Valley Trough Calculator — GRP, Lead & Dry Valley

GRP (fibreglass) dry valley troughs are the most common and cost-effective choice for modern UK tiled and slated roofs. Lead is used for traditional or conservation work — but use Code 5 (2.24mm) as the minimum for valleys: BS 6915 and the Lead Sheet Association advise that Code 4 is too thin for valley gutters and will fail. Use Code 6 (2.65mm) on exposed sites or large/high-rainfall catchments.

The standard supply length for GRP and dry-valley troughs is 3,000mm (3m), confirmed by Klober (KR966000, BBA 12/4928), Marley and Hambleside-Danelaw. They are lightweight, rot-proof and easy to cut. For a typical 4m valley you would need 2 sections with a pitch-based end-lap. Lead is different — it is supplied in rolls and laid in length-limited bays (1.5m for Code 5, 2.0m for Code 6) per BS 6915 to control thermal creep.

The end-lap between GRP/dry-valley sections is set by roof pitch, not a flat figure. Using the Klober KR966000 datasheet: 150mm for rafter pitches 40°–60°, 200mm for 22.5°–39.5°, and 250mm for 17.5°–22° (17.5° is the minimum pitch for a GRP dry valley). Hambleside-Danelaw quote up to 300–350mm on the shallowest profiled-tile pitches. Lead bays lap 150mm with welts/steps. Always lap in the direction of water flow — upper section over lower.

Tile-and-a-half tiles are wider tiles (approximately 1.5× normal width) bonded in at valley and verge junctions so the raking cut never leaves a narrow sliver that would be weak and prone to cracking. This calculator orders one tile-and-a-half per course per side as a conservative upper bound (BS 5534 setting-out). Note that on double-lap plain tiling the tile-and-a-half is bonded into alternate courses, so the actual manufactured-tile requirement is usually lower — order to the cutting layout.

Yes. GRP/dry-valley troughs sit on continuous valley boards (or ply) that run the full length of the valley with a board each side of the centre, plus a batten to support the trough flange. This calculator reports the continuous board run (both sides). This differs from field battens at 450–600mm centres — the valley itself needs continuous support so the trough cannot deflect under load. The trough is then nail-fixed at 500mm centres each side (Klober/Hambleside-Danelaw).