What is a roof valley?

A roof valley is the internal angle formed where two sloping roof planes meet, running diagonally from the point where the two ridges or planes intersect down to the eaves. Valleys occur at L-shaped or T-shaped roof plans, at dormers, and where extensions meet the main roof. They channel the combined rainwater from both adjoining slopes into a concentrated flow, making them one of the most waterproofing-critical parts of a roof. Valleys must be lined with a continuous, durable weatherproof material.

What are the different types of valley?

An open valley leaves the lining material (lead, zinc, or GRP) visible as a channel between the two roof slopes - tiles or slates are cut to follow the valley line at each side. A secret (or closed) valley hides the lining under the tiles or slates by interlacing them across the valley, with the lining concealed below - this gives a neater appearance but requires careful cutting and lapping. A swept valley (also called a laced valley) uses purpose-made shorter tiles interlaced across the valley to follow its diagonal line. A valley tray uses a purpose-made GRP or aluminium channel laid under the tiling to channel water.

What is a valley rafter?

A valley rafter is the diagonal structural timber that runs from the point where two roof planes meet (the internal corner) down to the wall plate, forming the structural spine of the valley. Jack rafters from each of the two adjoining roof slopes frame into the valley rafter from either side. Valley rafters must be deeper than the common rafters framing into them, typically by 50mm, as they carry loads from two slopes. A valley board (a wide flat board) is fixed to the top of the valley rafter to provide a bed for the valley lining.

Valleys are the most leak-prone part of a roof because they receive concentrated water flow from two large roof areas. Common causes of valley leaks include: lead lining that has crept (thermally expanded downhill over time) and pulled away from its fixings or underlay at the top; GRP valley trays that have cracked or delaminated from age; tiles cut too close to the valley gutter leaving insufficient clearance; debris (moss, leaves) blocking the valley and causing water to back up under the tiles; and insufficient valley lining width. A minimum 150mm width of lining should remain visible on each side of a clear open valley.

Roofing noun

Valley

/ ˈvali /

Also known as: roof valley, valley gutter, open valley, secret valley, swept valley

Definition

A valley is the internal angle where two sloping roof planes meet, channelling rainwater from both slopes down to the eaves. Valleys occur wherever an L or T-shaped roof plan creates an internal corner - at side extensions, dormers, and changes of roof direction. Because they carry concentrated water flow, valleys are the most waterproofing-critical part of a tiled or slated roof and must be lined with lead, zinc, GRP, or a proprietary valley tray system to prevent leaks.

In practice

A valley is formed structurally by a valley rafter (deeper than the common rafters) running diagonally down from the internal ridge intersection to the wall plate, with a valley board on top to provide a flat bed for the lining. Lead-lined open valleys (Code 4 or Code 5 lead, minimum 600mm wide) are the most durable option - correctly formed lead valleys can last 60-100 years with good detailing. GRP valley trays are a common modern alternative - quicker and cheaper to fit, with a typical 25-year service life.

For swept (or laced) valleys - where purpose-made narrower tiles interlock across the valley line - no visible lining is required, but an underlay is still essential below the tiles. Tiles or slates on either side of an open valley must be cut to follow the valley line with a minimum 75mm clearance from each cut edge to the centre of the gutter to prevent tiles directing water under the lining. Blocked valleys from moss, leaves, and debris are a very common cause of roof leaks - annual inspection and clearance is recommended.

Building Regulations

Relevant PartPart C - Resistance to weather

Lead standardBS EN 12588 - Lead and lead alloy sheet for building

Min lining width600mm total; min 150mm visible gutter each side

Lead codeCode 4 minimum (1.8mm thick) for valley gutters

Approved Document C requires roofs to resist weather penetration - valleys are a critical waterproofing junction and must be correctly detailed to comply. The Lead Sheet Association and NFRC publish detailed guidance on valley lining specifications. On listed buildings and in conservation areas, lead valleys are often required by planning conditions even where GRP would otherwise be acceptable. Lead valleys in excess of 1.5m in length must be formed in separate bays with movement joints to prevent cracking from thermal expansion and contraction.

Full Building Regulations guidance

Valley

Just so you know...