Underfelt Collar for Roof Windows: What It Does and Why You Probably Already Own One

A roofer fits two roof windows into your new extension. Six months later, water marks appear on the plasterboard reveal under one of them. He blames the seal, then the flashing, then the slate. Strip back the plasterboard and the real culprit is sitting there: the breathable membrane was cut short and laid on top of the underfelt collar, not tucked beneath it. Every drop of wind-driven rain that gets behind the tiles is now running straight off the membrane into your loft instead of being channelled around the window frame. The fix is a full strip-back of tiles, battens, and membrane on both sides of the window. The collar itself cost £28 – £41. The remediation costs over a thousand.

The underfelt collar is the cheapest, most-overlooked component of a roof window installation. Most homeowners don't even know it exists because Velux includes it free with their Pro+ flashing kits. But understanding what it does, why it matters, and how to verify it's been installed correctly takes ten minutes and prevents the most common roof window defect on the UK forums.

What it is and what it's for

An underfelt collar is a pre-cut sheet of breathable membrane shaped to wrap around a roof window frame. It bridges the gap between the main roof underlay and the window itself, creating a continuous secondary waterproofing layer at one of the most failure-prone junctions on any pitched roof.

The collar sits between two waterproofing systems. Above it, the metal flashing kit (the visible aluminium or lead-coloured surround you see from outside) handles water at tile level. Below it, the breathable membrane keeps wind-driven rain out of the rafter zone. The collar joins these two systems. Without it, there's a gap right around the window where water can bypass both layers and run straight onto your ceiling joists.

Every roof window manufacturer makes their own collar. Velux calls theirs BFX. Fakro calls theirs XDP. Dakea (a Velux Group brand) calls theirs RUC. Keylite has a "Felt Collar". They all do the same job but they are not interchangeable. Each is shaped to fit one specific brand's window frame profile, and each is required for that brand's warranty to remain valid.

The collar has two structural elements. The membrane skirt wraps around all four sides of the window frame and laps onto the main roof membrane. A galvanised steel drainage channel sits across the top of the window. When water hits the collar from above, the channel catches it and directs it sideways around the window, then down both sides to the lower membrane. Without the channel, water would pool on top of the upper batten and eventually find its way into the loft.

Do you actually need to buy one separately?

This is the question that catches most homeowners out, and the answer depends entirely on which flashing kit you've ordered.

Velux Pro+ kits (EDW 2000-series and EDJ 2000-series) include the BFX underfelt collar AND the BDX insulation collar as standard. If you've ordered an EDW 2000, EDJ 2000, EDP 2000, or EDL 2000 flashing kit at £109 for an MK04 size or £134 for SK06, both collars are already in the box. Don't buy a standalone BFX as well. You'll waste £28 – £41.

Velux basic kits (1000-series, e.g. EDW 1000) do NOT include either collar. These are the budget flashing kits, typically somewhat cheaper than the equivalent Pro+ version. If you go this route, you must buy the BFX collar separately (and arguably the BDX as well). By the time you've added them, the Pro+ kit is usually cheaper or the same price. The 1000-series exists mainly for installations where the collar isn't needed (older roofs without breathable membrane, certain commercial applications). For a homeowner extension, buy the Pro+.

Fakro flashing kits typically do NOT include the XDK collar set. Fakro sells flashings (EHV, ESV, EZV-A series) and the XDK insulating collar set as separate products. Budget for both. The XDK at £57 – £68 bundles the XDP underfelt collar with the XDS interior airtight collar in a single set.

Keylite, Dakea, and Roto follow the same pattern as Velux: premium flashing kits include collars; basic kits don't. Read the product description carefully, or call the supplier and ask directly which kit includes which accessories.

The collars are NOT cross-compatible. A Velux BFX will not fit a Fakro window. A Fakro XDP will not fit a Roto. Each is shaped to the specific window frame's profile and dimensions. Trying to substitute one for another either produces gaps or voids the manufacturer's warranty (often both).

Why the collar matters even when it's "optional"

Building Regulations don't mandate an underfelt collar. The window itself, correctly fitted with a flashing kit, technically satisfies Approved Document C requirements for moisture resistance. So strictly speaking, you can install a roof window without a collar and still pass Building Control inspection.

This is why some roofers, particularly older trades who learned their craft before breathable membranes became standard, treat collars as optional extras. They fit the window, fit the flashing, skip the collar, and the inspector signs it off because the visible installation looks correct.

The problem shows up later. Roof window manufacturers' guarantees (the 2025 Velux version, but the same terms apply going back to 2015) explicitly exclude "use of incompatible spare parts or accessories" and void cover for "defects from installation contrary to instructions." Installation instructions for every standard window in the GGL/GGU range require the appropriate underfelt collar (or its inclusion via a Pro+ flashing kit). No collar means no warranty. If you have a leak in year 4 of your supposed 10-year guarantee and the manufacturer discovers no collar was fitted, they walk away. So does Fakro. So does every other major manufacturer.

The second reason matters more in practice: condensation. Even on roofs without a breathable membrane (older 1930s-1950s housing without underlay), the collar still serves a purpose. The insulation collar, which always ships with an underfelt collar pre-fitted inside it, prevents warm internal air from condensing on the cold underside of the metal flashing in winter. Without the collar, you get water dripping inside the loft on cold mornings even though no rain has fallen.

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The three collar layers explained

Velux sells three distinct collar products that work together as a layered system. Most homeowners only ever encounter the BFX (because it ships with every Pro+ flashing kit), but understanding all three explains why your roofer might recommend adding the others.

BFX (the underfelt collar). This is the one this guide is mostly about. It's the membrane junction at the same depth in the roof build-up as your main breathable underlay. It uses a diffusion-open polypropylene skirt with a galvanised steel drainage channel across the top. It defends against wind-driven rain that gets behind the tiles. £28 – £41 standalone, free with every Pro+ kit. Required for Velux warranty.

BDX (the insulation collar). Fits between the window frame and the rafter, filling the structural gap with about 15mm of foam insulation. Always ships with a BFX collar pre-installed inside it, so if you buy a BDX you don't need a separate BFX. Costs £40–£50 as a separate purchase, and is included in every Pro+ kit alongside the BFX. Defends against thermal bridging and the condensation that forms on cold metal flashings in winter. Green Building Forum members tend to dismiss the 15mm foam as thermally feeble, but the consensus is "a lot better than nothing" for the cost.

BBX (the vapour collar). A fully welded rubber gasket that sits inside the window frame, forming an airtight barrier between the warm interior and the unheated roof zone. Not bundled with anything. Optional even by Velux's standards. Becomes important on Passivhaus-spec or near-Passivhaus extensions where airtightness is being formally tested. For a standard kitchen extension, you can skip it.

For nearly every homeowner extension, the answer is: buy the Pro+ flashing kit, get the BFX and BDX included automatically, skip the BBX unless your designer specifically calls for it.

Sizing: matching the collar to your window code

Underfelt collars are size-specific. A collar made for an MK06 window will not fit a CK04 or a UK08. The collar size code matches the window size code exactly. When ordering, the rule is: same letter pair, same number.

Velux size codes follow a fixed pattern. The two letters indicate the width category (CK = 55cm wide, MK = 78cm wide, PK = 94cm wide, SK = 114cm wide, UK = 134cm wide). The two-digit number indicates the height (01 = 70cm, 04 = 98cm, 06 = 118cm, 08 = 140cm, 10 = 160cm). So an MK06 is 78cm wide by 118cm tall. The BFX collar for that window is also coded MK06 and only fits that exact window.

Fakro, Keylite, and other brands have their own size systems but follow the same logic: collar code matches window code. When you order replacement collars or buy spares, check the size sticker on your existing window frame, not the rough opening size. The two are not the same.

How it gets installed (and what to check)

You won't be fitting this yourself. But the installation sequence is short, the failure modes are well-documented, and a five-minute inspection while the membrane is still visible saves you from defects that don't show up for years.

The collar goes on after the window frame has been fixed to the rafters and trimmer joists, but before the metal flashing kit is applied. The full sequence on a tiled pitched roof:

Roof structure ready. Rafters trimmed for the window opening, trimmer joists doubled, breathable membrane in place across the rest of the roof but cut around the window opening with enough material to dress up against the frame. The membrane edges around the opening should be loose, not stretched.

Window frame fitted. The window's outer frame (sash removed) is screwed to the doubled rafters and the trimmer joists at top and bottom, with corner brackets shimmed to bring the frame parallel and level. This is structural; the frame must not move once collared.

Collar positioned. The pre-cut underfelt collar is placed over the frame from above. The drainage channel sits at the top, with adhesive strips on the underside of the channel sticking down to the frame's top edge. The membrane skirt drops down all four sides.

Membrane dressed beneath the collar. Here's the critical step that gets missed. The main roof membrane must be lifted up, the collar's lower skirt dropped behind it, and the membrane then turned up at least 75mm against a fixing batten installed for that purpose. The collar's skirt sits ON TOP of the membrane on the upper side (so water runs from the collar onto the membrane and away), but the membrane's upstand is BEHIND the collar at the sides and bottom (so water can't track behind the frame). DuPont Tyvek's technical manual and BS 8000-6:2023 both specify the 75mm upstand.

Pleats stapled. The collar has pleated edges along the sides. Each pleat is stapled to the nearest rafter or batten with three staples. Without staples the collar can lift and crease in wind.

Flashing kit fitted. Once the collar is in place and the lower battens have been positioned (typically 80mm from the underside of the window for the lower batten, 45mm above the window head for the upper batten), the metal flashing components are slipped into position around the frame.

Tiles laid. Standard tiling proceeds across the rest of the roof, with the flashing kit's apron, sides, and head pieces interleaved with the tiles in the manufacturer-specified sequence.

The whole window install, including collar and flashing, takes a competent roofer a full day for a single MK06 or PK06 window. Two CK06 windows can usually be done in a day. Faster than that and corners are being cut.

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What to check before the tiles go on

Once the tiles are laid, the collar is invisible and any defect will only show up as a leak months or years later. The window five minutes to inspect during the build is worth thousands in avoided remediation. Walk up onto the scaffold (or stand on the loft platform looking up) and confirm:

The collar is present. This sounds obvious, but the most common failure mode is no collar at all. You should see a sheet of dark membrane wrapping around the window frame at all four sides, with a metal channel across the top.

The membrane is BENEATH the collar, not on top. Lift the collar's lower edge gently. The main roof membrane should be turned up against a batten with the collar's skirt sitting in front of it (when viewed from outside) so water runs collar -> membrane -> eaves. If the membrane is laid on top of the collar, water runs INTO the building. This is the single most common forum-reported defect.

The membrane upstand is at least 75mm. Measure from the top of the rafter to the top of the membrane upstand against the batten. Less than 75mm and water can splash over the top of the upstand into the rafter zone. This is specified in BS 8000-6 and the DuPont Tyvek technical manual.

Pleats are stapled. The collar's side pleats should each have three staples into a batten or rafter. Loose pleats lift in wind, crease, and develop pinhole leaks.

The drainage channel sits flat. The galvanised steel channel across the top should be horizontal, with the membrane skirt continuing under it across the rafter line. A buckled or twisted channel doesn't drain properly.

The collar sits tight against the window frame. Adhesive strips on the inside of the channel should be firmly stuck to the frame's top edge. Any visible gap between collar and frame is a water path.

If any of these checks fail, raise it before the battens go on. Once the tiles are down, fixing it requires stripping back to bare rafters on at least one side of the window.

Cost summary

For most homeowners building an extension with a new roof window installation, the collar cost is bundled into the Pro+ flashing kit and you don't see it as a line item. Where you do see it as a separate cost is when buying a basic flashing kit, replacing a damaged collar, or specifying a brand other than Velux.

For a typical kitchen extension with two MK06 windows on a Pro+ flashing kit, the collar component of your bill is effectively zero (already inside the £109 kit price). For two windows on basic kits, you'd add roughly £28 – £41 per window for the BFX, plus another £40–£50 per window if you also wanted the BDX insulation collar. At which point you've spent more than the Pro+ kit would have cost.

Where to buy

Specialist roofing retailers are the cheapest source. Roofing Outlet, Roof Giant, JJ Roofing Supplies, and The Skylight Company all stock the full installation product range with next-day delivery on most sizes.

Builders' merchants (Travis Perkins, Jewson, SIG) carry the popular sizes (MK04, MK06, SK06) but rarely the full range. Worth checking if you're already collecting other roofing materials. Special-order sizes typically arrive in 2-5 working days.

For Fakro, Keylite, and Dakea collars, go direct to the manufacturer or use a brand-specialist supplier. Loft Shop carries the full Fakro accessory range. Keylite sells direct with free delivery. Dakea is sold through specialist roofing outlets.

Alternatives

The honest answer is: there are no good alternatives to a manufacturer-matched underfelt collar. Some installers improvise by cutting strips of breathable membrane and dressing them around the window frame manually, taped at corners with membrane lap tape. This works mechanically but voids the window manufacturer's warranty, and the corners are extremely difficult to seal reliably. The pre-cut shaped collar exists because doing it by hand is fiddly and prone to error.

A few specific situations call for a deviation:

Roofs with no breathable membrane. On older houses being extended, or on traditional slate-on-sarking-board roofs without underlay, the drainage channel serves no purpose because there's no membrane below it to drain onto. In this case, fit the insulation collar (with underfelt collar integrated) anyway for the condensation control, but the drainage channel can be left dry. Don't skip the collar entirely; the warmth-condensation argument still applies.

Heritage and conservation windows. Some conservation window ranges use a different collar shaped to fit the heritage frame profile. Don't substitute a standard collar for a heritage-profile one. The fit will be wrong.

Stacked or coupled windows. When two or more windows are installed side-by-side or top-to-bottom in a single trimmer, manufacturers supply specific multi-window collars. The 500mm separation rule for stacked windows requires specific accessories; don't assume two single collars will work.

Where you'll need this

• Roof structure - rafter trimming and trimmer joists for the window opening must be complete before the collar can be fitted
• Roof covering - the collar sits within the membrane and battening sequence, between window frame and tile-level flashing
• Windows and doors - roof window selection determines which collar size and brand you need

These materials and components appear in any extension or loft conversion project that includes a roof window, regardless of project type.

Common mistakes

Membrane laid on top of the collar instead of beneath it. The single most common defect on UK roofing forums. Water hitting the membrane runs down ON TOP of the collar, then drips off the collar's lower edge straight into the rafter zone. The membrane must be tucked behind the collar at the sides and bottom, with the collar's skirt sitting in front of it. Five-minute fix during install. Day-long stripout if discovered after tiling.

No collar fitted at all. Some older roofers treat the collar as an optional extra, particularly on no-membrane roofs where the drainage channel seems pointless. The manufacturer warranty disagrees. No collar means no warranty cover, regardless of whether the roof leaks.

Wrong size collar ordered. CK06 and MK06 sound similar but the windows are 23cm different in width. The collar must match the window's exact size code, which is stamped on a sticker visible inside the window frame.

Generic or substituted-brand collar fitted. A Fakro XDP will not fit a Velux frame profile. Even if it did, fitting a non-matching part to a window voids the guarantee.

Membrane upstand less than 75mm. Without enough upstand, water can splash over the top of the membrane against the batten. BS 8000-6:2023 and the DuPont Tyvek technical manual both specify the 75mm minimum.

Pleats not stapled. Loose pleats lift in wind, crease, and over time develop tiny pinhole tears at the crease lines. Three staples per pleat into the nearest batten or rafter is the standard requirement.

Buying a standalone underfelt collar when you've also bought a Pro+ flashing kit. The Pro+ kit already includes both the underfelt and insulation collars. Read the flashing kit's "what's in the box" listing carefully before adding accessories to the basket.