VELUX flashing kits for tile and slate

The flashing is the metal skirt that surrounds the window and marries it to the surrounding roof covering. It is not decoration and it is not optional. Its job is to carry rainwater coming down the roof around the window and back onto the tiles or slates below it, so that nothing collects against the frame and nothing runs behind the covering. A window without its flashing is a hole with a lid on it.

A VELUX kit comes as four pieces: a top section that sheds water arriving from above, two side sections that channel it down past the frame, and a bottom apron that returns it onto the roof below the window. The apron is the piece that has to bend itself around the shape of the covering, and that is why tile and slate need different kits. Everything above the apron is broadly similar. The bottom is where the covering profile is either accommodated or it is not.

The flashing is aluminium, supplied as standard in a grey mill finish, with darker anthracite and black options where a roof wants the metalwork to sit back quietly rather than catch the eye. On a slate roof in particular, a dark flashing reads far better than bright aluminium, and that choice is worth making at the order stage rather than living with afterwards.

The difference comes down to depth. A profiled tile, the interlocking concrete or clay pantile that covers most twentieth-century roofs, stands proud of the roof surface. Its contour can be anything up to around 120mm from the trough to the crown. Water runs in the valleys of that profile, and a flashing has to reach down into those valleys and seal against them. So a tile flashing has a deep, corrugated apron with enough material to press into the contours and follow them across the width of the window.

Slate is the opposite. A natural or fibre-cement slate is a thin, flat plate, typically no more than 8mm thick, laid in smooth overlapping courses with no profile at all. There are no valleys to reach into. A slate flashing therefore has a shallow, nearly flat apron, because a deep corrugated one would leave gaps under it where the covering is flat. Put a tile flashing on a slate roof and it lifts and gaps. Put a slate flashing on a deep profiled tile and it cannot reach the troughs, and water runs behind it. The two are not interchangeable, and no amount of care on installation day rescues the wrong choice.

Plain tiles sit between the two. A plain clay or concrete tile is flat like a slate but thicker, often up to around 14mm, and laid double-lapped. It takes its own flashing rather than either of the other two, so a period roof in plain clay tiles is a third category, not a variation on the profiled-tile one.

VELUX names its flashings with a four-letter code, and once you can read it the whole range makes sense. The letter that matters most is the third one. W means a profiled covering, L means a flat or slate covering, P means plain tiles. An E at the front marks it as a standard single-window flashing, and a K in the second position marks a combination kit for more than one window. J and N in the third position are the recessed versions, which sit the window deeper into the roof.

These are the kits you will actually be choosing between on a house roof.

The pattern to hold onto is that W and J are your tile options, L and N are your slate options, and the recessed pair, J and N, are the same coverings installed to a lower-profile finish. Read the third letter and you know whether a flashing is meant for your roof before you look at anything else.

A flashing kit is ordered to match both the window and the roof, so three measurements settle the whole choice. None of them needs specialist equipment, though the last one is easier to judge from the roof than from the ground.

• The covering. Profiled tile, plain tile or slate. This picks the letter. If you are unsure whether a tile counts as profiled or plain, the test is whether the surface has a raised contour you could run water along. A pantile does, a flat plain tile does not.
• The profile depth. On a profiled tile, the height from the bottom of the trough to the top of the crown. Standard tile flashings cope up to around 120mm, which takes in almost every domestic concrete tile made. A deep clay pantile on an older roof is the one to actually measure, because a small number sit beyond the standard range and need a specific solution.
• The window size code. Every VELUX window has a size code, such as MK04, SK06 or UK08, and the flashing is bought to the same code. The letter is the width and the number is the height, so the kit is dimensioned to your exact opening rather than cut to fit on site.

Pitch matters too, though it rarely changes the kit. Standard flashings are designed for a roof pitch between 15 and 90 degrees, which covers effectively every pitched roof you will meet on a house in south Essex. Below 15 degrees a roof needs a specific low-pitch arrangement with additional underfelt to keep the run-off moving, and that is a different order rather than a standard kit. A very shallow rear addition is worth checking against that limit before anything is bought.

The recessed kits, EDJ for tile and EDN for slate, do the same weatherproofing job as the standard ones but set the window roughly 40mm deeper into the roof. The visual result is a window that sits almost flush with the covering rather than standing proud of it, with only a thin line of frame showing above the tiles or slates. On the roof it looks less like an addition and more like part of the original.

That lower profile is partly a matter of taste and partly a matter of thermal detailing, because setting the window deeper improves the run of insulation around it. But on some roofs the appearance is the deciding factor rather than a preference. The Leigh Cliff and Leigh conservation areas carry an Article 4 Direction that removes the usual permitted-development rights for changes to roofing materials and windows, so a roof window there needs a planning application where it normally would not, and the planners will look closely at how it sits. A recessed slate flashing in a dark finish, sitting low and quiet in a slate roof, is a far easier case to make than a standard kit standing proud in bright aluminium. Clifftown, with its Victorian estate, and Leigh Old Town, with its older fishing-cottage core, ask the same kind of restraint.

None of this is a reason to assume a recessed kit is always better. On an ordinary 1930s tiled roof away from a conservation area, a standard flashing is perfectly right and simpler to install. The point is that the finish is a choice, and on a period or protected roof it is a choice worth making deliberately.

Two windows side by side, or one above another, are not two separate flashings butted together. They take a combination kit, coded with a K, that weatherproofs the group as a single unit and manages the water running between the openings, which is where a pair of individual kits would let it collect. EKW handles a tile group and EKL a slate one, following the same letter logic as the singles.

The gap between the windows is fixed by the kit rather than chosen freely. A standard side-by-side combination sits the windows around 100mm apart, and that spacing is built into the flashing so the metalwork between them drains properly. Slimmer spacings exist for specific effects where two windows are meant to read almost as one, but the standard gap is the one most roofs use. It is worth knowing before the rafters are set out, because the window spacing and the flashing have to agree, and the time to get them to agree is at the design stage rather than after the timber is cut. A run of windows over a south-facing loft room, arranged to bring an even wash of light across the ceiling, is a combination job from the start.

The flashing is the visible weatherproofing, but it is not the only layer keeping water out. Underneath it, around the frame, sits a set of collars that most people never see and that do a good deal of the work. The underfelt collar ties the window into the roofing membrane below the tiles, so that any water getting past the covering in a driving storm is guided back onto the felt and away down the roof rather than into the timber. On an installation with insulation, an insulation collar wraps the frame to carry the thermal line of the roof continuously around the opening, which is what keeps the reveals warm and free of condensation.

These parts come as their own kits, the underfelt and drainage collar for older roofs and the combined insulation and underfelt collar for insulated ones, and a complete installation uses them together with the flashing. It is the reason a properly installed VELUX window stays watertight through decades of Essex weather rather than depending on the outer flashing alone. If an older window has begun letting water past a tired flashing, the sound answer is to replace the unit and its full set of collars as a single new installation, so the whole detail is renewed at once rather than worked around piecemeal. A worn seal on the roof surface with no window involved is roofing work, and a roofer is the right call for that.

The housing stock across this stretch of coast sorts fairly neatly into the three flashing families, which makes the choice easier than it first looks. The enormous 1930s semi belt, which runs through so much of Southend and the surrounding towns, is almost all interlocking concrete tile, so those roofs are EDW or, where the look matters, EDJ territory. The Victorian and Edwardian terraces and seafront villas along the cliff, and much of the older housing in the conservation areas, are natural slate, which puts them on EDL or the recessed EDN. Plain clay tiles turn up on period properties and in the weatherboard villages out towards Rochford and Maldon, and those take EDP. Once you have identified which of the three a roof is, the rest of the specification follows.

A VELUX window is at its best in a pitched slope, and getting the flashing right is the foundation of any pitched-roof installation that stays dry. It is also only half the specification. The flashing settles how the window sits in the roof and how the rain is managed around it. The glass settles how the room underneath actually feels, and on a south-facing slope across Southend, with the estuary open to the south and long sunlight hours, that is a real question. A window can be flawlessly weathertight and still turn a loft bedroom into somewhere nobody wants to be at six on a July evening, which is where the solar-control glazing and its G-value earn their place. Weatherproofing and solar control are two separate decisions, and both belong on the same order.

When you are specifying a job, three things are worth confirming before anything is bought: the covering type and therefore the flashing letter, the window size code so the kit matches the opening, and whether a standard or recessed finish is wanted, which on a conservation-area roof may be settled for you. We install to current Building Regulations, including the thermal requirements of Part L and the safety-glazing requirements of Part K, and we make the Building Control notification to Southend-on-Sea City Council, or the relevant local authority for your address, as part of the work. If you want the full picture on the windows themselves, the VELUX window installation service page sets it out, and when you are ready to price a specific roof you can ask us for a specification with the flashing and the glazing set out separately for each window.