Pitched Roof Windows (Velux-style): Size Codes, Flashing Kits and the BFX Collar Detail That Stops Leaks

Your roofer fits two Velux windows over a new vaulted kitchen ceiling. Everything looks perfect. Six weeks later, first heavy rain, water drips through the plaster reveal and stains the fresh paint. The tiles are fine. The flashing kit is fine. The fault is behind the plasterboard: the breathable roof membrane was cut as a simple rectangle around the window opening and the BFX underfelt collar wasn't lapped correctly under the membrane. Water tracks down the membrane, hits the cut edge, and runs inward instead of outward. Strip the tiles, strip the plaster, redo the collar. Two days of work and a claim on your roofer's warranty that Velux will refuse because the window wasn't installed to their instructions.

This failure is common enough that it appears in two-thirds of the community forum threads about Velux leaks. It's also entirely preventable if you know what to look for before the battens and tiles cover the membrane permanently. Pitched roof windows are one of the cheapest ways to bring light into an extension or loft conversion, but the install detail matters more than the window itself.

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What it is and what it's for

A pitched roof window is a glazed unit designed to sit in the plane of a sloping tile or slate roof. The frame and sash pivot on a central or top-hung hinge. Velux is the dominant UK brand (so much so that "Velux" is used generically for any pitched roof window), but Fakro and RoofLITE+ are genuine alternatives. All three are built on the same install principles: cut an opening in the roof structure, trim the surrounding rafters, fit the window frame, wrap it in a flashing kit matched to your tile type, and integrate the roofing underlay with a proprietary underfelt collar.

These windows are Permitted Development under Class C of the General Permitted Development Order 2015 as long as the window protrudes no more than 150mm above the roof plane, side-facing windows are obscure-glazed and fixed shut if the sill is below 1700mm internally, and the property isn't listed or in a conservation area with an Article 4 direction removing PD rights. Building Regulations approval is still required even when planning isn't, because you're cutting rafters and changing the thermal envelope.

They work on pitches from 15 degrees up to 90 degrees depending on the flashing kit specified. Below 15 degrees, no Velux-style window is rated. If your extension roof is a shallow mono-pitch at 10 to 14 degrees, you need a different product (either a flat-roof rooflight or a bespoke aluminium rooflight).

Size codes: what the letters and numbers mean

Every Velux window has a size code printed on the frame and in the product name: GGL MK04 2070. Reading this code is the first skill any homeowner buying roof windows needs, because flashing kits, blinds, installation products, and replacement sashes all have to match the window's size code exactly.

GGL is the product family (white-painted pine centre-pivot, the most common). Other families: GPL (top-hung), GGU (white polyurethane, good for bathrooms), GPU (top-hung polyurethane), GGL-S (sound-insulated).

MK04 is the size code. The letter indicates the frame width group; the number indicates the height position within that group. The eight width groups you'll see in UK merchants are:

Letter	Frame width	Typical use
CK	55cm	Small bathrooms, stairwells, narrow rafter bays
FK	66cm	Compact loft rooms
MK	78cm	Most common residential size
PK	94cm	Larger loft rooms, mid-size kitchens
SK	114cm	Kitchen extensions, vaulted ceilings (escape-compliant)
UK	134cm	Feature openings, panoramic views

The number suffix goes from 00 (short) to 10 (tall). So MK04 is a 78 x 98cm window. MK06 is 78 x 118cm (taller, same width). SK06 is 114 x 118cm (wider, same height as MK06).

2070 is the glazing code. This is the suffix nobody explains clearly:

• 2070: standard double-glazed with laminated inner pane, U=1.3 W/m²K. Meets current Part L for extensions and replacement windows. Still Velux's entry-level spec in 2026.
• 2066: triple-glazed, U=1.0 W/m²K, 37dB acoustic, anti-dew and easy-clean coating. The default choice for new builds and high-performance extensions.
• 2068: triple-glazed, U=1.1 W/m²K, slightly less acoustic/coating spec than 2066 but cheaper.
• 207030: INTEGRA solar-electric variant (2070 glazing with motor, controller, rain sensor).

For a new extension, specify 2066. The triple-glazed premium over 2070 is roughly [Unknown price: ] per window (indicative, varies by merchant), and you'll notice the difference in both warmth and noise the first time it rains hard.

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Types and opening mechanisms

Four opening types matter for UK extensions.

Centre-pivot (GGL, GGU) is the default. The sash rotates around a central horizontal hinge. You can turn it 180 degrees for cleaning the outside pane from inside. Cheapest option. Good for most roof pitches.

Top-hung (GPL, GPU) hinges at the top and opens outward like an awning. Gives unobstructed views and keeps the sash clear of your head when leaning out. Costs about 15-20% more than centre-pivot at the same size. Preferred for kitchen extensions where you want to lean through to check the gutter or for loft bedrooms with a low ceiling.

Combination or "cabrio" (GDL) pairs a top-hung upper sash with a lower element that swings out to form a railing. Creates a mini-balcony effect. Specialist product, expensive (around £2,500–£3,500+), and rarely used on extensions.

Electric or solar (INTEGRA) is a centre-pivot with a motor, rain sensor, and remote control. Mains-powered (GGL INTEGRA) needs an electrician's first-fix cable to the window. Solar-powered (GGL Solar INTEGRA) includes a small photovoltaic cell on the frame and needs no wiring. Solar is the sensible choice when retrofit cabling would mean chasing plaster. At 2026 retail prices around £734 for an MK06, INTEGRA costs roughly £120 more than a manual triple-glazed equivalent. Worth it for out-of-reach windows above vaulted ceilings where you'd otherwise need a pole to open them.

Flashing kit selection: the decision that makes or breaks the install

The flashing kit is a set of pressed metal aprons and side flashings that weather-seal the gap between the window frame and the roof covering. Velux makes around ten different flashing codes, each matched to a specific roof-covering profile. Using the wrong kit is the most common beginner mistake on roof window installs. The code is written on the kit's box as EDW, EDP, EDL, EDN, and so on. Here's what each one is actually for:

You buy the flashing kit in one of three bundle levels.

• Standard (EDW 0000 or similar): flashing only, no collars. Don't buy this unless you already have a BFX underfelt collar and BDX insulation collar.
• Pro (EDW 1000): flashing plus BFX underfelt collar.
• Pro+ (EDW 2000): flashing plus BFX and BDX. This is what you want. Buying the components separately always costs more.

At 2026 trade pricing, Pro+ kits run £109 for MK04 and £134 for SK06 including VAT. Pro kits without BDX save you about £40 but leave a thermal gap around the frame that your builder will want filled anyway, so the Pro+ premium is money well spent.

The BFX collar detail that causes most leaks

The BFX underfelt collar is a pleated sleeve of breathable membrane that wraps the roof window frame. It makes a continuous watertight seal between the breathable roof membrane already laid over the rafters and the window itself. It's a small component that ships with every Pro and Pro+ kit, and it's the single most important piece of the install.

The rule is simple and almost nobody explains it. The main breathable roof membrane must be dressed UNDER the BFX collar, not over it. Water that tracks down the membrane surface needs to run off the membrane, hit the collar, and drain out through the collar's built-in transverse drainage gutter. If the membrane sits on top of the collar, water runs under the collar and ends up behind the plasterboard reveal.

On a new-build roof with the membrane already laid, your roofer cuts the rectangular opening, peels the flaps outward, sets the window frame in, and wraps the BFX collar around the frame. The collar's four sides then dress over each cut flap of the main membrane, not under it. Velux publishes a short official video demonstrating this on YouTube (search "VELUX BFX underfelt collar") and a longer install walk-through with their technician Mark Dymond. Watch both before your roofer does the install, not after.

Forum threads document what happens when this is wrong. One DIYnot thread describes a roofer who treated the membrane as "a simple rectangle around the opening" with no collar at all; water flooded through during the first rain during tiling. A second thread shows a homeowner whose BFX was fitted but wasn't tight to the frame. The flashing kit wouldn't seat correctly, and the result was water ingress at the head flashing.

Brand comparison: Velux vs Fakro vs RoofLITE+

Three brands dominate the UK pitched roof window market. All three install with similar flashing principles, but pricing, glazing options, and warranty terms differ.

The pricing surprise is Fakro at the triple-glazed SK06 size. Confirmed 2026 retail for the FTP-V P5 triple is higher than Velux's equivalent GGL SK06 2066 at £769. The widespread assumption that Fakro always undercuts Velux holds for double-glazed entry-level units but breaks at premium spec. If you're buying triple-glazed, price-check both brands at your specific size before assuming.

RoofLITE+ is the genuine budget option. It's manufactured in Hungary by Altaterra, part of the VKR Group (the same corporate parent as Velux, but operating independently). It isn't "Velux-owned" in the literal sense, even though sibling brand positioning is accurate. At roughly half the price of Velux for a comparable double-glazed unit, RoofLITE+ is a sensible choice for a garage conversion or utility room where the window's performance is secondary. For a kitchen extension where the window is a feature, spend the extra on Velux or Fakro triple-glazed.

Building Regulations: U-values, escape, ventilation

Three sets of rules apply to every pitched roof window install. Your building control officer will check all three at completion.

Part L (thermal performance) sets the maximum whole-unit U-value. For a new dwelling, the notional limit is 1.6 W/m²K. For a replacement window in an existing dwelling or a new window in an extension, the limit tightens to 1.4 W/m²K. Velux 2070 double-glazed at U=1.3 meets both. Velux 2066 triple-glazed at U=1.0 comfortably exceeds both. Fakro P5 and RoofLITE+ TRIO triple-glazed products are similarly compliant. Don't let a builder fit an older 2-series or non-specified glazing as a substitute without confirming the U-value meets 1.4 for your context.

Part B (fire safety and escape) applies if the window serves a habitable room in a loft conversion or any storey where there's no protected stairwell. The window must provide a clear opening of at least 450mm high by 450mm wide AND a minimum 0.33m² clear area, all three conditions simultaneously. The sill must be no more than 1100mm above finished floor level. The window must stay open unaided. Not every size code meets this: an MK04 at 78 x 98cm provides around 600 x 820mm clear opening when fully rotated, which exceeds the requirement. Smaller CK and FK sizes don't. If the window is for loft-room escape, specify MK04 or larger and confirm the clear aperture from the manufacturer data sheet before ordering.

Part F (ventilation) requires background ventilation in habitable rooms. Most Velux windows include a built-in ventilation flap at the top of the sash that meets the background ventilation requirement for a single small room. Don't rely on window trickle vents alone for an open-plan kitchen extension; the overall ventilation calculation has to include the other openings and any mechanical extract.

How to work with it

You won't be installing this yourself. A competent carpenter or roofer does the install in half a day to a day per window on a new-build roof, longer on a retrofit where existing tiles need stripping back. What you need to understand is the sequence, because each stage has a check-point where mistakes become expensive to fix.

Structural trimming

Cutting an opening in a pitched roof means cutting through one or more rafters. The load those rafters carried has to be redirected around the opening by trimmer rafters: the rafters immediately each side of the opening are doubled up (two rafters bolted together), and head and sill trimmers (horizontal members) close off the top and bottom of the opening and transfer the cut rafter load across to the doubled side rafters.

Your structural engineer or competent builder decides how the trimmers are sized. On a standard rafter size (typically 100 x 47mm or 125 x 47mm C16 timber), doubling the two side rafters is usually enough for a single MK or SK-sized window. Larger openings, multiple windows in a row, or a pair of windows trimmed out to share a single opening may need deeper section trimmers or engineered joist (I-joist) reinforcement. Your building control officer will want to see this structural detail at strip-out, before plasterboard hides it.

Opening sizing

The framed rough opening must be 40mm wider than the window's external frame width and 80mm taller than the frame height, with the extra 80mm at the bottom for batten clearance. For an MK06 (78 x 118cm external), the rough opening is 820 x 1260mm. This clearance gives space for the BDX insulation collar and for the BFX collar to sit correctly. Cutting the opening to the exact frame size (a common mistake) leaves no room for the collars and the flashing kit won't seat properly.

Tile and batten clearance

Above the top of the window (the head), there must be a minimum 60mm clearance between the window's top flashing drain gully and the nearest tile above. Below this, water draining off the top flashing backs up behind the tile and runs sideways. Several DIYnot threads document leaks from installs where this clearance was only 10mm. On a steep roof at 35 to 45 degrees, 60mm is easy. On a shallow 15-degree extension roof, it's tight, so check your tile gauge before finalising the vertical position of the window.

Interior reveal

Once the roof is watertight, the interior reveal has to be lined. The four internal faces between the window frame and the ceiling plasterboard get lined with plasterboard, taped, skimmed, and painted. This is typically 2-3 days of plasterer time per pair of windows, and it's the cost that roofer quotes almost always exclude. Budget £300–£750 per window on top of the install quote for this finish.

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How much do you need

For a typical single-storey extension with a vaulted ceiling, two MK06-size windows mounted either side of the ridge is the standard spec. That gives good daylight penetration without creating heat-gain problems in summer. On a larger open-plan kitchen extension (30m² or more) you might go to two SK06 windows or a pair plus a single MK06, depending on roof geometry and ridge-line position.

Rule of thumb for daylight planning: roof windows typically give around 2.5 to 3 times the daylight of a vertical window of the same glazed area, because they face upward toward a larger portion of the sky. One well-sized roof window (MK06 or larger) brightens a 20m² extension meaningfully. A pair transforms it.

Lead times on stock sizes (MK04, MK06, SK06 in white-painted pine with 2070 or 2066 glazing) are typically 1-2 weeks (around 7-14 working days) from UK merchants. Non-stock sizes (UK10, combination units, specialist glazing) are quoted with longer lead times and can extend several weeks beyond the stock-size figure depending on supplier and factory schedule. Order early.

Cost and where to buy

Budget for the full cost of a window install in four line items: unit, flashing kit, labour, and interior finish. Roofers almost always quote the first three and leave the fourth to the plasterer.

Unit only. At 2026 trade retail inc VAT: £359 for a Velux GGL MK04 2070 double-glazed. £612 for a Velux GGL MK06 2066 triple-glazed. £769 for a Velux GGL SK06 2066 triple-glazed. Solar-electric INTEGRA MK06 runs around £734.

Flashing kit (Pro+, profiled tile). £109 for MK04. £134 for SK06. If buying separately, the BDX insulation collar alone is £40–£50 (same price across all sizes, already includes a BFX). Always buy the Pro+ bundle instead of separates.

Supply and install labour (new opening). £900–£1,800 per window for a standard new-cut install on a pitched roof in profiled tile. Simple replacement of an existing like-for-like window is cheaper at £550–£1,200. Complex jobs (trussed rafters needing engineered reinforcement, scaffolding, awkward access, shallow pitches) can push past the top of that range and into bespoke quote territory.

Scaffolding. Most roof window installs need an external working platform. Cherry pickers won't do it safely on anything above single-storey. Budget £1,200–2,500 for scaffold hire for the duration of the roofing work.

Interior reveal. See the Interior reveal note in the install sequence above; the same per-window plasterer cost applies in the all-in budget.

For the standard extension spec (two MK06 triple-glazed with EDW Pro+ flashing, installed over a vaulted ceiling), all-in realistic cost in 2026 is £3,200–£5,000 for the pair including ceiling reveal, before scaffolding. If someone quotes you well below that range all-in for two Veluxes over a vaulted kitchen ceiling, they've missed something.

The existing graph entry for £750–£1,750 reflects the simpler supply-and-install case where the opening already exists and the ceiling reveal is a separate trade. Use it as a sanity-check against builder quotes for straightforward replacements.

Where to buy

Specialist roofing suppliers carry the full range at the sharpest prices. Roofing Outlet, JJ Roofing Supplies, Roof Giant, and Burton Roofing all stock Velux, Fakro, and RoofLITE+ with next-day or 2-3 day delivery. Trade accounts get another 5-15% off list. Most UK extensions buy windows from one of these.

Builders merchants (Travis Perkins, Jewson, SIG Roofing, MKM) stock Velux and sometimes Fakro. Useful if you're consolidating a large order. Prices tend to be higher than the online specialists unless you have a long-standing trade account.

Wickes and Toolstation stock a narrow range of the most common Velux sizes. Screwfix carries some flashing kits. Fine for a late top-up if something is missing but not the sourcing channel for a new window.

Velux direct sells through their website but at full list price. Useful for accessories (blinds, remote controls, specialist glazing) that specialist merchants don't stock, less useful for windows themselves.

Alternatives

Bespoke aluminium rooflight: the large-format fixed aluminium rooflights (3-6m+ in one pane) that define modern kitchen extensions. Not a direct substitute for a Velux. A single large bespoke rooflight replaces 2-3 Veluxes with one uninterrupted pane, costs £6,000–£12,000 for a real extension-size unit plus specialist install, and doesn't open for ventilation. Use a Velux if you want openable windows with moderate daylight at moderate cost. Use a bespoke rooflight if the roof opening is the architectural centrepiece and natural ventilation comes from elsewhere.

Flat-roof rooflights (including pyramid, walk-on, opening flat rooflights): a different product for a flat or very-low-pitch roof. Velux makes the CFP and CVP ranges for flat roofs (up to 15 degrees). Korniche, Atlas, Keylite and Roof Maker all compete in this space. Don't fit a pitched-roof window on a flat roof or vice versa; the flashing and weathering principles are fundamentally different.

Fakro FTP: genuine Velux alternative, same install method, longer warranty (15 years on glass and frame). Best value at entry-level double-glaze. Flashing kit codes differ from Velux but perform the same function.

RoofLITE+: budget option from the VKR Group. Good for secondary spaces where the window spec isn't the priority. Accessories and blind range narrower than Velux.

Where you'll need this

• Roof covering: install follows roof structure and membrane, before tiling completes
• Windows and doors: roof windows sit in the same regulatory and specification conversation as vertical glazing

These windows appear in any extension or loft conversion where the roof slope needs daylight and ventilation, and in renovations where a loft becomes a habitable room under Part B escape rules. The knowledge above is portable across project types.

Common mistakes

Ordering a flashing kit that doesn't match the tile profile. EDW on plain tiles doesn't seat flat. EDP on profiled concrete interlocking tiles leaves gaps. Always identify the tile type before ordering the kit, and if the roofer is ordering for you, confirm the kit code in writing.

Skipping the BDX insulation collar. Pro (EDW 1000) and Standard (EDW 0000) kits don't include the BDX. Buying them as a cost saving leaves a 40mm gap around the frame that has to be filled with mineral wool stuffed in from below, which cold-bridges. Pay for the Pro+ kit or buy the BDX separately. The BDX price is captured in the Cost section above and ships with a BFX already inside.

Membrane laid OVER the BFX collar instead of under it. Covered above. This is the single most common failure mode and responsible for most new-install leaks.

Not doubling the trimmer rafters. On a standard rafter size, a single cut rafter at the head and sill of the opening isn't enough to carry the load of the cut section around the opening. Doubled trimmers are cheap and fast. Skipping them gets flagged at building control structural inspection.

Underestimating the interior reveal cost. The plasterboard return around the four internal faces of the window is not included in most roofer quotes. It's a plasterer's job and needs 2-3 days per pair of windows including taping, skimming and paint. The per-window figure shown in the Cost section is realistic.

Using "EDZ" in orders or specifications. The code is legacy. Current equivalent for profiled tiles is EDW. Some older roofers still use the term, which is fine in conversation, but on the order sheet make sure the SKU code matches a current product.

Ignoring the 60mm head-flashing clearance. At the top of the window, water draining off the top flashing gully needs a clear 60mm gap before the next tile. Shallow-pitch extensions run this close. Check tile gauge against window position before fixing the rough opening location.

Assuming triple-glazed is automatically cheaper than Velux from Fakro. Fakro P5 SK06 triple-glazed is priced above Velux GGL SK06 2066 triple-glazed at current 2026 retail. Price-check at your specific size and spec before assuming brand-based savings.