The flashing kit is ordered at the same time as the window, against the covering that is actually on your roof, and it cannot sensibly be chosen on the day from whatever is in the van. This page is the decision table for the coverings we meet within thirty miles of Worthing.
The one measurement that decides it
Profile depth. A straight edge across the covering and a rule down to the lowest point of the profile, taken on the slope where the window is going.
Manufacturers state the maximum profile depth each kit will accommodate, and that figure is what the choice hangs on. Everything else follows from it.
Guessing is where the errors come from. A tile that looks flat from the ground frequently has ten or fifteen millimetres of profile, and one that looks deeply profiled is sometimes within a flat kit’s range. From the roof anybody can tell; from the ground nobody can.
Two families, and where the line falls locally
Kits come in a flat family for coverings that lie close to the plane, and a profiled family for coverings with a defined shape and a void under each course. Why the two are not interchangeable, and what happens when they are substituted, is set out on the flashing kits page in the roof window section.
What matters here is where the line falls on the roofs in this county, because the covering that sits on the boundary is the one people get wrong.
Slate, plain tile and shingle are comfortably flat. Pantile and deep interlocking concrete are comfortably profiled. The awkward ones are the shallower interlocking tiles used on some 1960s and 1970s estates, which look flat from the ground and carry ten to fifteen millimetres of profile, and hand made peg tile, which is nominally flat and behaves like neither because it is cambered.
The coverings, and the kit each takes
| Covering | Where | Kit | Notes |
|---|---|---|---|
| Concrete interlocking tile | Post-war estates everywhere | Profiled | The easy case. Regular, known profile |
| Machine made plain tile | Mid century and later | Flat | Regular gauge, straightforward |
| Welsh slate | Hove, Heene, central Worthing terraces | Flat | Uniform, and recessing is usually right |
| Hand made clay peg tile | The Weald and the villages | Flat, adjusted | Gauge varies course to course |
| Clay pantile | Scattered, more toward the east | Profiled | Deep S profile, generous void |
| Horsham stone slab | The Weald and the scarp | None. Dressed lead | Graded courses, no repeating detail |
The easy case
A concrete interlocking tile roof on a trussed rafter house from the 1960s onward is the most common roof we work on and the most straightforward to flash.
The tiles are uniform, the gauge is constant, the profile depth is a known published figure, and the kit is designed for exactly this. Measure, order the profiled kit in the right size, and the junction goes together as designed.
It is worth stating plainly, because most of what is difficult on this page belongs to the older stock.
The hard case
Hand made peg tile is a flat covering, so it takes a flat kit, and the kit then has to be adjusted rather than fitted as supplied.
The tiles vary in thickness, they are cambered so they sit on head and tail rather than lying flat, and the gauge changes as you go up the slope. Soakers get cut to the courses actually present. Where a kit cannot be persuaded, the junction is made in dressed lead as it would be at a chimney on the same roof.
The case with no kit at all
Horsham stone slab takes no manufactured kit, and it is worth being direct about that rather than implying a workaround.
The courses are graded, so the gauge changes continuously up the slope. The slabs vary in thickness within one roof. The outlines are riven and irregular. A kit assumes a repeating course and this roof has none.
The junction is made in lead, dressed on site to the individual slabs. It is skilled leadwork, it takes longer, and it is the only method that works.
The kit is chosen against a specific window
A flashing kit is made for a particular window range and a particular size code, not for a size in millimetres.
Which means it cannot be chosen before the window is, and it cannot be carried over from the unit being replaced. The data plate identifies what is coming off; the order identifies what the kit has to suit. Those are two different references and they are frequently not the same.
It is also why a kit left over from another job is almost never the right kit, however similar it looks in the van.
What the supplier needs from you
Six pieces of information produce the right kit first time, and every one of them comes off the roof rather than off a drawing.
- The window range and size code being ordered.
- The covering, named precisely: not “tile” but concrete interlocking, plain tile, hand made peg tile or pantile.
- The measured profile depth in millimetres, taken on the slope the window is going in.
- The measured roof pitch.
- Standard installation or recessed, since these are different kits.
- One unit, or a group needing a combination kit, and if a group, how many and in what arrangement.
Getting three of the six and guessing the rest is how a kit arrives that nearly fits.
Checking what arrives
The kit is opened and checked against the order before the roof is opened, rather than on the roof on the day.
What gets checked: the right family for the covering, the right size for the window, and every component present. Kits come as several parts and a missing side section is not something to discover with the covering off.
The same five minutes catches the more common problem, which is the right kit to the wrong specification: standard coating where marine grade was ordered, or a standard kit where a recessed one was specified.
The collars go on the same order
Two components share the order and neither is part of the flashing kit: the underlay collar, which connects the cut underlay to the frame, and the insulation collar, which carries the insulation up to it.
Both have their own pages and both are separate line items. What belongs here is that they are the two most commonly left off an order, and neither can be added once the covering is back on.
We put all four on one order: window, flashing kit, underlay collar, insulation collar. It is the simplest way to make sure the third and fourth are not the ones forgotten.
Marine grade, on the order rather than after
Across the coastal strip the kit is specified marine grade coated with A4 stainless fixings, for the same reasons every exposed metal component on a coastal roof is.
The point that belongs on this page is that it is an ordering decision rather than a fitting one. A coating cannot be applied to a kit afterwards, and a kit fitted with plated fixings has plated fixings for its life. It costs very little on the order and nothing at all if it is remembered.
Batten depth, and the thing that catches people out
A flashing kit assumes the covering sits at a particular height above the frame, and that height is set by the battens and the underlay beneath it.
On a roof with counter battens, or with a thick modern underlay over an old batten gauge, or where a previous repair has doubled a batten, the covering sits higher than the kit expects. The soakers then sit lower relative to the tiles than they should, and the lap that should be generous becomes marginal.
It is not usually a reason to change the kit. It is a reason to look at the batten arrangement while the covering is off rather than assuming it, and to adjust the setting out of the window in the roof so that the kit sits where it was designed to.
Lead, and where it still belongs
Most current kits are aluminium with a formed skirt, and lead has not disappeared from this work.
Some kits include a lead or lead substitute apron for the section that has to be dressed to the shape of the covering, because a preformed piece cannot follow a hand made tile or a deep pantile closely. On the older coverings here that dressed section is doing the real work.
Lead is dressed rather than bent, it work hardens if it is overworked, and a lead apron beaten thin at a corner is a lead apron that will split within a few winters. It is one of the places where an experienced hand is visible in the finished job and invisible in a photograph.
Two windows, two coverings, two kits
Older houses here frequently carry more than one covering, because a rear addition was roofed differently or a repair used whatever was available.
Two windows on one job but on different coverings need two different kits. The profile is measured on each slope separately and each kit is specified against its own, and ordering one kit for both because they are on the same house is an error we do find.
A window spanning a change of covering is a worse idea again, and is almost always avoidable by moving it a course or two.
Lead times, and where the kit sits in them
Kits are generally more available than windows, which creates a specific trap.
Where a window has a lead time and the kit does not, it is tempting to order the window now and the kit later. What that risks is the kit being ordered against a remembered figure rather than a measured one, weeks after anybody was last on the roof.
We order both together off the same survey, so the profile measurement and the kit specification are never separated by a month.
Keeping the record for next time
The kit reference is worth writing down, because in twenty five years somebody will want it.
A replacement window in the same opening needs a kit for the same covering at the same profile, and the reference on the original order answers that in seconds. Without it, somebody is back on the roof with a straight edge and a rule.
We leave the window specification, the size code and the flashing kit reference with the completion paperwork. It costs nothing now and it saves a visit later.
How the measuring is done on a VELUX
We measure the profile depth and the pitch at survey rather than estimating them, specify the kit against what is actually on the roof, and order it with the window along with both collars. On old irregular coverings we adjust the kit to the courses present, and on Horsham stone we dress lead rather than pretending a kit will do.
The window keeps the weather out. The kit is what keeps it out at the edges, and it is chosen by the covering rather than by convenience.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.