Lead Valley Gutters: Open vs Closed Valleys, Minimum Width, Code Selection and Drip Details

Summary

A valley is formed wherever two roof slopes meet at an internal angle. Water from both slopes converges at the valley and must be directed to the eaves or gutter without infiltrating the roof structure. In UK domestic roofing, the most common solutions are the open lead valley (lead exposed between cut tiles on each side), the swept or laced valley (tiles interleaved to shed water without exposed lead), and the mitred valley (tiles cut and butted). Lead is by far the most durable material for open valleys and is the material of choice on traditional and heritage roofs.

Valley gutters are among the highest-stress leadwork details because they combine large catchment areas, concentrated water flow, and full thermal cycling. The volume of water channelled through a valley on a heavy rainfall day can be substantial — a 30 m² roof area draining to a single valley gutter means every millimetre of rainfall delivers 30 litres into that gutter. Insufficient gutter width causes water to back up and overtop the lead, soaking the roof structure. Insufficient bay lengths with inadequate drip details cause the lead to creep, crack, and eventually leak.

A common misconception among homeowners (and some less experienced contractors) is that any valley detail that "looks fine" is performing adequately. Lead valley failures are characteristically slow and insidious: creep cracking begins at the fixed laps, water infiltration begins as a slow seep into the sarking felt beneath, and by the time damp shows at the ceiling the structure may have been wet for years. Correct sizing, code selection, and drip detailing from the outset is the only way to ensure the valley performs for its intended 60-year lifespan.

Key Facts

• Open valley — lead exposed, tiles cut to a straight line each side; most common in UK domestic work; allows inspection and maintenance
• Swept/laced valley — tiles interleaved at the valley angle; no lead exposed; traditional finish; more labour-intensive; tile breakage risk over time
• Mitred/closed valley — tiles cut and butted at the valley centre; no lead exposed; least durable; not recommended on new work
• Woven/bonded valley — tiles interlocked alternately from each slope; traditional; appropriate for plain tiles and hand-made clay tiles on heritage work
• Minimum gutter width (open valley) — 125 mm clear at the head of the valley (apex); widens toward the eaves as catchment area increases; Lead Sheet Manual provides a table based on roof area and pitch
• Code 4 minimum for domestic valleys — standard for domestic pitched roofs with moderate catchment
• Code 5 for larger catchments — required where the valley collects from a total roof area of approximately 40 m² or more (verify against Lead Sheet Manual table)
• Maximum bay length (Code 4) — 1,500 mm; longer bays will creep and crack
• Maximum bay length (Code 5) — 2,000 mm; the greater thickness provides more resistance to thermal stress
• Drip height (minimum) — 50 mm; the upstand at the upper edge of each bay that laps over the drip of the bay below
• Drip overlap — upper bay laps over lower bay by a minimum of 50 mm at the drip; typically 75 mm
• Valley board — a purpose-made timber board running the full length of the valley rafter to provide a flat, solid surface for the lead; typically 150–225 mm wide depending on valley width; must be fixed before lead installation
• Tilt fillet — a triangular timber fillet at the head of the valley where it meets a wall or ridge; used to tilt the lead and prevent water tracking under the lead at the apex
• Valley gutter outlet — at the foot of the valley, the lead must terminate over or directly into the eaves gutter; a lead apron or outlet piece is used to prevent water dropping behind the fascia
• Thermal movement — Code 4 lead in a 1,500 mm bay will move approximately 3.1 mm between winter and summer extremes; drip details must accommodate this movement without the lap closing completely

Quick Reference Table

Note: These are indicative figures. Always consult the Lead Sheet Manual valley width table using actual roof area and pitch.

Detailed Guidance

Open Valley Gutter Construction

An open valley requires a continuous valley board running the length of the valley rafter. The board provides a flat, solid substrate for the lead; without it, the lead would follow the gap between rafters or sarking boards and would develop uneven bearing that causes cracking.

The valley board is typically 25 mm thick softwood, kiln-dried, treated against rot. Width depends on the valley gutter design: at minimum, the board must accommodate the full gutter width (125–200 mm) plus the lead laps under each tile course. A 150–200 mm board is common for domestic work; wider boards are needed for large catchments.

The lead valley liner is cut to bays of maximum 1,500 mm (Code 4) or 2,000 mm (Code 5). Each bay is folded with an upstand (welt) at the upper end (the drip detail) and is fixed along the edges under the tiles. The lead is clipped at the edges rather than nailed through the face: nails through the face of lead are a known failure point, as the nail hole becomes a stress concentration under thermal cycling.

A lead clip (also called a tack or retaining clip) is a small strip of lead folded over the edge of the valley liner and fixed to the valley board with a single nail. Clips are spaced at approximately 450–600 mm centres and are hidden under the tile margin. The clip retains the valley liner at its edges while allowing it to move longitudinally.

Drip Details: Why They Matter and How to Form Them

The drip is the critical detail that allows each bay of valley liner to move independently while maintaining a watertight joint. Without drips, a continuous run of lead would attempt to act as a single piece; with drips, each 1,500 mm bay can expand and contract freely.

A drip is formed at the upper end of each lower bay. The lead is turned up into a step — minimum 50 mm high — and dressed to be vertical. The upper bay slides over this step and laps it by a minimum of 50 mm (preferably 75 mm). The upper bay is not fixed to the lower bay; it is free to slide down over the drip as it contracts in cold weather and up over it as it expands in summer.

Common failures at drips include:

• Drip too shallow (less than 50 mm): the upper bay slides off the drip during expansion and the lap opens
• Upper bay nailed through to the lower bay: thermal movement is constrained and the lead cracks at the nail
• Drip formed with a fold instead of a step: a folded drip collapses under foot traffic or when roof tiles are walked on during maintenance

On a long valley (say 6 m), three or four bays are needed (four bays at 1,500 mm maximum). That means three or four drip details to form. The labour for forming drips correctly is a real cost and should be allowed for in the quote.

Calculating Valley Catchment Area

The catchment area feeding a valley gutter determines the minimum gutter width and the minimum lead code. Catchment area is calculated from the plan area of each roof slope draining to the valley.

For a simple hip roof, the catchment area feeding a valley is approximately:

Catchment = 0.5 × roof width × valley length × cos(pitch angle)

In practice, most leadworkers and specifiers use the simpler rule of thumb: measure the plan area (horizontal projection) of each slope draining to the valley and sum them. For a domestic roof with slopes of 15 m² and 12 m² meeting at a valley, the total catchment is 27 m², which falls in the Code 4, 150 mm minimum width category.

For complex roofs with multiple drainage areas converging, or where dormer cheek flashings drain into a valley, add each tributary catchment to the valley total before selecting gutter width and code.

Valley Board and Tilt Fillet Requirements

The valley board must be:

• Fixed firmly to the rafters with no movement; any rocking board will cause the lead to work and crack at the fixing points
• Flat along its surface; any twist will cause the lead to bridge, leaving an unsupported span that will puncture under foot traffic
• Treated against rot; the valley board sits in a condensation-prone location and must not decay beneath the lead

At the head of the valley where it meets a ridge, a tilt fillet is used. This is a triangular timber batten nailed to the valley board at the apex. The fillet raises the tail of the first bay of lead slightly, directing water flow away from the joint and preventing water tracking up behind the lead at the apex under hydrostatic pressure. The fillet height is typically 25–40 mm.

At any abutment at the head of the valley (e.g. a valley meeting a chimney or a parapet), a back gutter or step flashing is required to redirect water from the wall face into the valley. This detail is often poorly executed on refurbishment work: the valley is replaced but the back gutter at the head is left in its original (degraded) state, creating a leak point that may be attributed to the new valley work.

Common Failure Modes and How to Avoid Them

Laps too short at drips: the upper bay rides over the drip initially but contracts in winter to expose a gap. Always achieve a minimum 75 mm lap at drips, not the 50 mm minimum.

Insufficient gutter width: most commonly seen on older properties where the original valley was narrower than current guidance requires. On replacement work, widen the valley board if necessary — a 25 mm strip of additional boarding costs almost nothing compared to a callback.

No drips on long runs: occasionally seen where a leadworker has tried to run a single 3 m bay to save time. A 3 m bay of Code 4 will develop creep wrinkles within a few years and eventually crack through at the stress concentrations.

Valley board inadequately supported: if the valley board is not fixed to every rafter it crosses, it will spring under foot traffic, causing point-load stress in the lead above.

Tiles cut too close to the lead: if tiles are cut to within 50 mm of the valley centre, mortar and debris from the cut edge falls into the gutter and blocks it. Tiles should be cut to leave a clear gutter of the specified minimum width, with the cut edge a minimum of 50 mm from the valley centre line.

Outlet detail omitted: at the foot of the valley, the lead must direct water clearly into the eaves gutter. If the lead simply terminates at the fascia, water will run behind the fascia in heavy rainfall. A lead apron or extended tongue piece lapping over the back of the gutter resolves this.

Swept and Laced Valleys: When to Use Them

Swept and laced valleys are appropriate for heritage work and for roofs where the visual character requires no exposed lead line. They rely on the careful interleaving of tiles to shed water, with an underlayer of lead or underlay providing secondary protection.

A swept valley requires tiles that can be cut to an angle without excessive breakage; plain tiles and handmade clay tiles are well-suited. Concrete tiles can be swept but the cut edge appearance may be considered unsatisfactory on heritage work.

For an LCA-qualified leadworker, a swept valley still requires a lead valley board liner (typically Code 4) underneath the tile interleave: the tile weaving provides the primary weathering but the lead provides the backup for wind-driven rain. On new work, it is more common for architects and surveyors to specify an open lead valley because it is inspectable and more easily maintained.

Frequently Asked Questions

Can I use a proprietary plastic valley system instead of lead?

Proprietary GRP or UPVC valley systems are available and widely used on budget domestic work. They are typically acceptable under Building Regulations for new work and carry product warranties. However, they are generally not accepted on heritage or listed building work, and their lifespan (20–30 years typically) is shorter than a correctly installed lead valley (60+ years). Some principal contractor frameworks and insurance requirements specify lead for high-value properties.

How do I determine the correct valley width for my specific roof?

Use the valley width table in the Lead Sheet Manual (published by the Lead Sheet Association). The table gives minimum clear gutter width as a function of catchment area in m² and roof pitch. If the Lead Sheet Manual table is not available, apply the indicative figures in the table above and err on the side of a wider gutter.

Can I lay lead valley liners directly on the sarking felt without a valley board?

No. Lead must be laid on a continuous, flat, solid substrate. The valley board is not optional. Lead laid directly on sarking felt will have no support at the rafter gaps, will flex under thermal movement and foot traffic, and will crack at the unsupported spans within a few years.

What is the minimum pitch for a lead valley gutter?

There is no absolute minimum pitch in BS 6915 for valley gutters (unlike gutters and flat roofs which have minimum fall requirements). However, on very shallow pitches (below about 20°) the water velocity through the valley is lower and the risk of debris accumulation blocking the gutter increases. On shallow pitches, increase the gutter width above the minimum to provide a safety margin.

Do I need to use Code 5 just because the roof is large?

Not necessarily. The determining factor is the total catchment area draining to the specific valley, not the overall roof area. A large roof with multiple valleys may have a relatively small catchment per valley. Use the Lead Sheet Manual table to calculate the required width and code for each valley individually.

Regulations & Standards

• BS 6915:2001 (with amendments) — Design and construction of fully supported lead sheet roof and wall coverings; contains valley gutter width tables, bay length limits, and drip detail guidance

• BS EN 12588:1999 — Rolled lead sheet standard; Code 4 and 5 specifications

• Building Regulations Approved Document C — Resistance to moisture; valley gutter performance as part of the roof weathering system

• NHBC Standards Chapter 7.2 — Pitched roofs; valley gutter requirements for new-build warranty

• Lead Sheet Association Lead Sheet Manual — Definitive valley gutter sizing tables, drip detail illustrations, and code selection guidance

• Lead Contractor Association — LEADSKILLS Level 2 covers valley gutter installation in detail

• BSI BS 6915 — Full standard text

• bs en 12588 lead sheet — Code numbers 3–8, thicknesses and colour coding

• lead soakers and cover flashings — Soaker and cover flashing details at abutments

• lead dormer flashings — Dormer back gutters, cheek flashings, and step-and-cover details

• lca leadskills qualification — LEADSKILLS Level 2 covers valley gutter installation