Lead Fixings and Tacks: Copper Clout Nails, Stainless Clips, Spacing and Avoiding Electrolytic Corrosion

Summary

Lead sheet is one of the most durable roofing materials in existence — correctly installed leadwork on a church or historic building can last 100 years or more. That longevity depends entirely on correct detailing, and one of the most common causes of premature failure is the wrong fixing. Tradespeople who have spent years on other roofing materials sometimes reach for whatever nails or screws are on the van. With lead, that habit is expensive. Galvanised nails, zinc-plated screws, and ordinary bright wire nails will all react with lead in the presence of moisture, creating an electrolytic cell that accelerates corrosion of both the fixing and the surrounding lead.

The problem is invisible until it is too late. The reaction begins at the point of contact and works outward through the metal. By the time a roof starts to lift or weep, the fixings may be almost entirely consumed. The correct fixing materials — copper and stainless steel — avoid this by sitting in a compatible or neutral position in the electrolytic series. This article covers the full specification: fixing types, dimensions, clip spacing, substrate considerations, and how to avoid bi-metallic contact elsewhere in the assembly.

This guidance applies to all leadwork trades: roofers installing gutters, valleys, dormers and flat roof coverings; plumbers working on flashings, soakers, aprons and parapet cappings; and any trade carrying out repairs or extensions to existing lead sheet work. The LCA Manual (available from the Lead Sheet Association, now trading as Midland Lead) is the definitive industry reference, and BS 6915 is the British Standard for design and construction of sheet lead coverings.

Key Facts

• Approved fixing metals — copper and stainless steel only; no zinc, no galvanised, no aluminium, no mild steel
• Copper clout nails — 40mm × 3.35mm is the standard size for fixing lead tacks and clips to timber; annular ring shank preferred for improved pull-out
• Stainless steel grade — austenitic stainless steel grades 304 or 316; grade 316 preferred in coastal environments due to superior chloride resistance
• Lead tack dimensions — minimum 50mm × 50mm, cut from Code 3 lead sheet (1.32mm thick, 14.97 kg/m²)
• Clip spacing — rolls — maximum 500mm centres along the length of a roll or bay
• Clip spacing — drips — at least one clip per bay within 50mm of the drip edge
• Exposed locations — reduce clip spacing to 450mm centres in exposed or high wind locations (NHBC and LCA guidance)
• Number of clips per sheet — a 1.5m-wide bay with two rolls requires a minimum of four clips top and bottom
• Stainless steel clips — factory-made lead or stainless retaining clips are available as an alternative to site-cut lead tacks; minimum 50mm × 25mm stainless flat bar at 1.5mm thickness
• Self-tapping screws — A4 stainless steel self-tapping screws used where clips fix to metal substrates (e.g. copper or stainless steel gutters); minimum 30mm length
• Pull-out values — timber — square-sawn C16 softwood at 38mm minimum depth offers approximately 0.6–0.8 kN pull-out per 40mm clout nail (verify with structural engineer for exposed/high-load applications)
• OSB pull-out — OSB/3 at 18mm offers lower pull-out than sawn timber; minimum two nails per clip recommended
• Plywood pull-out — WBP plywood at 18mm offers similar pull-out to OSB/3; again minimum two fixings
• Never nail through lead sheet directly — nailing through the body of the sheet (rather than through a clip or tack) creates a fixed point that prevents thermal movement and leads to fatigue cracking
• Bi-metallic contact — even without direct fastener contact, lead resting on a dissimilar metal deck without isolation can still corrode over time through run-off or galvanic coupling

Quick Reference Table

Detailed Guidance

Why Electrolytic Corrosion Happens

Electrolytic (galvanic) corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte — in roofing terms, that electrolyte is rainwater. The metals form a galvanic cell, and the less noble metal (the anode) corrodes preferentially. In the electrolytic series, zinc sits well below lead; when zinc contacts lead in the presence of water, the zinc corrodes rapidly. Galvanised nails and screws are zinc-coated mild steel, so both the coating and eventually the steel beneath are attacked.

The rate of corrosion depends on the relative surface areas of the two metals and the conductivity of the moisture present. A small zinc fixing (anode) in contact with a large lead sheet (cathode) concentrates the attack on the fixing — the nail wastes away while the lead may appear undamaged until the fixing pulls through. In industrial or coastal atmospheres where rainwater has higher conductivity, the reaction is faster.

Copper sits much closer to lead in the electrolytic series. Copper is slightly more noble than lead, meaning lead is the anode and copper the cathode — lead does experience very mild corrosion at direct copper contact, but the rate is negligible in practice. The LCA Manual confirms copper as the preferred fixing material for lead sheet, used for generations on historic buildings without issue. Stainless steel is also acceptable; it is effectively inert in the electrolytic series and does not react meaningfully with lead.

Copper Clout Nail Specification

Standard copper clout nails for leadwork are 40mm long with a 3.35mm shank diameter and a large flat head (approximately 9mm diameter). The large head distributes the fixing load across the clip or tack and prevents the nail pulling through soft Code 3 lead. Annular ring shank nails are preferred over smooth shank because the rings provide significantly better withdrawal resistance — important in exposed locations where wind uplift creates sustained pull-out forces on clips.

Nails should always be driven into the substrate at a slight angle (5–10 degrees) away from the direction of lead movement to reduce the risk of the nail working loose as the sheet expands and contracts thermally. Two nails per clip is the minimum for Code 3 tacks; single nail fixing is only acceptable for Code 4 or heavier lead tacks with a larger footprint.

Copper nails are softer than steel and can deform if driven with a heavy hammer. Use a medium-weight hammer (about 500g) and drive carefully. Overdriving a copper nail through thin Code 3 lead will tear the tack. If the nail bends during driving, extract it and use a fresh nail rather than trying to straighten it.

Lead Tack Dimensions and Cutting

A lead tack is simply a small piece of Code 3 lead sheet, cut to size on site and nailed to the substrate. The minimum dimensions are 50mm × 50mm. In practice, 75mm × 50mm tacks are common because the extra length provides a larger contact area with the overlying sheet and reduces the risk of the tack being exposed at the edge of the bay.

Cut lead tacks with a sharp lead knife or tin snips — never with an angle grinder. Grinding generates lead dust and fume, which is a COSHH hazard. The cut edge should be clean and straight. Fold a 10mm lip along the edge that will engage with the lead sheet; this folded lip hooks under the sheet edge and prevents it lifting in wind without creating a rigid fixing point that would prevent thermal movement.

When cutting tacks from scrap lead sheet, check the code weight of the scrap — lead tacks cut from Code 3 or Code 4 offcuts are ideal. Avoid very thin offcuts from Code 3 sheet that has been stretched during dressing; stretched lead is work-hardened and more brittle.

Clip Spacing Rules

The 500mm maximum spacing rule for clips applies along the full length of a bay. For a standard 1.5m-long bay, this means a minimum of three clip positions along each roll (at the top, middle, and bottom). In practice, many experienced leadworkers use four positions — at the top, one-third down, two-thirds down, and at the drip edge — which gives a working spacing of approximately 375mm and provides additional security.

At drips, at least one clip must be positioned within 50mm of the drip edge. This prevents the sheet edge curling upward under wind suction, which is the most common mode of lead sheet failure in exposed conditions. Some LCA guidance recommends double clipping within 50mm of drip edges in exposed locations.

In locations classified as severe or very severe exposure (as defined in BS 8104 for driving rain indices, or where the site is within 3km of an open coast at elevations above 100m), reduce the clip spacing to 450mm and add a clip within 25mm of each drip edge. For very exposed upland sites, consult the LCA Manual for additional guidance on heavier code weights and increased fixing frequency.

Substrate Considerations: Timber, OSB, and Plywood

Lead sheet should be laid over a smooth, firm substrate capable of providing adequate nail pull-out. The three most common substrates are:

Square-sawn softwood boarding (typically 25mm finished thickness, C16 grade or better): This remains the best substrate for lead sheet. The grain provides good nail engagement, the surface is stable, and any slight unevenness can be planed. Boards must be dry (moisture content below 18%) before lead is laid, as green timber can shrink and cause lead to buckle. Boards should be laid with a 3mm gap between them to allow for seasonal movement.

OSB/3 (Oriented Strand Board): Acceptable as a substrate but with lower nail pull-out values than sawn timber. OSB/3 at 18mm thickness has adequate pull-out for standard clip loads, but two nails per clip are recommended rather than one. OSB must be the exterior-grade (OSB/3 or OSB/4); interior-grade OSB/2 swells and delaminates when wet. A ventilated void beneath OSB decking is strongly recommended to allow the board to breathe.

WBP plywood: Similar performance to OSB/3. Use minimum 18mm WBP (Weather and Boil Proof bonded) plywood. Plywood is dimensionally more stable than OSB and provides slightly better pull-out values. Avoid hardboard, chipboard, or MDF as lead substrates — all absorb moisture and fail structurally.

For all timber substrates, ensure the boards or panels are positively fixed to the roof structure. Inadequate structural fixings beneath the lead substrate transfer all wind uplift loads to the lead clips and can result in catastrophic failure.

Avoiding Bi-Metallic Contact Elsewhere in the Assembly

Even if the lead fixings are correctly specified, electrolytic corrosion can still occur if the lead sheet itself is in direct contact with incompatible metals. Common situations to check:

• Lead flashing against a zinc-coated steel wall profile: Separate with a strip of 2mm-thick EPDM or neoprene tape, or use a lead-lined bituminous felt isolating layer between the lead and the zinc surface.
• Lead apron flashing over an aluminium window frame head: Aluminium corrodes very rapidly in contact with lead. Always introduce an isolating layer — neoprene tape or bituminous felt minimum 75mm wide is the standard detail.
• Lead valley next to galvanised steel nails in timber battens: Where lead valley lining is in contact with nailed battens, use stainless steel nails in the battens within the overlap zone.
• Lead secured with stainless steel clips to a copper gutter: This combination is acceptable; stainless and copper are both noble metals and any galvanic action is minimal.

A useful rule of thumb: if the metal is shiny silver and you cannot confirm it is stainless steel, assume it is zinc-coated and isolate. When in doubt, use bituminous felt between the lead and any unknown metal surface.

Frequently Asked Questions

Can I use galvanised nails if I cannot get copper ones?

No. Galvanised nails in contact with lead will fail within 2–5 years in typical UK conditions, faster in urban or coastal environments. This is not a matter of preference — it is a straightforward corrosion mechanism that has caused countless premature leadwork failures. Copper clout nails are stocked by all roofing merchants and specialist leadwork suppliers. If you are caught short on site, stainless steel A4 screws are an acceptable substitute; do not use what is on the van.

Do lead tacks need to be nailed or can they be held by the overlying lead?

Lead tacks must be positively fixed to the substrate with copper clout nails. A tack that is simply trapped between the substrate and the overlying lead sheet is not secured against wind uplift. In any storm condition, unsecured tacks will pull free, allowing the sheet edge to lift and ultimately causing water ingress and potential sheet loss.

My substrate is a metal deck — how do I fix clips to it?

On a profiled metal deck, fixing into the steel is impractical. Instead, lay a continuous layer of roofing-grade insulation board or timber firring pieces on the deck and fix the clips into the timber overlay. The timber layer must itself be positively fixed to the metal deck using appropriate metal deck fixings. Never rely on the weight of lead sheet alone to hold insulation down in exposed conditions.

How do I know if my existing leadwork has the wrong fixings?

Carefully lift the edge of a sheet bay at the drip. If you can see clips or tacks, examine the fixing nails. Zinc-coated nails will typically show white or grey corrosion products (zinc oxide) and may be visibly wasting. Lead tacks with rusted nails suggest mild steel. If the tacks are loose, crumbly, or separating at the nail hole, this is evidence of galvanic failure. Document the findings and advise the client that re-fixing with correct materials is required.

What if my stainless clips were fitted with galvanised nails?

This combination is common in poor-quality work. The stainless clip is fine, but the galvanised nail fixing it to the timber will corrode, eventually allowing the clip to pull free. If the existing clips are otherwise sound (not cracked or deformed), they can be re-fixed using copper clout nails after removing and discarding the corroded galvanised nails. Check the nail hole in the timber — if it has enlarged due to movement, plug with marine epoxy or relocate the clip slightly.

Regulations & Standards

• BS 6915:2001+A1:2014 — Design and construction of fully supported lead sheet roof and wall coverings; primary British Standard for specification of lead fixings

• LCA Manual (Lead Contractors Association) — Industry code of practice for lead sheet work; covers all aspects of fixing specification, spacing, and substrate requirements

• Building Regulations Approved Document A — Structural requirements including wind uplift resistance; relevant to substrate fixing adequacy

• COSHH Regulations 2002 — Control of Substances Hazardous to Health; relevant when cutting lead and generating dust (see also lead recycling and waste)

• BS 8104:1992 — Code of practice for assessing exposure of walls to wind-driven rain; used for determining exposure category for clip spacing decisions

• Lead Sheet Association Technical Manual — Full technical guidance on lead sheet installation including fixing specifications

• HSE COSHH Essentials for Lead — Occupational health guidance relevant to handling lead sheet

• NHBC Standards Chapter 7.1 — Roofing standards including leadwork; clip spacing and exposure categories

• lead and other metals compatibility — Full guidance on bi-metallic contact and separation details

• lead bossing techniques — Hand-forming techniques that avoid penetrating the sheet

• lead recycling and waste — Disposal of lead offcuts and COSHH controls for cutting

• lead on listed buildings — Heritage considerations and LCA certification