Roof valleys and valley flashing

How it works

1. valley gutter with a central rib and upturned edges
2. roof covering
3. sealing strip on both sides of the valley
4. continuous boarding or close battening (gaps under 10 cm)
5. tiling battens
6. fixing with clips at the edges only, never through the centre of the valley

Drawing no.

KV-02

Title

Roof valley, cross-section

Scale

diagram, exposed width in mm

Revision

14 October 2026

Water from both slopes runs to the centre line of the valley and down it to the eaves. On both sides the covering stops a little short of the centre to leave a clear channel. Under the edges of the covering there is a sealing strip that stops driven rain, snow, dust, insects and birds from getting under the covering.

Types of valley

Installation rules

• Substrate. There must be continuous boarding or close battening under the valley. Ruukki gives gaps between battens of less than 10 cm down the slope, so the valley can carry snow and ice.
• Underlay. In the valley the roofing underlay is laid with laps alternating from both sides, so that there are three layers at the valley. The underlay is never pierced at or near the centre of the valley.
• Laps between lengths. MASLEN gives a 200 mm lap (not 100 mm as for other flashings), with sealant at the joint.
• Fixing with clips along the edges, not with a nail or screw through the surface.
• Exposed width 200–300 mm once the covering is on (Ruukki).
• Sealing strip stuck to the upper edge of the valley gutter on both sides.
• At the ridge the valley can be lapped over the ridge.

The most common mistakes

Maintenance

Valleys clog with leaves and needles faster than the rest of the roof. Check them at least in spring and autumn, especially if there are trees near the house. A blocked valley spills water over the edge of the sheet and under the covering.

Cost

Plechy Majer (14 April 2026) lists a valley among the system components for its metal roofing at €12 per m incl. VAT (materials). A made-to-measure valley is priced by girth and material. Labour is usually charged together with the rest of the roof flashing; Numiq (14 May 2026) gives €12–35 per m excl. VAT for roof flashing.

Questions and answers

How wide should a valley gutter be?

For non-standard valleys made to measure, Ruukki gives a girth of 625 to 1,250 mm and an exposed valley width of 200 to 300 mm once the covering is on. The width depends on the area draining into the valley and on the pitch.

Why must a valley never be nailed through the middle?

All the water runs down the middle of the valley. Any hole there lets water straight into the roof. Valleys are therefore fixed with clips along the edges.