A leaking Velux window is one of the most common roofing complaints we receive from Northampton homeowners — and one of the most frequently misdiagnosed. The standard response of applying sealant around the frame rarely resolves the underlying problem, which is almost always in the flashing kit rather than the window itself. This guide explains the most common causes of Velux leaks on Northampton properties and what a proper fix looks like.
Most Velux problems in Northampton - leaks, misting, draughts - trace to a failed flashing kit, failed sealed unit or degraded weatherstrip, all repairable without replacing the whole window. Leaks usually mean the wrong flashing kit was used for the tile profile. Flashing replacement costs £250-£500.
The Most Common Cause: Wrong Flashing Kit
The flashing kit is the lead and aluminium collar that seals the junction between the Velux window frame and the surrounding tile surface. Every Velux model requires a specific flashing kit matched to the tile profile — the EDW kit for plain tiles and small interlocking profiles, the BDX kit for larger interlocking tiles, the EDJ for profiled tiles, and so on.
Installing a Velux with the wrong flashing kit for the tile profile is the single most common cause of Velux leaks on Northampton properties. The mismatch means the flashing collar does not sit flush against the tile surface — gaps exist where water can track in under the collar and find the junction between the frame and the roof structure.
This problem is particularly common on the 1930s clay plain tile roofs in Kingsthorpe, Abington and Spinney Hill. Clay plain tile has a specific camber and overlap dimension that requires the correct EDW or BDX specification. A Velux installed with a flashing kit designed for concrete interlocking tile will sit incorrectly on the clay surface and produce a leak at the first significant rainfall.
The fix for a wrong flashing kit is to replace the flashing kit with the correct specification — not to sealant over the gap. A correctly specified replacement flashing kit for a standard Velux on a clay plain tile roof in Northampton costs £250 to £500 installed and will last the remaining life of the window.
Failed Sealed Unit
The sealed unit is the double-glazed pane within the Velux frame. When the hermetic seal between the two panes of glass fails, moisture enters the air gap and produces the characteristic fogging or condensation visible between the glass panes.
This is not a roof leak — no water is entering the building from outside. However, it is commonly confused with one, particularly if the condensation is heavy enough to produce drips on the inner sill. Failed sealed units are the second most frequent Velux problem we are called to diagnose on Northampton properties.
The fix is sealed unit replacement — removing the pane from the frame and installing a new sealed unit. This is an independent repair from the frame and flashing, costs £200 to £400 depending on size, and does not require scaffold access in most cases as it can be done from inside the loft. It restores the window to full function without any further intervention to the roof.
Deteriorated Weatherstrip
The weatherstrip is the rubber seal that runs around the perimeter of the Velux opening sash, compressing against the frame when the window is closed. Over time — typically 15 to 25 years — this rubber degrades, loses its elasticity, and no longer forms a complete seal. The result is water ingress at the frame perimeter during driving rain or heavy precipitation.
Weatherstrip failure is more common on older Velux installations across Northampton's 1990s and early 2000s housing than on more recent ones. Properties in Hunsbury, Sixfields and the later-developed estates in this area are beginning to see weatherstrip issues as their Velux installations age.
Weatherstrip is replaceable on most Velux models — the replacement part is available from Velux directly or through roofing suppliers and clips into the frame groove. Replacement costs £100 to £200 installed. A deteriorated weatherstrip that is producing leaks should be replaced promptly rather than monitored — the water ingress at the frame perimeter can track into the loft insulation and cause damp problems that are disproportionate to the size of the gap.
Frame Deterioration
On older Velux windows — those more than 20 to 25 years old — the frame itself can deteriorate to the point where repair is no longer viable and full window replacement is necessary. The most common causes of frame deterioration on Northampton properties are: prolonged water ingress from a failed flashing kit that has been left unaddressed; direct UV degradation on south-facing installations with no shade; and physical damage from impact or storm events.
Frame deterioration is the least common cause of Velux problems and the most expensive to address — full window replacement including new frame, sealed unit and flashing kit typically costs £600 to £1,200 depending on size and tile profile. However, it is often the consequence of a failed flashing kit or sealed unit that was not addressed promptly.
For Northampton properties with Velux installations from the 1990s that have never been serviced, a professional inspection is worthwhile. We can assess whether the flashing, sealed unit and weatherstrip are in adequate condition or whether any element requires attention. The cost of a repair to any one of these components is modest compared to the cost of frame replacement driven by neglected moisture ingress.
Free surveys across all NN postcodes. Call 0160 428 1189 or request a quote online.
Diagnosing a Rooflight Leak Properly
When a rooflight leaks, the instinct is to blame the window — but the water is usually getting in around it, not through it. The commonest culprit is the flashing kit where the unit meets the surrounding tiles or slates: if it was fitted incorrectly, sized for the wrong covering, or has aged and lifted, water tracks underneath. Condensation is another red herring — a rooflight in a bathroom or kitchen can drip with condensation that looks exactly like a leak but is a ventilation issue. Proper diagnosis means checking the flashing, the surrounding tiles, and the internal conditions before condemning the window. In most cases a re-flash restores watertightness at a fraction of the cost of a new unit. We only recommend replacing the rooflight itself when the frame or seals have genuinely failed — which, on a decent unit, is usually after 20 years or more.
The key thing to remember about a leaking rooflight is that the window itself is rarely the problem — it's almost always the flashing around it, which usually means a re-flash rather than a costly replacement. If you've got a Velux or skylight letting water in, or you're planning a new one and want it fitted so it stays watertight for the long term, a quick look will tell us whether it's a simple re-flash or a genuine unit failure. We'll give you the honest answer and the cheaper fix wherever the cheaper fix is the right one.
We cover rooflight and skylight work right across Northampton and the surrounding Northamptonshire towns — from the terraces of Semilong, St James and Far Cotton to the estates of Blackthorn, Duston and Kingsthorpe, and out to Wellingborough, Kettering, Daventry and the villages. Wherever you are, the approach is the same: an honest assessment first, a written itemised quote with no call-out fee, tidy work by a local team, and a 10-year workmanship guarantee. If anything in this guide matches what you’re seeing on your own roof, a quick free survey will give you a clear, no-pressure answer on what — if anything — needs doing.
Frequently Asked Questions
Last reviewed: July 2026