Soakers, aprons and head flashings explained

The three pieces, and the order water meets them

Water on a pitched roof travels downhill, so read the set from the top. It arrives first at the head flashing, which is tucked under the course of tiles or slates above the window and turned up behind them. That piece carries water sideways and delivers it onto the side flashings.

The side flashings carry it down past the frame, each piece overlapping the one below, and deliver it onto the apron. The apron takes everything and sheds it onto the covering underneath, far enough down the slope that it cannot run back under the tile it has just left. Three components, one continuous route, and no point at which water is asked to travel uphill.

The apron at the cill, and the distance it has to reach

The apron is the piece people can actually see, a shaped tray across the bottom of the frame that turns up against the cill and lays down over the covering below. It is the one part of the set carrying the whole roof’s worth of water off the unit.

Its dressed length down the slope is not arbitrary. On a plain tile roof at 40 degrees it needs to land at least one full course below the frame so its lower edge is over a tile that is itself lapped twice. On a shallower pitch it gets longer, because water leaves the apron with more sideways momentum and less downward pull. Manufactured aprons for a low pitch kit are visibly deeper than the standard part, and that extra depth is the whole difference between the two products.

The apron also has to be dressed to the profile of what it lands on. On a flat covering it lies flat. On a pantile or a deep concrete profile it has to be worked down into the trough of every tile it crosses, because a flat apron bridging a roll leaves a gap the width of a finger with sky at both ends.

Soakers: one piece per course, cut and interleaved

A soaker is a small L-shaped piece of lead or coated aluminium, one leg turning up against the side of the frame and one leg lying under the slate or tile beside it. There is one soaker per course, and each one laps over the head of the soaker below by the same amount the covering itself laps.

They exist because slate and plain tile are laid in a double lap with a level gauge, so there is nowhere for a continuous flashing to sit without standing proud. Interleaving one piece per course keeps the covering flat and puts a hidden metal channel down each side of the opening. Nothing of it is visible from the garden, which is why a soakered side is the neatest detail available on a slate roof.

Cutting them is slow. On a Cotswold roof, where the gauge changes course by course up the slope, every soaker is a different size and each one is cut on the scaffold rather than taken from a bundle. That is a real cost difference and it gets said at survey.

Continuous side flashings, and where a kit uses one instead

A continuous side flashing is a single formed length running the full height of the frame, turned up against the side of the unit and dressed out over the covering. Manufactured kits for profiled coverings use these rather than soakers, and there is a good reason for it.

On a concrete interlocking tile or a pantile, the covering already has depth and a side channel of its own. A continuous flashing sits down in the tile’s own drainage route and works with it, and there is enough profile depth to hide the metal. The flashing is dressed into the tile profile with a bossing stick or, on a preformed part, arrives already shaped for a named tile family.

On a flat covering a continuous side flashing would stand proud of the slates for two metres, which looks wrong and lifts every slate it crosses. That is why the same manufacturer sells two different side details for what looks like the same window.

The head flashing, and the course it has to vanish under

The head flashing is the piece nobody sees and the piece most often shortened. It laps under the covering above the window by at least 150mm, and on any pitch below about 25 degrees that figure goes up.

Getting it right depends on where the frame sits relative to the batten gauge. If the head of the frame lands 40mm below a batten, the flashing has almost nothing to tuck under and the installer is tempted to face-fix it and rely on sealant. That is the joint that lets go in ten years. The correct answer is to move the frame position at setting out so the head lands in a sensible place in the gauge, which is decided before the covering comes off, not after.

The head flashing also has to be turned up at the back, behind the batten or against the frame, so that water arriving with wind behind it hits a vertical face rather than running straight over a horizontal edge.

Fixed at the head only, and the reason for it

Lead moves. A one metre length expands and contracts measurably between a frosty night and a July afternoon on a south-facing slope, and it does that cycle several thousand times over the life of a roof.

Fix it at the top edge and it slides freely along its length as it moves. Fix it at both ends, or nail it down each side, and the metal has nowhere to go, so it works against its own fixings until it splits along the nail line. Cracked lead running in a straight line parallel to a row of nail heads is one of the most reliable findings when an old unit is taken off, and it is always a fixing pattern problem rather than a material problem.

Piece lengths are limited for the same reason. Code 4 lead in a flashing is kept to about 1.5 metres per piece, and anything longer is made up as two lapped pieces rather than one heroic length.

The frame’s own drainage channel, and what has to feed it

Every manufactured roof window has a drainage channel formed into the outside of the frame, usually a moulded gutter behind the cladding on each side that runs down to a discharge point at the cill.

That channel is designed to take the small quantity of water that always gets past a covering, and it only works if the flashing pieces are dressed into it rather than over it. Side flashings turn into the channel. The underlay tongues turn into it too. Get that wrong, lay the side flashing so it discharges over the outside of the frame instead, and the water bypasses a working gutter and lands on the timber below.

This is also why the cladding pieces at the sides of a VELUX or Fakro frame go on in a set order, with each clipped over the one below it. Details of the frame components themselves are on the VELUX window installation page.

The underlay underneath everything

Below the flashing set is the roofing underlay, and it is the second line rather than a redundancy. Some water always crosses a pitched covering in wind, and the membrane is what carries it down and out at the eaves.

At an opening the membrane is cut in a shape rather than slit in a straight line. A tongue is taken up over the head of the frame and fixed to the batten above so water on the membrane is carried over the window rather than into it, and the side pieces are dressed down into the frame’s own channel. The full detail of that layer, including what to do with old bitumen felt and with sarking boards, sits on the underlay and sarking around a pitched rooflight page.

Recessed installation, and what it does to the set

A recessed kit sets the frame roughly 40mm deeper into the covering so the glass sits closer to flush. It looks better on a slate roof and it changes every piece of the flashing set.

The apron becomes deeper because it starts lower. The side flashings have a taller upstand against the frame. The head flashing has further to reach before it gets under the course above, so the batten gauge above the opening matters more than it does on a standard depth installation. On a plain tile roof at a modest pitch the recessed detail can run out of room entirely, and the honest answer is that the standard depth kit is the one the roof will take.

Where the covering is too thick for any kit at all

Cotswold stone slate is 20 to 30mm thick and laid in diminishing courses, so the gauge changes every course up the slope. No manufactured flashing kit is drawn for that, because a kit assumes a repeating module and a covering a few millimetres thick.

The answer is a hand-formed lead set: an apron bossed to sit over stone rather than tile, soakers cut individually to each course depth, and a head flashing sized to whatever the gauge above the opening happens to be. Even then a standard frame stands proud of a 25mm slate in a way that reads as an object bolted onto the roof, which is exactly the problem a conservation rooflight was designed to solve. The conservation rooflight page sets out how those frames sit low enough to make the flashing work.

Abutments, verges and valleys, where a set runs out of room

A flashing set needs clear covering on all four sides. Put the unit too close to something and one of the three pieces has nowhere to go.

Against a wall abutment there is already a stepped flashing and a cover flashing in the brickwork, and a window side flashing arriving into that junction has to be worked as one detail rather than two overlapping ones. At a verge the side flashing needs room before the undercloak or dry verge unit starts. In a valley the two are competing for the same water, and a window set closer than about 600mm to a valley gutter puts a lot of concentrated flow across the side of its own frame. Position is decided with those clearances measured, not eyeballed from the ground.