Calculate GRP troughs, lead rolls, or valley tiles for roof valleys
Roof valleys are the internal angles where two sloping roof surfaces meet, channelling rainwater from a large area of roof down to the gutters. They are one of the most leak-prone areas of any roof, which makes choosing the right valley system and ensuring correct installation absolutely critical. In the UK, there are three main approaches to forming roof valleys: GRP (glass-reinforced polyester) troughs, lead-lined valleys, and tile valleys (also called laced or swept valleys). GRP valley troughs are the most popular choice for domestic roofing in the UK. Made from moulded fibreglass, they are lightweight, rot-proof, and quick to install. Standard GRP troughs come in 3-metre lengths and are overlapped by a minimum of 125mm at each joint. They sit on top of the valley boards and the roof tiles are cut to line up with the edges of the trough, leaving a visible channel that carries water to the eaves. Lead-lined valleys represent the traditional premium option. Using Code 5 lead (2.24mm thick, 25.40 kg/m2), lead valleys are formed from sheets laid in bays of no more than 1.5 metres to allow for thermal expansion. The standard width is 600mm, with the lead dressed under the tiles on each side. Lead valleys have an expected lifespan of 100 years or more, making them the most durable option, but they are significantly more expensive than GRP and require skilled leadwork to install correctly. The Lead Sheet Association publishes detailed installation guidance. Tile valleys (laced or swept valleys) use specially cut main roof tiles to form the valley, with each course of tiles sweeping across the valley line. This creates a subtle, visually appealing valley with no visible trough or channel, but it requires considerable skill to execute and is the most labour-intensive option. Tile valleys are most commonly seen on high-end properties, listed buildings, and in conservation areas where a traditional appearance is important. This calculator helps you determine how many GRP troughs, lead rolls, or valley tiles you need based on your valley length, the number of valleys, and a waste allowance.
To calculate valley materials: 1. Measure the valley length in metres. This is the distance from the ridge (top) to the eaves (bottom) measured along the valley line itself, not the horizontal distance. If you cannot access the roof, you can estimate by measuring the internal angle of the building at ground level and calculating the slope length using the roof pitch. A valley on a 35-degree pitch roof with a 5-metre horizontal run from the ridge to the eaves is approximately 6.1 metres long. 2. Enter the number of valleys. Count every internal angle on your roof where two slopes meet. A simple L-shaped house typically has 2 valleys. A T-shaped house may have 4 valleys. A dormer window creates 2 small valleys on either side. 3. Select the valley type. GRP troughs are the most common and cost-effective choice for domestic properties. Lead-lined valleys are the premium option for longevity. Tile valleys give the most traditional appearance but require specialist installation. 4. Adjust the waste allowance. A 10% waste allowance is standard. GRP troughs cannot be returned once cut, so it is wise to have at least one spare trough per valley. For lead-lined valleys, waste is generated at the overlaps between bays. 5. Review the results. For GRP troughs, the calculator shows the number of 3-metre troughs needed. For lead-lined valleys, it shows the number of 3-metre lead rolls required. For tile valleys, it shows the number of special valley tiles at approximately 3 per metre of valley length. All quantities include the waste allowance.
The valley calculation depends on the valley type: Total valley length = valley length (m) x number of valleys For two 5-metre valleys: total valley length = 5 x 2 = 10 metres. For GRP valley troughs: troughs per valley = valley length / 3 (standard 3m trough length), rounded up. This is calculated per valley to account for the fact that each valley starts with a fresh trough. Total troughs = troughs per valley x number of valleys. For a 5m valley: ceil(5 / 3) = 2 troughs per valley. For 2 valleys: 2 x 2 = 4 troughs. For lead-lined valleys: lead rolls = total valley length / 3, rounded up. Lead is supplied in 3m rolls (though it is installed in bays of no more than 1.5m). For 10m total: ceil(10 / 3) = 4 rolls. For tile valleys: valley tiles = total valley length x 3 (approximately 3 valley tiles per metre), rounded up. For 10m: ceil(10 x 3) = 30 valley tiles. Quantity with waste = base quantity x (1 + waste percentage / 100), rounded up. With 10% waste on 4 GRP troughs: ceil(4 x 1.1) = ceil(4.4) = 5 troughs. The GRP trough calculation is done per-valley rather than from the total length because each valley starts with a new trough at the top. You cannot use a leftover piece from one valley in another valley, so calculating per-valley and then multiplying gives a more accurate material estimate.
When installing GRP valley troughs, ensure the valley boards are straight and level before laying the troughs. A twisted or bowed valley board will cause the trough to sit unevenly, creating potential water ponding and leak points. The tiles on each side of the valley should be cut to leave a minimum 125mm-wide channel in the centre of the trough for water flow. For lead-lined valleys, always follow the Lead Sheet Association's guidance on bay lengths, clips, and underlaps. Lead valleys should be installed over a suitable underlay and fixed with copper or stainless steel clips (never ferrous fixings, which cause galvanic corrosion). In Scotland, Building Standards may have specific requirements for valley detailing -- check with your local authority. For all valley types, ensure the valley extends beyond the fascia line at the bottom to discharge water cleanly into the gutter.