A leaking Velux window is one of the most misdiagnosed roofing problems we encounter in Inverness. The leak appears at the window frame and everyone assumes it's the glass seal — but in the vast majority of cases we inspect, the actual failure is the flashing collar. Here's how to tell the difference.
When a Velux window lets water in, there are two distinct failure points — and they have completely different causes and fixes:
The EDW and BDX flashing systems used on most Velux installations involve a lead or aluminium collar dressed around the frame and overlapping the surrounding tiles. This collar expands and contracts with temperature — and in Inverness, the annual temperature differential is one of the widest in the UK. A roof surface can reach 50°C+ in direct July sun and drop to -10°C in a January frost. That's a 60°C swing, repeated every year.
Over 15–25 years of this cycling, the collar gradually works loose from the tile course above it, and from any mortar or sealant used at the frame junction. Once it lifts even marginally, Highland-driven rain — which is wind-driven and penetrating rather than simply falling vertically — tracks underneath and into the roof space.
Flashing collar failure: the leak appears directly at or above the window frame during or after rain, often tracking down the wall below the window. The gap between the frame and the tiles above may be visible from outside if you can get close enough. Water tends to appear in the loft rather than just inside the room.
Glass seal failure: condensation builds up between the glass panes (double-glazed units), or water enters specifically at the glass edge in a consistent location regardless of wind direction. The window may feel draughty at the seal line.
We frequently take over from other roofers who have attempted to fix Velux leaks with external silicone sealant applied around the frame. This is a patch, not a repair. Silicone applied over a failed flashing collar holds for one or two Highland winters before the thermal cycling cracks or separates it. The underlying problem — an incorrectly seated or failed flashing system — remains.
The correct repair is to remove the tiles around the window, strip the existing flashing kit, and install a new Velux-specification EDW or equivalent flashing collar correctly dressed to the roof pitch and tile profile. This is a half-day job that we carry the standard kit sizes to complete in a single visit for most common window sizes.
If your Velux has been patched and is still leaking, call us for a correct diagnosis. We carry standard flashing kits for same-day repair on most common window sizes.
Call 01463 262835After years of Velux work across Inverness and the Highlands, most call-outs fall into one of three categories. Understanding which applies to your window saves time and avoids paying for the wrong fix.
Category 1 — Flashing collar failure (most common). The EDW or BDX flashing kit around the window frame has lifted, cracked or separated. Water enters from above the window frame and tracks down. This accounts for approximately 70% of the Velux repair calls we receive. The fix is full flashing kit replacement — typically a half-day job at £200–£450 depending on window size.
Category 2 — Glass unit seal failure. The double-glazed unit has failed — misting between panes indicates the hermetic seal is gone. This is a glass replacement, not a roofing job. Velux supply replacement glass units for most current and legacy models. Cost is typically £150–£350 for the glass unit plus fitting, depending on model and age.
Category 3 — Condensation and ventilation. Heavy condensation on the window interior — not between the panes — indicates inadequate roof space ventilation rather than a flashing or glass failure. The loft is too damp, and the Velux glass is the coldest surface where condensation deposits. This requires ventilation improvement, not window repair. We assess ventilation cross-flow as part of every Velux survey and flag this if it applies.
The manufacturers' design life for Velux flashing systems is typically quoted as 20–25 years under normal UK conditions. In Highland conditions, we see first failures at 15–20 years — roughly 20–25% earlier than the design life. The reason is the thermal cycle.
A roof surface in Inverness in July can reach 50°C in direct sun. The same surface in January frost can reach -12°C. The lead or aluminium flashing collar cycles through this 60°C differential every year — expanding and contracting as the temperature moves. Over 15–20 years of this cycling, the bonding between the flashing collar and the surrounding tiles or the window frame begins to fail.
Installations where the original flashing was not correctly dressed to the roof pitch — a common installation error where the collar is not properly angled to shed water — fail faster because water is actively sitting against the joint rather than being directed away. We see this on a significant proportion of the failed flashings we replace, particularly on roofs installed in the 1990s and early 2000s when quality control on Velux installation was less consistent.
Velux produces flashing kits to different specifications depending on the roof type, pitch and tile profile. The standard EDW kit is appropriate for most Inverness pitches. For low-pitch roofs (15–25 degrees), the EFL or equivalent low-pitch kit provides better drainage geometry. For flat-to-pitched roof conversions, the EKW or equivalent kerb-mounted system is required.
We carry the most common EDW sizes in stock for same-day installation on most standard Velux window sizes. Less common sizes and Fakro, Keylite or Roto equivalents are ordered on survey and installed within 3–5 working days typically.
One upgrade worth considering on any Highland Velux re-flashing: the manufacturer's extended lead collar option where available. The standard kit uses an aluminium collar in many configurations. Lead, where it can be specified, handles the Highland thermal cycling better and has a longer service life at this latitude. We will advise on this where it is a viable option for your window configuration.
A slow Velux leak that is ignored or repeatedly patched causes progressive damage to the roof deck beneath the window. Water tracking down from a failed flashing collar saturates the batten and sarking immediately above the window, causes softening and eventual rot in the timber, and in severe cases tracks along joists into the ceiling below. By the time a homeowner presents with a serious Velux leak, the damage is almost always more extensive than the window itself.
The repair cost scales with how long the leak has been running. A flashing replacement carried out when leaking is first noticed: £200–£450. A flashing replacement plus batten and sarking repair after 3–5 years of slow leak: £500–£900. A flashing replacement plus structural timber repair after sustained water damage: £800–£1,800+. We make this clear not to alarm, but because the economics of prompt repair versus delayed repair are significant.
"Two previous roofers had patched the flashing with silicone. Both failed within a winter. This article explained exactly why. New flashing kit fitted correctly — dry ever since."
"Thought it was the glass seal because that's where I could see the water. This article made me reconsider. Was the flashing all along. Saved me buying a new window unit unnecessarily."
"Cradlehall property, 1997 build. Velux had been weeping for two years. Thermal cycling point in this article resonated — we're up on the plateau and get wide temperature swings. Correct diagnosis, clean repair."
Almost always because the repair addressed the symptom (sealing the visible gap) rather than the cause (a failed flashing collar). Silicone patches on failed flashings last 1–2 Highland winters before failing again.
A full Velux flashing collar replacement costs £200–£450 depending on window size and roof pitch. We carry standard kit sizes for same-day repair. Call 01463 262835.
Misting between double-glazed panes indicates failed unit seal — the glass unit needs replacing, not the flashing. We can advise whether this is a Velux warranty issue or a standard repair.
From outside (safely from the eaves or a window), look for a visible gap between the flashing collar and the tile course above. From inside the loft, look for water staining on the timber above and to the sides of the window frame. In heavy rain, you may hear water movement in the roof space above the window before it becomes an interior drip.
Velux product warranties cover manufacturing defects but not installation failures or thermal-cycling degradation, which is the primary cause of Highland flashing failures. If your Velux is less than 5 years old and the flashing has failed, it may indicate an installation error that we can assess and potentially claim through the original installer.
We do not recommend DIY Velux installation in Highland conditions. Correct flashing installation requires understanding the pitch-specific collar geometry, tile profile interaction and flashing code specification. Incorrectly fitted flashings typically fail within 2–5 Highland winters. Professional installation with a guarantee is significantly better value over the service life.
For standard window sizes where we carry stock, typically 2–3 windows per day. For less common sizes requiring ordered kits, we schedule a survey visit and an installation visit separately. We will confirm the timeline at survey based on your specific window models.
No call-out fee. No obligation. A straight answer from a local roofer.
01463 262835