If you have been quoted for lead and the quote says "code 4" or "code 5", you may be wondering what the number means and whether it matters. It does. The code is the single most important specification on a lead job, because it sets the thickness of the sheet, and thickness decides how well the metal copes with movement, weather and wear. A flashing in too thin a code can crack in a few winters. The same flashing in the right code can outlast the brickwork it is fixed to. Here is how the system works and how the codes are matched to the job.
What a lead code is
Lead sheet for roofing is made to a British and European standard, BS EN 12588, which defines a series of thicknesses numbered from code 3 to code 8. The higher the number, the thicker and heavier the sheet. Each code has a colour, printed as a stripe on the back of the roll, so a leadworker can tell at a glance what they are holding. The industry body that publishes the guidance on which code to use where is the Lead Sheet Training Academy, whose recommendations most good leadworkers follow.
Nominal thicknesses run from about 1.3 mm for code 3 up to about 3.6 mm for code 8, with the codes in between rising in steps. The difference may look small on paper, but a code 5 sheet carries far more metal per square metre than a code 3, and it behaves quite differently under temperature change and foot traffic.
| Code | Colour stripe | Approximate thickness | Typical use |
|---|---|---|---|
| 3 | Green | 1.3 mm | Soakers, small vertical flashings, some cladding |
| 4 | Blue | 1.8 mm | Flashings, aprons, dormer cheeks, hips and ridges |
| 5 | Red | 2.2 mm | Valleys, gutters, flat roofs, bays |
| 6 | Black | 2.7 mm | Larger flats, gutters and exposed bays |
| 7 | White | 3.2 mm | Heavy-duty gutters and exposed roofs |
| 8 | Orange | 3.6 mm | Specialist and heavy work |
Treat the table as a guide and not a specification for your roof. The actual code depends on the detail, the length of the piece, the exposure and the building.
Why thickness matters
Lead expands and contracts with temperature. Over a year, and over each day, every piece of lead moves. Thin sheet stretches more easily and fatigues sooner: each cycle work-hardens the metal at the point of greatest stress until a crack forms. Thicker sheet is stiffer, stands up better to wind lift and foot traffic, and tolerates more cycles before fatigue. But it is also heavier, dearer and harder to dress into tight corners, so a thicker code is not simply better.
The art lies in choosing the lightest code that will do the job safely, and then limiting the length of each piece so movement is shared across joints and never concentrated at one. Our guide to how long lead flashing lasts shows how often cracking traces back to a wrong code or an over-long run.
Code 3: soakers and light work
Code 3 is the thinnest roofing lead in common use. Its natural home is small, hidden, light-duty pieces such as soakers, the folded pieces slipped under slates or tiles at an abutment. Soakers are short, protected from the sun and not walked on, so thin lead copes. Our page on soakers and step flashing shows how they work. Code 3 is not used for exposed flashings, valleys or anything that sees weather directly.
Code 4: the general-purpose flashing
Code 4 is the workhorse for exposed flashings and aprons around chimneys, cover flashings along walls, small dormer cheeks and hip and ridge rolls. It is stiff enough to resist wind and thick enough to last, yet light enough to dress around steps and corners. When someone quotes for replacing chimney leadwork, code 4 is the usual answer for the flashings, apron and stepped sides, with perhaps code 5 for a back gutter behind the stack.
Code 5: valleys, gutters, flats and bays
Code 5 steps up for places where water runs and movement is larger: valley linings, parapet and box gutters, back gutters, small flat roofs and bay window roofs. A valley carries the run-off of two slopes, so it gets constant duty. A bay roof is exposed and has many joints. The extra thickness of code 5 gives these surfaces the stiffness and life they need. Our page on lead bay roofs and canopies shows how code 5 is laid there, and what a lead valley is explains the valley detail.
Codes 6 to 8: heavy duty
Codes 6, 7 and 8 are used on larger flat roofs, long gutters, exposed locations and specialist work such as church and cathedral roofs. They are heavy to carry and hard to bend, so they only appear where size or exposure demands it. On an ordinary house, code 6 is occasionally specified for a large flat or a gutter that takes a lot of water, and codes 7 and 8 almost never.
Length limits and why they matter
The code is half the specification. The other half is the maximum length of each piece. The Lead Sheet Training Academy publishes recommended maximum lengths for each detail and code, based on how far a piece can expand without stress. In general, flat roofs and gutters are limited to shorter lengths than flashings, and the lengths shrink as the pitch drops. A piece cut longer than the guidance, even in the right code, will creep and crack.
That is why a good leadworker puts in drips, rolls and laps at set intervals. They are not decoration. They break the roof into lengths that can move independently. If a quote shows a single large sheet where the guidance calls for several, ask why.
What happens with the wrong code
- Too thin for the position. The lead stretches under its own weight on a vertical surface, thins at the corners and splits at the first hard winter.
- Too thick for a tight detail. The sheet will not dress cleanly, leaving gaps at corners and a bulky edge that lifts.
- Right code, wrong length. Thermal movement builds at the ends and tears the fixings.
- A uniform code used everywhere. A job priced as "all code 4" or "all code 5" has probably been simplified to save time, not matched to the details.
Cheap lead is not always cheaper in the long run. Code costs rise with thickness because of the weight of metal, so there is pressure to specify low. A fair quote shows the code for each element, for example code 4 for the flashings and code 5 for the back gutter.
A worked example: one chimney, three codes
Take a typical brick chimney on a pitched tiled roof, with the stack standing clear of the ridge. The apron across the lower face and the stepped flashings up each side would be code 4, dressed over the tiles and turned into the mortar joints. The soakers beneath the side flashings, slipped between the tiles, could be code 3 because they are covered. The back gutter behind the stack, which collects water running down the uphill side and carries it round, would be code 5, since it carries concentrated flow and is the hardest working piece on the chimney. A cover flashing along the top edge of the gutter might return to code 4.
The same chimney quoted in a single code, whatever the code, has been simplified. That may still work, but it is worth asking what drove the choice. If the answer is price, you are trading a lower bill today for a shorter life on the pieces that work hardest.
Codes and appearance
Thickness also changes how the lead looks once fitted. Thinner sheet shows ripples and the outline of whatever it is laid over, such as the edges of tiles or rough brick joints. Thicker sheet holds a straighter, cleaner line, which is why visible work on period frontages is often specified a code higher than strength alone would demand. Where lead is seen from the road, for instance on a bay or a parapet, the extra thickness is partly a decision about appearance. On a chimney at the back of the house, the cheaper reading of the guidance is usually fine.
Fixings matter as much as the sheet. Copper nails or stainless steel are used with lead, because galvanised nails corrode where they pass through. Clips are cut from the same code or a thicker one, so they do not fatigue before the main sheet does.
What to check on a quote
Ask for the code for each element in writing. Ask the maximum piece lengths and where the drips or rolls will be. Ask how the top edge is fixed and what the lead sits on. Compare quotes on code as well as on price, because two quotes for "new lead flashing" that differ in code are not the same job. Published UK guides put replacing chimney leadwork at an average near £570 within a range of £350 to £920. Those are typical UK ranges, and the code for each piece is only settled at the survey, when the real quote is written.
Cost aside, heritage matters too. On a listed building or in a conservation area, the code and the detail may be set by the council's conservation officer. Our page on leadwork on period and listed homes covers it. If you are weighing lead against other metals, our guide to lead alternatives compares them.
How we specify
We choose the code for each piece after we have seen the junction, not before. We note the exposure, the length of run, the wall and the access, and write the code into the quote line by line. Photographs from the survey show your own junctions, and the price that follows is fixed on paper. The 10-year workmanship guarantee is part of the paperwork. For the whole picture of lead work, see our lead roofing and leadwork page, and to start, ring 01256 213956 or use WhatsApp.
