Roof valleys: planning the detail that carries water from two slopes

Rain drains across an ordinary roof area in separate streams. In a valley, runoff converges from both sides, while leaves and snow may restrict the channel. This makes the detail different from an ordinary sheet joint.

The usable valley width must suit the pitch, slope length and contributing roof area. A detail serving a small dormer may be inadequate for a large roof. A view from the yard cannot establish its suitability.

Follow the final drainage section too: water leaving the valley must enter the intended gutter. Correct flashing higher up cannot help if the stream overshoots the gutter or reaches an unsuitable eaves corner.

Before fitting the flashing, establish stable support, fixing positions and continuity of the roofing underlay. Water beneath the covering also needs an outlet. The membrane must not form a pocket that collects moisture.

During renovation, expose and inspect the timber beneath the old valley. Prolonged damp may be invisible from outside. First identify its cause and assess the structure; new metal over weakened support does not solve the underlying problem.

Arrange sections so the upper one sheds water onto the lower one. Lap dimensions and sealing depend on the system and pitch. A single overlap value should not be transferred indiscriminately between different roofs.

Fixings must not puncture the principal water channel at arbitrary positions. Long metal components expand and contract with temperature. The detail must accommodate this movement so rigid restraint does not deform the flashing.

The cut line must leave a clear drainage channel. Excessively projecting roof-tile sheets obstruct flow and cleaning, while too much clearance exposes the detail. Establish the edge position for the actual covering and roof geometry.

Small modules are easier to handle, but diagonal cuts still demand accuracy. Allow for offcuts with long sheets. Use approved tools, remove swarf and avoid leaving narrow ends without adequate support.

Select seals for the profile and their purpose: limiting wind-driven rain, snow or debris. They must not obstruct drainage or necessary ventilation. Filling the entire valley with foam defeats its essential function.

Investigate a leak before adding another layer of sealant. Sealant alone cannot correct a defective overlap or an undersized channel. Compatibility with the substrate, surface cleanliness and application conditions also matter.

Inspect the channel, flashing edges, coating and fixings, particularly after leaves have fallen. Arrange safe access. Walking on wet roofing or positioning a ladder so it damages the gutter introduces additional hazards.

Before repairs, photograph the whole valley area rather than only the visible stain. Water travelling beneath the covering may emerge below the defect. Acceptance of a complex roof should include the valley terminations and the actual drainage route.