Guide · Roof Window Installation
Soakers, aprons and head flashings explained
Every roof window in a tiled or slated slope is weathered by three things working together: an apron across the bottom, soakers up both sides, and a head flashing across the top. They arrive as a matched kit, they are fitted in a fixed order, and between them they turn an interruption in the covering back into a continuous water shedding surface.
Homeowners rarely see any of it once the job is finished, which is precisely why it is worth understanding. Nearly every problem that surfaces years after a roof window installation starts here rather than at the glass. This page takes the three components in the order water meets them and explains what each has to achieve.
The three parts, and the order they go on
Water arrives at the top of the opening and leaves at the bottom, so the flashing is built from the bottom up and each piece is lapped over by the one above it. That sequence is not a preference. It is the whole mechanism.
The apron goes on first, at the sill, sitting over the covering below the window. The soakers go on next, working up each side, each one lapping over the piece beneath it. The head flashing goes on last, tucked under the covering above the window and lapping over the top of the soakers. Fit any of them out of sequence and you have created a joint that water can walk into rather than over.
How water moves around an opening in a slope
Rain landing above a roof window does not stop at it. It runs down the covering, reaches the head of the opening and has to be diverted sideways, past the frame, and released back onto the covering below. The flashing is a diversion channel, not a seal.
That distinction matters because it explains why sealant has no role in a properly detailed rooflight. A sealed joint relies on adhesion, and adhesion fails eventually on a surface that expands and contracts through a forty degree annual temperature swing. A lapped joint relies on gravity and geometry, neither of which degrades. Where a roof window has been made watertight with mastic, someone has substituted a material for a detail.
The apron, and what it has to clear
The apron is the piece at the bottom of the window. It is a shaped skirt that comes off the base of the frame, extends down the slope, and is dressed onto the covering below so that water leaving the window is delivered cleanly onto tile or slate rather than behind it.
Its length down the slope is the thing to watch. On a shallow pitch water leaves the glass with less momentum and spreads sideways more, so the apron has to reach further down before it releases. On a deep profile covering it also has to sit into the profile properly, which is why aprons in a concrete tile kit are formed differently from those in a slate kit. The apron is the most exposed part of the whole assembly and the piece most often visible from the garden.
Soakers, and why they come in pieces
A soaker is a small preformed piece of metal that sits between two courses of the covering at the side of the window, turning up against the frame and out under the tile or slate. Rather than one long strip, the sides are made from a stack of individual soakers, one for every course of covering the window passes.
They are made in pieces for two reasons. The first is that the covering itself is laid in courses, and a continuous strip would have to be threaded under all of them at once, which is not possible without stripping the roof. The second is thermal movement: a two metre strip of metal against a frame expands and contracts far enough to work its fixings loose over a decade, and a stack of short pieces free to move against each other does not.
The head flashing, and the detail above the unit
The head is the hardest of the three and the one that fails most often when it is done badly, because it is where the largest volume of water arrives. The head flashing is a wide piece running the full width of the opening and a good distance beyond it each side, tucked up under the covering above the window and lapping down over the top soakers.
Two dimensions decide whether it works. The upstand behind the frame, which stops water being driven up and over the top of the flashing by wind, and the amount by which it disappears under the covering above. That second dimension has to exceed the headlap of the covering, or water travelling under the tiles will emerge below the top of the flashing rather than on top of it.
If water can get behind the head flashing, nothing further down the window matters.
Laps, and the direction everything overlaps
Every joint in a flashing assembly follows one rule: the upper piece laps over the lower piece, by an amount related to the pitch. On a steep slope water leaves quickly and a modest lap is sufficient. On a shallow slope water travels slowly, spreads sideways and is more easily driven back uphill by wind, so the laps increase.
The same logic runs through the whole roof. It is why headlap on slates increases as pitch decreases and why shallow pitch installations have their own set of rules. A flashing detail that is perfectly sound at 40 degrees can be marginal at 17.
Gauge and headlap, and how the covering sets the sizes
The number of soakers a window needs is not a guess. It is the height of the opening divided by the gauge of the covering, which is the vertical distance from one course to the next.
| Covering | Typical gauge | Soakers per side on a 1180mm window |
|---|---|---|
| Natural slate, 500 by 250 | Around 190 to 210mm | Six or seven |
| Plain clay tile | Around 100mm | Twelve or more |
| Concrete interlocking tile | Around 320 to 345mm | Three or four |
| Pantile | Around 300 to 345mm | Three or four |
This is the practical reason a kit is not interchangeable between coverings. A plain tile kit and a concrete interlocking kit for the same window contain a different number of soakers of a different shape, because the roofs they sit in are set out at completely different intervals.
Why the kit is matched to the profile, not the brand
The profile of the covering is what the flashing has to sit into. A flat covering allows the metal to lie tight against it. A deep profile leaves a trough beside every roll or rib, and the flashing has to be dressed down into that trough or water simply runs under the bridged section.
Each of the main Essex coverings has its own detailed treatment, covered separately on concrete interlocking tile roofs, plain clay tile roofs and natural slate roofs. What they share is that the kit is ordered against the covering measured on site, which is why a survey records profile depth and gauge rather than just noting the material.
Materials, and what each one does over time
Most proprietary kits are aluminium, either mill finish or coated, pressed into the profile required. Aluminium is light, holds its shape, resists corrosion well and does not need dressing in the way a soft metal does.
Lead is the traditional material and it is still the right answer in places, particularly where a window meets an awkward junction or where a conservation setting wants the appearance. It is soft enough to be worked into a complex profile by hand and it lasts a very long time when the sheet size and fixing are correct. Lead has its own rules on sheet code, drip formation and expansion, covered on lead flashing and weathering detail. Lead-free alternatives, usually laminated composites, are used where lead is inappropriate for the setting and are worked in a broadly similar way.
Recessed and standard installation depth
Roof windows are fitted at two depths, and the flashing kit differs between them. A standard installation sits the frame so the outer face is roughly in line with the covering. A recessed installation sets the whole unit lower into the roof structure so the glass sits closer to the plane of the tiles.
Recessed fitting reads better from outside, which is why it is common in conservation areas and on prominent elevations, and it is also more demanding to weather. The soakers and head flashing sit deeper relative to the covering, the gutter formed above the head has to work harder, and the tolerance for a poorly dressed lap drops. The kit is different and it is specified at order, not decided on the roof.
What sits underneath, and why it is not the same layer
The flashing is the outer defence. Beneath it, and beneath the covering generally, is the underlay, which catches anything that gets past. Around a rooflight the underlay is cut, folded and dressed onto the frame in a specific sequence so that any water reaching it is delivered back onto the outside of the covering below the window.
Both layers are needed and neither substitutes for the other. The detailing of that lower layer is covered on underlay and sarking around a pitched rooflight. What matters here is that the flashing kit is fitted after the underlay is dressed, and dressing the underlay after the flashing is in place is not possible.
Combination flashings for units in a group
Where two or more windows are fitted next to each other or above each other, the gap between them cannot be weathered by two separate kits. The soaker from one and the soaker from the next would meet in a valley with nothing under it.
Combination kits solve this with a purpose made section that sits between the frames and drains the gap. They come in fixed dimensions, which is the reason the spacing between grouped units is decided by the kit rather than by eye, and it is a point covered further on two rooflights side by side on one slope.
Windows near a valley, a hip or the ridge
The three flashing components assume a plain run of covering all the way round. Where a window sits close to a valley, a hip or the ridge, one of them runs out of room.
The usual working rule is to keep at least one full course of covering, and preferably more, between the flashing and the junction, so that each detail is complete before the next begins. Near a valley this is stricter, because a valley is already concentrating water off two planes and delivering it down a narrow channel. Placing a window where its apron discharges into a valley is asking one detail to handle the output of a large area of roof, and moving the unit half a metre along the slope removes the problem entirely.
Dressing, and what good work looks like
Dressing is the act of forming the metal down onto the profile of the covering so it makes contact rather than bridging. On a flat covering there is little to do. On a deep profile it is the difference between a working detail and a decorative one.
From the ground, good dressing reads as a flashing that follows the shape of the tiles, sitting down in every trough with no daylight visible under it. Poor dressing reads as a flat metal line running across a profiled roof, with shadow gaps under it every 300mm. Those gaps are open paths, and on an exposed Essex slope with wind driven rain coming off the estuary they will be found.
Fixings, clips and thermal movement
Flashings are held by clips and by the weight of the covering laid back over them, not by nails driven through the middle of a piece of metal. A fixing through the face of a flashing is a hole in the layer whose only job is to be continuous.
Movement is the reason. A metre of aluminium in full sun on a south facing slope changes length by a couple of millimetres between a January night and a July afternoon, and it does that every year for decades. Clips allow it. Rigid face fixing resists it until something tears. The same reasoning limits the length of a single piece of lead, which is why lead work is made up from multiple pieces with laps rather than one long sheet.
Defects you can see from the ground
A surprising amount can be assessed with binoculars from the garden. A flashing standing proud of the covering rather than sitting into it. A visible line of sealant along the top of the head flashing, which suggests the lap under the covering above is short. Tiles cut back further than they need to be around the sides, leaving the soakers over-exposed. Moss built up along the apron, which means water is sitting where it should be running.
None of those means water is coming in today. All of them mean the detail is working on a smaller margin than it was designed with, and margins are what get used up by an easterly gale in February.
Keeping the perimeter clear over the years
The flashing needs very little attention, which is the point of a lapped detail. What it does want is for the water to keep moving. Leaves collecting along the head flashing, moss growing in the trough beside the soakers and debris banking up behind the apron all slow the flow, and slow water finds its way sideways.
Ten minutes each autumn clearing the perimeter is the entire maintenance requirement for a correctly fitted kit. Most of the water problems that appear around roof windows years later began as a drainage path that quietly closed itself while nobody was looking at it.
What a quote should say about the flashing
A quote worth comparing names the kit, not just the window. It states the covering it is being ordered for, the profile depth where that applies, whether the installation is standard or recessed, and whether a combination kit is included where more than one unit is being fitted.
It should also make clear who is making the covering good around the opening, since re-laying hand-made clay tiles around a new window is slower and more skilled work than dressing a flashing onto a modern concrete profile. Those two lines on a quote account for a large share of the difference between two prices for what looks like the same window.
Keep reading
More on roof window installation
Rooflights in a trussed roof and what cannot be cut
Read the guide →Rooflight reveals and why splaying them matters
Read the guide →Two rooflights side by side on one slope
Read the guide →Rooflights on a shallow pitch below 15 degrees
Read the guide →Underlay and sarking around a pitched rooflight
Read the guide →
Get a quote
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements