Lead Roofing: A Guide to Valleys, Flashings and BS6915 Standard

Lead is the oldest roofing material still in active use in the UK — and for good reason. Correctly installed lead flashings, valleys, and dormer coverings outperform every modern alternative on lifespan. The problem is that lead work varies enormously in quality, and lead installed below the BS6915 standard fails early and expensively. Here’s what the standard means and why it matters. Call 0129 670 6028.

Where Lead Is Used on a Roof

Flashings. The weatherproofing seal between the roof slope and any vertical surface: chimney stacks, dormer walls, parapet walls, and abutments. Step flashings run up the side of chimney stacks; back gutters sit behind them; soakers tuck under individual slates or tiles at the junction. Together they form the watertight junction between roof and wall.

Valleys. The trough formed where two roof slopes meet. Open valleys (where the lead is visible as a channel) handle high water volumes better than closed valleys (where tiles or slates are cut close) and are the correct specification for most pitched roof applications.

Dormer coverings. Flat dormer cheeks and flat tops are commonly lead-covered on period properties. Lead is flexible enough to form complex shapes and expands and contracts predictably with temperature — essential for surfaces with significant thermal movement.

Bay windows. Bay roofs are frequently lead-covered on Victorian and Edwardian properties across Aylesbury's town centre. Bay lead commonly fails at the abutment with the main wall — the same junction where chimney flashings fail.

What BS6915 Specifies

BS6915 is the British Standard for the design and installation of lead sheet in building. It specifies: minimum lead thickness by application (Code 3 for soakers, Code 4 for flashings, Code 5 for valleys and flat coverings); maximum bay length (to accommodate thermal movement without buckling or splitting); minimum lap dimensions; fixing methods and spacing; and detailing at abutments and outlets.

The most common failures in substandard lead work are: lead that is too thin (Code 3 used where Code 4 or 5 is specified — it splits under thermal cycling); bays that are too long (lead expands by approximately 1.2mm per metre per 10°C temperature change — long bays buckle and split); and inadequate fixing that allows wind uplift. All of these are prevented by correct specification at the outset.

Lead Theft and Alternatives

Lead theft from roofs remains a significant issue in the UK. On accessible bays, porches, and church-style buildings, lead is stripped by opportunistic thieves and is difficult to secure without specialist deterrents. Alternatives to lead — including zinc, copper, and GRP-formed valleys — are appropriate in some applications and should be considered on high-risk properties. We advise on a case-by-case basis.

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Frequently Asked Questions

Correctly specified and installed lead work to BS6915 standard has a lifespan of 60–100 years in UK conditions. Lead on Victorian and Edwardian properties in Aylesbury that has never been disturbed or repaired incorrectly often still performs well after 100+ years. Lead that was installed below standard — wrong code, wrong bay length, inadequate fixing — can fail within 10–15 years.

A standard open lead valley replacement on a 3-bed semi, including scaffold, typically costs £500–£1,200 depending on valley length and access. Valley replacement on complex roofs or where structural repairs are also needed will be higher. Free survey provided.

For localised failures — a single joint that has lifted, or a small crack — targeted repair with lead wedges and sealant can be appropriate. Where the lead has corroded through or where multiple sections are failing, replacement is the better long-term choice. We survey and advise honestly rather than defaulting to replacement.

Code 3 (1.32mm) is correct for soakers under slates or tiles. Code 4 (1.80mm) is the standard for step flashings and back gutters. Code 5 (2.24mm) is correct for valleys, flat roof coverings, and bay tops. Using Code 3 where Code 4 or 5 is specified is a common cost-cutting measure that results in early failure. Any reputable contractor will specify the correct code in the written quote.

Lead Roofing: A Guide to Valleys, Flashings and BS6915 Standard

Where Lead Is Used on a Roof

What BS6915 Specifies

Lead Theft and Alternatives

Frequently Asked Questions

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