What type of valley trough should I use?

GRP (fibreglass) troughs are the most common and cost-effective choice for UK roofs. Lead code 4 or 5 is used for traditional or conservation area roofs. Dry valley tray systems are used with interlocking tiles and don’t require mortar.

How much overlap do valley troughs need?

GRP and lead valley troughs need a minimum 150mm overlap between sections. Dry valley trays typically need 100mm. Always overlap in the direction of water flow (top section over bottom).

What are tile-and-a-half tiles?

Tile-and-a-half tiles are wider tiles (approximately 1.5x normal width) used at valley junctions. They allow the valley cut to be made without leaving too narrow a tile piece, which would be weak and prone to cracking.

How long are GRP valley troughs?

Standard GRP valley troughs come in 1.5m sections. They are lightweight, rot-proof and easy to cut. For a typical 4m valley, you’ll need 3 sections with 150mm overlaps.

Do I need support battens for the valley?

Yes. Valley troughs sit on support battens (valley boards) fixed at approximately 600mm centres along the valley line. These provide a flat, even surface for the trough to sit on.

TradeCalculator

Valley Trough Calculator — GRP, Lead & Dry Valley

Calculate valley trough sections from roof geometry. Covers GRP, lead code selection, overlap allowances and tile-and-a-half counts.

Valley Length

Length of one valley from ridge to eaves

Number of Valleys

How many valleys on the roof

Valley Type

GRP is most common for modern roofs

Tile Gauge

Tile gauge (batten spacing)

Price Per Metre (optional)

How We Calculate This

This calculator determines the number of valley trough sections needed based on your valley length and the material type. It accounts for overlaps between sections.

The formula

Trough sections = Valley run ÷ (Section length − Overlap)

Tile-and-a-half count = (Valley run ÷ Tile gauge) × 2 sides

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.