Guide · Roof Window Installation
Lead flashing and weathering detail
Lead is the oldest weathering material still in everyday use on British roofs, and it holds its place for one reason: it can be worked cold into the exact shape of whatever is in front of it. A sheet of lead pressed into the profile of a deep concrete tile stays there. It does not spring back, it does not need heat, and it lasts longer than most of the components around it.
That workability is also why it is easy to get wrong. Lead moves as it warms and cools, and a sheet that has been fixed as though it will stay still will eventually split. This page is about the material and how it is used around a rooflight: the codes, the bay lengths, the fixings, the alternatives, and how to read a detail that has been up there for thirty years. The individual components in a kit, and how they overlap, are covered on soakers, aprons and head flashings. This sits under roof window installation.
Lead codes, and the numbers on the sheet
Milled lead sheet is graded by code, and the code is a thickness. It is also a colour, because every roll is marked with a coloured stripe so the grade is identifiable on site once it is off the pallet.
| Code | Thickness | Colour mark | Typical domestic use |
|---|---|---|---|
| Code 3 | 1.32mm | Green | Soakers, small cover pieces |
| Code 4 | 1.80mm | Blue | Aprons, side flashings, most rooflight work |
| Code 5 | 2.24mm | Red | Exposed positions, wider bays, valleys |
| Code 6 | 2.65mm | Black | Gutters, heavier weathering, some parapets |
| Code 7 | 3.15mm | White | Large flat areas and heavy duty work |
The codes come from an older system in which the number was the weight of a square foot of the sheet in pounds. It survives because it is short, unambiguous and understood by everyone on a roof.
Why code 4 is the domestic default
Code 4 is heavy enough to hold a dressed shape and stay where it is put, and light enough to work by hand without a struggle. For aprons and side flashings around a rooflight on an ordinary house it is the sensible grade, and it is what most proprietary kits use.
Going up a code is a considered decision rather than an automatic improvement. Code 5 is worth it on an exposed slope, on a wider bay, or where the flashing is going to take foot traffic during later work. Going down a code to save material on a main flashing is the wrong economy: thin lead fatigues faster and tears at the fold, which is the one place it must not.
Bay lengths, and the reason lead is not run in long sheets
Lead expands and contracts more than almost anything else on a roof, roughly 0.03mm per metre for every degree of temperature change. On a south facing slope in Essex, surface temperature can swing from below freezing in January to well over 50 degrees in July, and a long sheet moves several millimetres over that range.
If the sheet is restrained at both ends, that movement has nowhere to go and it turns into stress at the fixings. The answer is to break the run into bays with a lap or a joint between them, so each bay is free to move on its own. For a cover flashing in code 4 the working maximum is around 1.5 metres per piece, and for code 5 slightly more.
Thermal movement, and the failure it causes when ignored
Lead that is over-restrained fails in a recognisable way. A fine crack appears along the line of a fold, usually at a fixing, and it grows over successive seasons until it is a split you can see daylight through. It is called fatigue cracking and it is entirely avoidable.
The diagnostic is worth knowing because it is often blamed on the wrong thing. A rooflight with a wet mark appearing at one corner after several dry years is rarely a failed unit. More often it is a long piece of lead that was run in one length and has now split where it was pinned. Correct bay lengths at installation cost nothing and remove the failure mode entirely.
Lead does not wear out; it gets held too tightly and tears.
Lead clips, welts and how a sheet is held down
The head of a flashing is fixed and the rest of it is allowed to move. That is the underlying principle, and everything else follows. Fixings are placed along the top edge only, at reasonable centres, so the sheet hangs and expands downwards.
The free edge is held against wind uplift with clips rather than nails. A clip is a strip of lead or a compatible metal, fixed to the substrate and turned over the edge of the sheet, so the sheet is restrained but not pinned. Where two bays meet on a slope, the joint is a lap of at least 100mm on the steeper pitches, increasing as the pitch falls. On very shallow work a welted joint is used instead, with both edges folded together and dressed flat.
Dressing lead, and the tools that shape it
Lead is worked with wooden tools, not metal ones. A dresser for general shaping, a bossing stick and a mallet for working a corner, and a setting-in stick for putting a clean fold along a line. Wood spreads the load and does not cut into the surface.
The skill is in moving the material rather than stretching it. Lead that is thinned by being beaten in one place is weaker exactly where it was worked hardest, which at a rooflight corner is the point taking the most water. A properly bossed corner has the same thickness as the sheet it came from, achieved by gathering the material rather than driving it.
Chases, cover flashings and where a joint is made
Where a rooflight upstand meets a wall, or where a kerb meets a vertical face, the weathering is made in two parts. An upstand carries water off the horizontal surface and turns up the vertical one. A cover flashing comes down over it from above, tucked into a chase cut in the masonry or into a brick joint.
The two are not fixed to each other. The overlap between them, usually 75mm to 100mm, is what weathers the joint, and leaving them separate is what allows both to move independently. Sealing them together with mastic to make them look tidy converts two free components into one restrained one, and it is a very common way of building in a future split.
Wedges, pointing and the top edge of a flashing
The top edge of a cover flashing goes into a chase around 25mm deep, is turned back on itself to form a lip, and is held with lead wedges driven into the chase at 300mm to 450mm centres. The wedges do the holding. Only then is the chase pointed.
Pointing without wedges is the shortcut that gives it away. Mortar alone has nothing to grip, the lead moves behind it, and within a few winters there is a fine gap along the top of the flashing that takes water directly into the wall. A modern flexible pointing compound over properly driven wedges is a durable answer; the compound on its own is not.
Patination oil, and the white staining it prevents
New lead oxidises unevenly in its first weeks, and rain running off it carries a white lead carbonate that stains everything below. On a rooflight that means streaks down the tiles and, on a rear extension, down a rendered wall or a glazed elevation.
Patination oil is applied to new lead as it goes on, in a thin even coat, and it prevents that first flush of staining while the surface develops its own stable grey. It takes a few minutes. Skipping it produces marks that take a couple of years to weather away and which nobody looks at without assuming something is wrong.
Run-off from lead onto other materials
Lead run-off is mildly acidic and it does not sit well with everything it lands on. Two combinations are worth knowing about on a domestic roof.
The first is aluminium. Water running off lead onto an untreated aluminium surface, such as a proprietary trim or a rainwater component, will corrode it over time, so the two are separated or the aluminium is protected. The second is any bituminous or single ply flat covering below a lead detail, where a long-term drip in one spot slowly marks the membrane. Neither is a reason to avoid lead. Both are a reason to think about where the water goes after it leaves it.
Lead-free flashing materials, and how they compare
There are several alternatives now, and the good ones are genuinely good. Most are a reinforced aluminium or polymer mesh laminate with a bonded surface, supplied in rolls, and they stretch to take the shape of a covering rather than being dressed into it.
They are lighter, they carry no theft risk, and on a deep profile tile a self-adhesive expanding flashing forms the profile faster and more consistently than hand dressing. What they give up is longevity: a well laid lead detail can run to a century, and no laminate claims that. For most rooflight work the proprietary kit supplied with the unit will already have made this choice, and both routes are used.
| Consideration | Milled lead | Lead-free laminate |
|---|---|---|
| Working life | Very long, measured in decades | Shorter, typically warranted in years |
| Forming | Dressed cold by hand | Stretched and rolled into profile |
| Weight | Heavy, needs bay lengths | Light, longer runs possible |
| Theft risk | Real on accessible roofs | None |
| Staining | Needs patination oil | No run-off staining |
Proprietary flashing kits, and the lead inside them
A rooflight flashing kit is a boxed set of pre-formed components sized to the unit and matched to the covering. Depending on maker and model, the pliable pieces in that box are either milled lead or a lead-free equivalent, and the rigid pieces are usually a coated aluminium.
The kit is designed and tested as an assembly, which is the argument for using it complete rather than substituting pieces. Where a job genuinely needs something outside the kit, at an awkward abutment or where a unit sits close to a chimney stack, that is where hand-worked lead earns its place alongside the kit rather than instead of it.
Laps, and the minimum overlap on each pitch
Every horizontal joint in a lead detail has a lap, and the lap increases as the pitch decreases, because water moves more slowly and wind has more chance to drive it back up. As a working guide, a pitch above 30 degrees takes around 100mm, a moderate pitch takes 150mm, and below about 15 degrees the lap grows again and a welted joint becomes preferable.
The same logic governs how far a flashing turns up an upstand and how far it is dressed down onto the covering. Shallow slopes are covered in their own right on rooflights on a shallow pitch, where the laps stop being a detail and become the governing constraint.
Lead on a listed or conservation roof in Essex
In Thaxted, Coggeshall, Saffron Walden and the older cores of Maldon and Chelmsford, the flashing material is sometimes a matter of consent rather than preference. Conservation officers generally expect lead on a historic roof, dressed in traditional bay lengths, with a slim flush frame set into the plane of the covering.
That combination, a conservation rooflight weathered in code 4 lead, is the detail that usually gets approved. It is slower to install than a modern kit and it is also the detail most likely to still be sound when the building changes hands twice. Establishing what is expected before anything is ordered is quick and avoids an expensive change of direction.
Handling, health and site practice with lead
Lead is safe to work with under ordinary controls and hazardous if those controls are ignored. The route into the body is ingestion and dust rather than skin contact, so the practice is straightforward: no eating or drinking with lead on the hands, wash before breaks, and avoid creating dust by grinding or burning it on site.
Offcuts are collected rather than left, both because loose lead on a roof finds its way into a gutter and because it has scrap value and attracts attention. On an accessible roof, that last point is worth designing around, and it is one of the reasons lead-free products have taken so much of the low-level market.
Reading a lead detail that has aged
A sound old flashing has an even grey surface, sits flat to the covering, and has visible bay joints at sensible intervals. It may look dull and it may carry some surface roughness, and neither means anything is wrong.
What is worth a second look is a fine dark line along a fold, a lifted or rippled edge with no clips holding it, a chase where the pointing has gone and no wedges are visible, or a single sheet running three or four metres with no joint in it. Those four observations describe most of the flashing trouble on domestic roofs, and all four are set at installation rather than by the weather. It is the same principle as everywhere else on a rooflight: the unit is the small half of the job, and the perimeter decides how long it stays dry.
Keep reading
More on roof window installation
Rooflights in a trussed roof and what cannot be cut
Read the guide →Soakers, aprons and head flashings explained
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 →
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