. It appears on cold nights whether or not it is raining. It is worst in bathrooms, kitchens and bedrooms with the door shut. It forms on the glass itself and runs down, rather than arriving at the frame. And it improves noticeably the moment you start using the background ventilation flap. A genuine leak follows rain and typically shows at a corner or along the head. Where it is a real leak, the flashing is the culprit far more often than the unit. Manufacturers make covering-specific flashing kits — one for slate and thin flat coverings, another for profiled tiles, another for deep profiles — and using the wrong one, or dressing lead over the top because the right one was not ordered, is the commonest cause of a leaking roof window. The top corners fail first, because that is where the kit is taking water from the whole slope above and diverting it round the unit. The second commonest cause is the underlay. The roofing membrane has to be dressed and sealed around the opening to form a gutter above the window that takes water arriving from higher up and diverts it either side. Where that was cut and left, water running down under the covering goes straight into the opening — and no amount of work to the external flashing will stop it. Failed seals are the third, and they belong to older units. Gaskets harden with age and stop sealing, which lets driven rain past when the wind is in the right direction. On a unit of any real age that is usually the point where replacement makes more sense than repair — and a straight swap into the existing opening is far less work than people expect, because the major manufacturers have kept frame sizes stable for decades. One last check: make sure it is the window at all. Water travels along battens and rafters before it drops, so a stain near a roof window can easily originate from a valley or a flashing several metres away, with the window simply the nearest thing to blame. That is why the diagnosis starts in the loft rather than at the frame.

Question 3

Water round my roof window

Accepted Answer

Most 'leaking' roof windows are condensing rather than leaking — glass is the coldest surface in the room and humid air runs off it onto the sill. Where water genuinely is getting in, it is nearly always the flashing kit rather than the window, and it typically shows at the top corners. The first thing to establish is whether water is coming in from outside or forming on the inside, because they look identical once there is a wet sill and a stained reveal. The tells for condensation are reliable. It appears on cold nights whether or not it is raining. It is worst in bathrooms, kitchens and bedrooms with the door shut. It forms on the glass itself and runs down, rather than arriving at the frame. And it improves noticeably the moment you start using the background ventilation flap. A genuine leak follows rain and typically shows at a corner or along the head. Where it is a real leak, the flashing is the culprit far more often than the unit. Manufacturers make covering-specific flashing kits — one for slate and thin flat coverings, another for profiled tiles, another for deep profiles — and using the wrong one, or dressing lead over the top because the right one was not ordered, is the commonest cause of a leaking roof window. The top corners fail first, because that is where the kit is taking water from the whole slope above and diverting it round the unit. The second commonest cause is the underlay. The roofing membrane has to be dressed and sealed around the opening to form a gutter above the window that takes water arriving from higher up and diverts it either side. Where that was cut and left, water running down under the covering goes straight into the opening — and no amount of work to the external flashing will stop it. Failed seals are the third, and they belong to older units. Gaskets harden with age and stop sealing, which lets driven rain past when the wind is in the right direction. On a unit of any real age that is usually the point where replacement makes more sense than repair — and a straight swap into the existing opening is far less work than people expect, because the major manufacturers have kept frame sizes stable for decades. One last check: make sure it is the window at all. Water travels along battens and rafters before it drops, so a stain near a roof window can easily originate from a valley or a flashing several metres away, with the window simply the nearest thing to blame. That is why the diagnosis starts in the loft rather than at the frame.

Question 4

Roof window leaking in the corners

Accepted Answer

Most 'leaking' roof windows are condensing rather than leaking — glass is the coldest surface in the room and humid air runs off it onto the sill. Where water genuinely is getting in, it is nearly always the flashing kit rather than the window, and it typically shows at the top corners. The first thing to establish is whether water is coming in from outside or forming on the inside, because they look identical once there is a wet sill and a stained reveal. The tells for condensation are reliable. It appears on cold nights whether or not it is raining. It is worst in bathrooms, kitchens and bedrooms with the door shut. It forms on the glass itself and runs down, rather than arriving at the frame. And it improves noticeably the moment you start using the background ventilation flap. A genuine leak follows rain and typically shows at a corner or along the head. Where it is a real leak, the flashing is the culprit far more often than the unit. Manufacturers make covering-specific flashing kits — one for slate and thin flat coverings, another for profiled tiles, another for deep profiles — and using the wrong one, or dressing lead over the top because the right one was not ordered, is the commonest cause of a leaking roof window. The top corners fail first, because that is where the kit is taking water from the whole slope above and diverting it round the unit. The second commonest cause is the underlay. The roofing membrane has to be dressed and sealed around the opening to form a gutter above the window that takes water arriving from higher up and diverts it either side. Where that was cut and left, water running down under the covering goes straight into the opening — and no amount of work to the external flashing will stop it. Failed seals are the third, and they belong to older units. Gaskets harden with age and stop sealing, which lets driven rain past when the wind is in the right direction. On a unit of any real age that is usually the point where replacement makes more sense than repair — and a straight swap into the existing opening is far less work than people expect, because the major manufacturers have kept frame sizes stable for decades. One last check: make sure it is the window at all. Water travels along battens and rafters before it drops, so a stain near a roof window can easily originate from a valley or a flashing several metres away, with the window simply the nearest thing to blame. That is why the diagnosis starts in the loft rather than at the frame.

Why is my skylight leaking?

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