What is a roof window flashing kit?
A flashing kit is the set of preformed metal pieces that joins a roof window to the roof covering around it. It ships as a separate boxed product, ordered against the window’s own size code and against the profile of the tile or slate it is going into, and it is roughly half of what makes an installation watertight. It is not a sealant, not a tape and not something formed on site out of a roll of lead. Every piece in it is a shaped component with a defined lap, and it works by directing water rather than by blocking it. The wider installation sequence is on the roof window installation page, and the kit for each Gloucestershire covering is set out in detail under flashing kits by roof covering.
Four pieces, and the job each one is doing
Open a kit and there are four groups of components inside, plus fixings and a set of instructions specific to that window size.
- The apron, a single preformed piece that sits at the cill and lies over the covering below the window. Water running off the glass lands on top of the tiles rather than behind them.
- The side pieces, either a stack of individual soakers to be interleaved course by course, or a pair of continuous side flashings, depending on the covering. These take water arriving from either side and carry it out onto the roof.
- The head flashing, which tucks under the course of tiles above the window so water coming down the slope crosses onto metal and is thrown sideways past the frame.
- The corner pieces and cover profiles, which close the junction between the apron and the sides, and hide the fixings.
Assembled correctly the four form a continuous tray around the frame with every joint lapped in the direction the water is already travelling. Nothing relies on adhesion and nothing relies on a bead of anything. That principle is described further on the soakers, aprons and head flashings page.
Why it is a manufactured product rather than site work
Lead has been dressed around openings by hand for centuries and a good leadworker can still do it. What a kit brings is repeatability. The pieces are pressed to a known profile, the laps are already the correct length, the fixing positions are pre-punched, and the whole set is dimensioned to the exact frame it belongs to.
There is a second reason, which is the guarantee. A manufacturer will warrant a window installed with its own kit and to its own instructions. A window weathered with site-formed metalwork is a different conversation with the manufacturer if anything is ever queried. On a job where the workmanship carries a ten-year guarantee and the unit carries the maker’s warranty, keeping those two documents clean is worth the cost of the boxed kit, which is modest against the labour of forming the same detail by hand.
The two things the kit has to match
Ordering a kit needs two pieces of information and getting either wrong means the box is the wrong box.
The first is the window size code, because the apron, the head flashing and the side pieces are all dimensioned to that frame. The second is the profile height of the covering, measured from the top of one tile to the top of the next where they overlap. That single number decides which family of kit is needed: a flat covering such as slate or a plain tile, a low-profile tile, or a deep profile such as a pantile or a bold-roll concrete tile. Slate at 5 to 8mm and a deep pantile at over 100mm need completely different geometry at the side of the frame, and a kit made for one will not sit down on the other. Measure it rather than name it, because a covering that looks like a plain tile from the ground is not always one.
The three families, in one place
Kits are grouped by that profile measurement, and the three families cover almost every roof in the county.
| Family | Profile height it covers | Side detail | Typical Gloucestershire covering |
|---|---|---|---|
| Flat coverings | Up to about 8mm | Individual soakers interleaved with each course | Welsh slate, clay plain tile, fibre cement slate |
| Profiled coverings | Up to about 120mm | Deep pressed side flashing dressed into the profile | Clay pantile, concrete interlocking tile, bold roll |
| Metal and single-ply | Standing seam, welts, membrane | Kerb or upstand rather than a kit | Cheltenham lead flats, zinc mansards, flat rear links |
A fourth variant, the recessed kit, sits the window roughly 30mm deeper into the roof so the covering runs closer to the glass and the unit reads flatter from the garden. It is a different box with deeper side sections and it is a specification decision made before the order, because the timber trimming changes with it.
Insulation collar and underlay collar, fitted first
Two components arrive alongside the flashing and often confuse people because they are not metal and they are not visible in the finished job.
The insulation collar is a preformed sleeve of mineral wool or foam that wraps the outside of the frame in a continuous ring, closing the gap between the frame and the trimmed timber with no voids. It is what stops the perimeter of the window becoming a cold ring, which is where black spotting appears at the top corners in February. The underlay collar is a shaped membrane apron that connects the roof underlay to the frame: taken up over the head and stapled to the batten above, dressed down the sides into the frame’s own drainage channel, and run out over the covering below the cill. That is the second line of defence, and it is the detail that decides whether a mark ever appears on a ceiling. Both go on before the flashing kit, and neither can be inspected afterwards.
Lead, aluminium and the coating on the visible faces
Most kits use a combination. The pieces that have to be dressed to the shape of a tile are lead or a lead substitute, because they need to be worked by hand into a profile. The pieces that stay flat, particularly the head and the cover profiles, are roll-formed aluminium with the same polyester powder coating as the window cladding, so the visible metal matches the frame.
Where lead is used it is code 4 for most positions and code 5 where a piece is exposed or forming a small gutter. It expands and contracts several millimetres across a metre run between a frosty dawn and a July afternoon on a south-facing slope, so every piece is fixed at the head only and left free along its length. Lead nailed down both sides tears itself along the fixing line within a few winters, and cracked lead along a row of nail holes is one of the most common findings when a thirty-year-old unit comes off a Gloucestershire roof. That is a fixing decision rather than a material failure.
What the kit does not do on its own
A correct kit fitted correctly still depends on three things around it. The cut tiles at each side of the opening have lost their nibs or their nail holes, so they need mechanical fixing with a clip, a new nail hole or a copper tingle. A cut tile bedded in mortar alone will loosen, and when it lifts in a gale the water goes straight onto the underlay at the one point where the underlay has already been cut.
The lap has to be right for the exposure. Head lap is a minimum of 150mm and more on a shallow pitch, and on the exposed elevations of this county the manufacturer’s minimum is a starting point rather than a target. Driving rain off the south west travels sideways across a slope and climbs laps that would be fine in still air, which is why laps go beyond the standard figure in the Forest of Dean around Coleford and Cinderford, on the wold tops, and in the funnelled Stroud valleys. And the pitch has to be inside the range the kit is rated for, because a kit for 15 degrees and a kit for 45 degrees are dimensioned differently.
Ordering the right kit for a Gloucestershire roof
The survey settles it. The covering is identified properly rather than by eye: not “slate” but Welsh slate at a stated size with a measured headlap, or a concrete interlocking tile with a 75mm minimum lap, or Cotswold stone slate in diminishing courses. Batten gauge is measured. Pitch is taken off a digital level. The profile height is measured with a rule at the lap.
Two Gloucestershire cases fall outside the standard boxes and both are worth knowing about early. Cotswold stone slate at 20 to 30mm thick, laid in diminishing courses on riven battens with a lime mortar torching underneath, is a covering no kit was ever drawn for. A conservation rooflight with its own lead detail is usually the right answer there, and that is on the conservation rooflight installation page. Central Cheltenham’s shallow upper mansard slopes behind parapets are often below the pitch any kit is rated for, so the unit becomes kerb-mounted and the membrane does the weathering instead. Everywhere else in the county, from the interwar concrete tile at Hucclecote to the clay plain tile on the vale terraces, there is a box with the right pieces in it, and the VELUX window installation page sets out how the kit code and the window code go on the same order.
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