Lead Flat Roof Coverings: Maximum Bay Sizes for Each Code, Drips, Hollow Roll and Dog-Ear Details

Summary

Lead flat roofs are one of the most durable roofing systems available — correctly laid and sized lead bays can last over 100 years without needing replacement. The reason they fail prematurely is almost always the same: bays that are too large for the lead code, causing fatigue cracking from repeated thermal expansion and contraction. Lead expands approximately 2.9mm per metre for every 100°C change in temperature. On a south-facing UK roof in summer, surface temperatures can reach 65–80°C; in winter they can fall to −10°C or below. An incorrectly oversized bay on a dark-covered roof can be cycling through 80°C+ temperature swings, causing the lead to work-harden and crack within a few years.

The bay sizing limits in the LCA Manual are not conservative estimates — they are the result of accumulated industry experience with failure patterns. Every millimetre over the maximum bay size shortens the service life of the roof. The most common error made by non-specialist roofers attempting lead flat work is simply rolling out long sheets the full width of the roof without installing intermediate rolls, then wondering why the lead splits along the roll lines within 3–5 years.

Code 5 is the minimum acceptable code for flat roof coverings. Code 4 was historically used on small bays but is no longer recommended by the LSA for flat work because the thinner sheet has less tolerance for fatigue and is more easily damaged during foot traffic and maintenance. Code 6 is used on large areas, accessible roofs, and where longer bay lengths are needed. Code 7 is rarely needed in domestic work but is appropriate for very large or frequently trafficked areas.

Key Facts

• Code 5 minimum — for all lead flat roof coverings; 2.24mm thick, 25 kg/m²
• Code 6 — for large areas, long bays, or accessible roofs; 3.0mm thick, 35 kg/m²
• Code 5 max bay — along slope — 2,250mm
• Code 5 max bay — across slope — 675mm
• Code 6 max bay — along slope — 3,000mm
• Code 6 max bay — across slope — 850mm
• Thermal expansion — lead expands 2.9mm per metre for 100°C temperature rise; 1.5mm/m for a 50°C rise
• Hollow roll height — minimum 50mm
• Hollow roll width — minimum 50mm (diameter of the roll former)
• Drip height — minimum 50mm for Code 5/6
• Welted edge — turned up 50mm and folded back on itself at upstand edges
• Copper clips — used to fix lead at drip edge; Code 4 lead clips or purpose-made copper clips at 450mm centres
• Minimum fall — 1:80 recommended for lead flat roofs; 1:60 preferred; 1:40 on small roofs
• Substrate — minimum 18mm WBP/exterior-grade plywood; tongue-and-groove boarding also acceptable
• Ventilation — adequate sub-lead ventilation required to prevent condensation in habitable spaces below
• Dog-ear corner — hand-bossed corner detail at internal/external angles where the lead cannot be folded cleanly
• LCA/LSA Manual — Lead Contractor's Association (Lead Sheet Association) Manual is the primary technical reference

Quick Reference Table

Detailed Guidance

Code Selection

Code 5 is the correct starting point for any lead flat roof specification. The additional thickness over Code 4 provides greater tolerance for fatigue, better resistance to accidental foot traffic damage, and a more stable sheet that drapes without creasing during installation.

Upgrade to Code 6 in the following situations:

• The roof area (total, not individual bay) exceeds approximately 15m²
• The roof is accessible (used as a balcony, walkway, or maintenance access)
• Bays need to be longer than 2,250mm along the slope to fit the roof geometry
• The roof faces south or south-west and is dark-coloured (higher surface temperatures accelerate fatigue on lighter code)
• High rainfall area where the gutter or drainage has restricted catchment capacity

Code 7 is very rarely specified in domestic work. It is appropriate for large accessible roofs in commercial contexts.

Bay Layout Planning

Before cutting any lead, the roof dimensions must be set out to determine the bay layout. The goal is to divide the roof into bays that do not exceed the maximum dimensions for the chosen code, with roll joints running up the fall direction and drip joints running across the fall direction.

Step 1: Measure the full roof width (across the slope) and divide by 675mm (Code 5) or 850mm (Code 6). Round up to find the minimum number of roll bays required. For example, a 2,400mm-wide Code 5 roof requires at least 4 bays (2,400 ÷ 675 = 3.55, round up to 4).

Step 2: Measure the roof length (along the slope) and divide by 2,250mm (Code 5) or 3,000mm (Code 6). This gives the number of drip steps required. For a 4,500mm-long Code 5 roof, two drips are needed creating three bays of 1,500mm each.

Step 3: Check that the fall is set in the substrate before any lead is laid. On a 4,500mm-long roof at 1:80, the top should be 56mm higher than the outlet.

Each bay is a separate sheet. Rolls run up the slope; drips cross it. The layout should be marked on the substrate before work starts.

Hollow Roll Construction

The hollow roll is the standard joint between adjacent lead bays along the slope direction. It is called "hollow" because the roll is formed over a round timber former that is removed after the lead is dressed, leaving a hollow barrel-shaped joint.

Former: typically a 50mm-diameter hardwood dowel or purpose-made timber roll former, minimum 50mm diameter. Some leadworkers use a 50mm × 50mm timber batten.

Construction sequence:

1. The left-hand bay sheet is dressed up and over the roll former, forming a full half-circle (180° wrap)
2. A Code 4 lead clip is fixed to the boarding, protruding under the former
3. The right-hand bay sheet is dressed up and over the roll, enclosing the full roll and lapping over the clip
4. The former is slid out from the end, leaving the hollow roll in place
5. The clip, now inside the roll, holds the roll down against wind uplift without restricting thermal movement

The finished roll must be a minimum 50mm in height above the roof surface and 50mm in diameter. Undersized rolls allow water to overtop in heavy rainfall. The roll is closed at the drip end by dressing the lead over and folding it into the drip.

Where wood-cored rolls are used instead (a batten that remains in place), the maximum bay across the slope is reduced to 500mm for Code 5 because the fixed core restricts thermal movement, increasing stress at the bay edges.

Drip Construction

The drip is a cross-slope step in the roof surface that separates adjacent bays along the slope direction and provides the primary thermal expansion joint for lengthwise movement.

The drip step must be a minimum 50mm in height for Code 5 and Code 6. The step is formed in the substrate by a timber upstand board fixed across the roof at the required spacing.

Construction sequence:

1. The lower bay sheet is dressed up the face of the drip board and over the top, with a copper-riveted edge or folded drip edge at the base
2. The upper bay sheet overlaps the top of the drip board, lapping a minimum 75mm over the drip
3. The overlap is held by a clip or tack at the top edge

The riveted or welted edge at the base of the drip prevents the lead sheet from rolling back due to thermal expansion or wind. The 75mm minimum overlap on the upper bay prevents wind-driven rain from tracking back up under the higher sheet.

Dog-Ear Corner Detail

At internal or external corners where the flat roof lead meets a wall or parapet, the lead cannot simply be folded flat without puckering — the geometry requires more material than a straight fold can accommodate. The dog-ear (or bossed corner) is the correct detail.

External (convex) corner: The lead is cut at 45° into the corner from the edge, and each triangular flap is dressed down separately onto the vertical and horizontal surfaces. The point where the two flaps meet is bossed (gently worked with a bossing mallet and stick) to produce a smooth, crack-free corner. No solder or weld is required or desirable at domestic scale.

Internal (concave) corner: The lead is cut to allow the material to overlap at the internal angle; excess is removed and the corner is bossed smooth. An internal corner that is forced flat without bossing will crack at the apex of the fold within a few cycles of thermal movement.

Dog-ear corners require skill and take time. They should be included explicitly in any quote for flat lead work. A rushed dog-ear done with a hacksaw and a hammer will fail within one or two winters.

Substructure Requirements

The timber substrate must provide a stable, smooth surface that supports the lead evenly without movement. Minimum requirements:

• Boarding type: 18mm WBP (exterior-grade) plywood preferred; 25mm tongue-and-groove softwood boarding acceptable but must be well-seasoned to prevent shrinkage gaps
• Board direction: boards run across the fall (perpendicular to the slope direction), so joints do not form gutters that direct water under the lead
• Fastening: all boards must be firmly fixed with no movement; loose boards create stress concentration in the lead above
• Surface finish: smooth; no nail heads, screw heads or splinters proud of the surface
• Fall: set in the substrate using firring pieces; never rely on packing under lead
• Separating layer: a layer of building paper or proprietary underlay should be placed between boarding and lead to prevent the lead bonding to the substrate and to allow movement

Condensation and Ventilation

Lead flat roofs over habitable spaces are at risk of interstitial condensation if the construction is not correctly detailed. The principle is the same as for any cold flat roof: warm moist air from the habitable space below must not reach the cold underside of the lead deck where it will condense.

Two approaches:

1. Cold deck (ventilated): insulation placed between ceiling joists below the deck; ventilation cross-path of minimum 50mm between insulation top and deck underside; ventilation equivalent to 1/150th of the roof area provided at eaves and opposite side. The LCA Manual specifies ventilation requirements for cold deck lead roofs.

2. Warm deck (inverted): insulation placed above the lead, held down by ballast (gravel or paving slabs). No condensation risk because the lead is on the warm side of the insulation. Minimum 80mm PIR or 150mm EPS insulation above the deck.

Warm deck is generally the lower-risk option for new construction. Cold deck requires meticulous vapour control and ventilation detailing. Under-ventilated cold deck lead roofs develop condensation on the underside of the boarding, which accelerates timber decay and can be misdiagnosed as a lead leak.

Frequently Asked Questions

Can I reroof over existing lead rather than stripping it?

No. Overlaying new lead on old lead is not acceptable practice. The old lead must be stripped to inspect the substrate, assess the fall, check for rot in the boarding, and ensure correct ventilation. Trapping old lead under new lead also creates a bonded sandwich that prevents the new lead from moving freely, causing premature fatigue failure.

My roof is only 800mm wide — can I use one sheet without a roll?

For Code 5, the maximum across-slope bay dimension is 675mm. An 800mm-wide roof requires two bays (two sheets) with a hollow roll between them. Using a single 800mm sheet would exceed the Code 5 maximum, and thermal expansion would cause the sheet to lift and pucker at its edges within a few seasons.

What fall is actually achievable on an existing flat roof?

On an existing structure, the fall is set by the joists, which are already fixed. On old flat roofs, joist falls of 1:120 or even flatter are common. In these cases, the minimum fall of 1:120 should be treated as the absolute floor; if the actual fall is lower than this, the firring pieces should be used to bring the substrate up to 1:120 as a minimum before laying lead.

How do I close the roll at the eaves?

At the eaves end (lower end), the hollow roll is closed by dressing the lead at the end of the roll over and folding it into the drip edge. The fold should be bossed smooth; no solder is required. At the top end (against a wall), the roll end is turned up behind the cover flashing and wedged.

Is soldering ever used on lead flat roofs?

Lead welding (burning) is occasionally used for outlet sump details and corner patches where bossing is not practical. Soft soldering is not appropriate for structural joints in lead flat roofs — the thermal cycling will fracture soldered joints within a few years. All expansion joints must be achieved by the mechanical details described above.

Regulations & Standards

• Lead Sheet Association (LSA) Manual — Specifies all bay dimensions, roll and drip details, code selection, falls, and substructure requirements for lead flat roofs

• BS EN 12588 — Specification for rolled lead sheet for building purposes; defines all code thicknesses and properties

• Building Regulations Approved Document C — Resistance to moisture; substructure and vapour control requirements

• Building Regulations Approved Document L (Conservation of fuel and power) — Insulation requirements for flat roofs over habitable space; affects warm/cold deck choice

• NHBC Standards Chapter 7.1 — Flat roofs and balconies; minimum lead code and bay sizing in domestic construction

• Lead Sheet Association — Rolled Lead Sheet: The Complete Manual — Primary industry reference for all flat roof bay sizes, codes and construction details

• BS EN 12588 — Rolled lead sheet specification

• NHBC Standards 7.1 — Flat roofs; minimum lead specifications

• lead thermal expansion provision — Full explanation of expansion coefficients, temperature ranges and why bay limits matter

• lead parapet gutters — Gutter design, falls and outlet sizing for the perimeter drainage

• lead chimney flashings — Code selection and upstand sizing principles