Lead flashing and weathering detail
Lead is used around rooflights because it does something no other common building metal does: it can be beaten cold into the exact shape of whatever it has to sit on, and it will hold that shape for a hundred years. A sheet of lead dressed down over the roll of a pantile takes the roll’s profile permanently. That malleability comes with a condition. Lead moves a great deal with temperature, it work-hardens if it is bent back and forth, and it fails in entirely predictable ways when it is fixed as though it were rigid. Almost every cracked flashing found when an old rooflight comes off was cracked by its own fixings rather than by weather. This page is about the metal itself: codes, sizes, movement, fixings, staining and the alternatives. How the three components of a flashing set fit together around a frame is on soakers, aprons and head flashings explained, and the wider job sits on the roof window installation page.
Milled lead to BS EN 12588, and what a code number is measuring
Rolled lead sheet for building work is manufactured to BS EN 12588 and sold by code number. The code is not a grade or a quality rating. It is a thickness, inherited from the old imperial system where the number was the weight of a square foot of the sheet in pounds.
| Code | Thickness | Weight | Colour marking | Where it belongs around a rooflight |
|---|---|---|---|---|
| Code 3 | 1.32mm | About 15 kg per square metre | Green | Soakers only, and code 4 is better even there |
| Code 4 | 1.80mm | About 20 kg per square metre | Blue | The standard for aprons, side and head flashings, and soakers |
| Code 5 | 2.24mm | About 25 kg per square metre | Red | Exposed slopes, larger aprons, and anything acting as a gutter |
| Code 6 | 2.65mm | About 30 kg per square metre | Black | Gutters, valleys and long dressed runs beside a unit |
| Code 7 and 8 | 3.15 and 3.55mm | About 36 and 40 kg per square metre | White and orange | Heavy leadwork, rare on a domestic rooflight |
Choosing a heavier code buys durability and resistance to damage. It does not buy the right to make a piece longer, which is the mistake people make with it. The length limits below apply to code 8 exactly as they apply to code 4.
The colour marking on the roll, and reading a delivery
Rolled lead sheet arrives with a coloured stripe or marking along the edge that identifies its code, and the roll carries a label with the code, the thickness and the manufacturer. It is worth checking on delivery, because 1.80mm and 2.24mm are not distinguishable by eye or by hand and a roll of code 3 in the wrong place around a rooflight is a very slow failure.
Reclaimed lead has no marking at all, and its thickness varies across a single sheet where it has been dressed before. It has a place in conservation work where the appearance of old lead matters, and it is measured with a gauge rather than assumed. On a new rooflight installation the material is new milled sheet of a stated code, and that code goes in the specification alongside the unit.
Thermal movement, and the number that every fixing rule comes from
Lead expands and contracts more than almost anything else on a roof. Its coefficient of linear expansion is around 0.0000297 per degree Celsius, which is roughly double that of steel.
Put real numbers on it. A 1.5 metre length of lead flashing on a south facing Gloucestershire slope might sit at minus 5 degrees on a January night and reach 55 degrees on a July afternoon under direct sun. That 60 degree swing moves the piece about 2.7mm along its length. It does that every day, and over the life of a roof it does it something in the order of twenty thousand times. Lead that is free to slide simply slides. Lead that is pinned at both ends has to absorb 2.7mm of movement somewhere in its own body, so it work-hardens at the restraint and eventually splits. That single figure is the origin of every rule that follows: fix at one edge, limit the piece length, and let the free edge move.
Maximum piece sizes, and the joint that has to be built in
The Lead Sheet Association’s guidance sets maximum piece sizes for exactly the reason above, and 1.5 metres is the working maximum length for a flashing in any code. Girth, meaning the developed width of the piece once it is unrolled flat, is limited too, generally to around 300mm for a flashing and less for a piece with a lot of bends in it.
Around a rooflight this bites on the side flashings of a tall unit. A 1400mm long window has a side flashing under the limit and can be made in one piece. A 1600mm unit cannot, and it is made as two pieces with the upper lapping the lower by at least 100mm. Fitters who dislike the joint sometimes run one heroic 1.8 metre length instead. It looks better on the day and it splits somewhere around year twelve. Where a run of two or more units side by side needs a shared flashing, the piece lengths are planned before the metal is cut, and combination arrangements are covered on two rooflights side by side on one slope.
Bossing against lead welding, and when each is right
Lead is shaped in two ways. Bossing is the traditional method: the sheet is worked cold with a bossing stick, a dresser and a mallet, stretching and gathering the metal so a flat sheet becomes a three dimensional corner with no joint in it at all. Lead welding fuses two pieces together with a filler rod of lead and an oxy-acetylene flame, producing a joint as strong and as thick as the parent metal.
Around a rooflight the corners of an apron are the decision point. A bossed corner has no joint and no heat affected zone, and on a small apron it is quick. A welded corner is dimensionally exact and is the better answer where the geometry is awkward, where the piece is code 5 or heavier, or where a bossed corner would thin the metal too much. What is not acceptable is a folded corner held with sealant, which is what turns up on hurried work and which opens as soon as the metal starts moving.
Nails, wedges and clips: the three ways lead is held
All three appear on a single rooflight installation, and each has its own spacing.
- Head fixing into timber. Copper or stainless clout nails along the top edge of the piece and nowhere else, so the metal hangs and slides. On a lead sheet head fixing the nails go in two staggered rows at close centres; on a light flashing a single row along the top edge does the job.
- Wedges into a chase. Where a flashing turns into masonry beside a rooflight set close to an abutment, the turn-in goes into a chase raked out to about 25mm and is held with folded lead wedges at no more than 450mm centres, then pointed.
- Clips on a free edge. A long free edge of lead lifts in wind. Copper or stainless clips, or lead clips of a heavier code, are fixed at around 500mm centres and reduced to 300mm on an exposed elevation such as a western slope in the Forest of Dean.
Plain steel and galvanised nails have no place in any of it. Galvanised zinc in contact with lead in a wet joint sets up a small electrolytic cell, the zinc goes first, and the fixing is gone long before the lead is.
Patination oil, and the run marks it prevents on the covering below
New lead reacts with rainwater and carbon dioxide to form lead carbonate, a loose white powder. It washes off in the first rains and runs down the slope, leaving white streaks across the tiles or slates below the new work. On a red clay roof or a dark Welsh slate those streaks are conspicuous and they take a year or two to weather away.
Patination oil applied to both faces of the lead on the day it goes in prevents the reaction from starting, by keeping water off the surface long enough for a stable protective film to form instead. It is a five minute job with a cloth and it is skipped constantly. It goes on all new lead, including the parts that will be buried, because condensation reaches the underside as readily as rain reaches the top. Where a rooflight sits high on a slope with a large area of visible covering below it, the oiling is not cosmetic detailing. It is the difference between finished work and work that looks unfinished for two summers.
Underside corrosion, and the lead that goes from below
The less familiar failure mode. Where lead is laid over a surface that traps moisture and there is no ventilation behind it, condensation forms on the underside and sits there. Organic acids from timber, particularly unseasoned or acidic species, dissolve into that moisture and attack the metal from behind. The top face looks perfect and the sheet is thinning underneath.
It matters at a rooflight where lead is being dressed over sarking boards or over a timber kerb rather than over a ventilated batten cavity. The answer is an isolating layer: a geotextile underlay or a building paper between the lead and the timber, which separates the metal from the substrate and gives any moisture somewhere to go. On flat roof kerb detailing the same principle applies, and the kerb build-up sits on the flat roof rooflight installation page.
Mortar, cedar and moss: the three things lead does not sit well against
Fresh cement mortar is strongly alkaline and it attacks lead. Where a flashing beside a rooflight has to be bedded or pointed against new mortar, the lead is isolated with a bituminous coating on the buried face, and pointing is done after the mortar has taken its initial set rather than bedding the metal into wet mix.
Water running off western red cedar shingles or cladding is acidic and will corrode lead over time, which comes up on the occasional timber clad extension in the Stroud valleys. And run-off carrying moss and lichen acids from a north facing tiled slope above a rooflight does something similar at a slower rate. None of these is a reason to use a different metal. They are reasons to know what is upstream of a flashing before specifying its code, and to go up a code where the run-off is aggressive.
Girth, and setting a flashing out before anything is cut
Girth is the flat, unfolded width a piece needs to be cut to before it is bent. Getting it right is arithmetic done on a scrap of board, and getting it wrong wastes an expensive material.
An apron for a rooflight might need 50mm turned up against the cill of the frame, 25mm of turn at the top edge for a fixing, and 150mm dressed down over the covering, plus an allowance at each bend. The girth is the sum, not the visible dimension. On a profiled covering there is an extra allowance because the sheet has to travel down into and back out of every trough it crosses, which on a deep roll tile can add 15 to 20 per cent to the girth across the width. That allowance is the reason a flashing over a Sandtoft bold roll takes noticeably more lead than the same flashing over a flat tile.
Milled, cast and sand cast lead, and where each belongs
Almost all building lead today is milled: rolled from a slab to a consistent thickness, uniform, and available in every code. It is what goes around a new rooflight.
Cast lead is poured onto a bed of sand, giving a sheet of varying thickness with a textured underside and a slightly rippled face. It costs considerably more and it is specified on historic buildings where the appearance and the working characteristics of the original material are part of what is being conserved. On a Cotswold stone slate roof at Painswick or Bibury, a conservation officer may ask for cast lead in a visible position. On a rear slope at Longlevens nobody is asking, and milled code 4 is the correct and honest answer. Machine cast lead sits between the two: cast rather than rolled, but to a consistent thickness.
Lead free alternatives, compared without loyalty
There are good reasons to reach for something other than lead, and there are jobs where lead remains clearly better.
| Material | What it is | Strengths | Limits around a rooflight |
|---|---|---|---|
| Milled lead, code 4 or 5 | Rolled lead sheet | Dresses to any profile, exceptional life, fully recyclable | Heavy, movement rules must be obeyed, attractive to thieves |
| Coated aluminium flashing | Thin aluminium with a polyester or bitumen coating | Light, comes in the manufacturer’s own kit, colour matched | Less forgiving over deep profiles, thinner section |
| Reinforced flexible flashing | Aluminium or polymer mesh in a butyl or bitumen carrier | Stretches over profiles, quick, no bossing needed | Relies on an adhesive bond and on surface preparation |
| Lead substitute sheet | Aluminium mesh laminated in a polymer | Dresses like lead, far lighter, no theft interest | Movement behaviour differs, and it looks like what it is up close |
| Copper or zinc | Formed metal sheet | Long life, suits a metal roof it matches | Cannot be bossed the way lead can, needs different jointing |
Manufactured rooflight kits are supplied with coated aluminium components and they perform properly when installed as drawn. Lead comes into its own where no kit exists: a thick stone slate roof, a hand formed set around a conservation unit, an awkward junction with an abutment, or a very deep tile profile. On a Cotswold roof there is no alternative to a hand-cut lead set, and that is set out on the conservation rooflight installation page.
Theft, and the visible lead on a Gloucestershire roof
Lead has a scrap value and roofs get stripped. Rooflight flashings are a poor target compared with a church roof or a bay window, because there is very little metal in them and it is fixed under a covering. It still comes up as a question, particularly on isolated properties in the Forest and the wolds.
Where it is a genuine concern, a lead substitute in the visible positions removes the incentive entirely, and security marking systems exist for larger areas of leadwork. On most domestic rooflight installations the honest answer is that the exposed lead amounts to a couple of kilograms of apron and it is not worth anybody’s night. It is worth raising rather than ignoring, because a customer who has had lead taken from a porch roof will ask.
Working with lead on a scaffold, safely
Lead is not hazardous to have on a roof and it is hazardous to eat or inhale. The practical rules on a rooflight installation are simple and they are followed as a matter of course: no cutting with an abrasive disc that generates fine dust, no burning or welding without extraction and proper ventilation, gloves while handling, and hands washed before eating or drinking. Off-cuts go into a bin rather than onto the ground where a child or a dog might find them.
The other practical point is weight. A roll of code 5 is heavy and awkward, and getting it up a scaffold in a Stroud valley lane where the wagon has stopped fifty metres away is part of what makes those jobs longer. Access is priced from what the survey saw, which is covered on the costs page.
What a finished lead detail is checked against
Before the covering goes back over it, the leadwork around a rooflight is inspected against a short list, and every item on it is invisible afterwards.
- Code correct for the position, and the material new milled sheet of a known thickness.
- No piece longer than 1.5 metres, with any joint lapped at least 100mm, upper over lower.
- Fixed along one edge only, with copper or stainless fixings and no galvanised steel anywhere.
- Free edges clipped at 500mm centres, tightened to 300mm on an exposed slope.
- Corners bossed or welded, with no folded corner relying on sealant.
- Every lap facing downhill, with at least 150mm of cover onto the covering below.
- Isolating layer between lead and any timber it lies directly on.
- Patination oil applied to both faces on the day.
- No lead bedded into wet cement mortar without an isolating coat.
None of that can be inspected once the tiles or slates are back on, which is precisely why it is checked at the time and photographed. The workmanship guarantee is written against details of exactly this kind, and the reason it can be written at all is that they are done in a fixed order every time.
Next on this subject.
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