Within the anatomy of a roof, few components are as critical as the valley trough. Often seen as just a simple channel for rainwater, the valley is (in reality) a critical point where two roof slopes meet. It handles a concentrated deluge of rainwater, endures significant thermal movement and is a crucial location for combating water ingress. A failure here could mean a catastrophic leak that can compromise an entire building, leading to structural damage, insulation failure and costly interior repairs.
For specifiers, architects and contractors, selecting the right valley trough is just a single part of a roofing specification. This guide provides a detailed checklist to simplify the process, helping you to specify the right valley trough for your next project. We’ll look at a modern GRP vs traditional valley trough comparison, highlighting why GRP valley troughs are the obvious choice for your next project.
What Does a valley trough Do?
A valley trough is a channel installed in the internal angle between two sloping roof surfaces. Its primary duties are:
- Water management – To efficiently collect and channel large volumes of runoff from the adjoining roof areas to the gutter system.
- Weatherproofing – To form a continuous, watertight seal at one of the roof’s most vulnerable points.
- Durability – To withstand environmental stressors like UV exposure, thermal expansion and potential physical impact.
- Aesthetics – To provide a clean, finished line that complements the roof’s design.
Why GRP Valley Troughs Excel
The root of the valley trough debate often centres on material choice. The rise of Glass Reinforced Polyester (GRP) has challenged traditional roofing materials. See below how GRP valley troughs perform and when to use GRP valley troughs.
Glass Reinforced Polyester (GRP)
GRP is a composite material made of a polyester resin reinforced with fine glass fibres. The benefits include:
- Zero corrosion – Impervious to rust and corrosion, making it ideal for harsh environments.
- Lightweight – Significantly lighter than all metal alternatives, reducing the load on the roof structure.
- Seamless and leak-proof – Manufactured in 3 metre lengths, reducing the number of vulnerable end laps and seams.
- Low maintenance – Does not require painting or protective coatings.
- Smooth finish – Offers a hydraulically smooth surface that promotes faster water flow, reduces debris trapping and resists moss build-up.
GRP vs Traditional Valley Trough Comparison
| Feature | GRP Trough | Traditional Troughs |
| Corrosion Resistance | Excellent | Fair to Good |
| Weight | Very Low | Can Be Very High |
| Ease of Installation | Easy – Preformed lengths ready to install | Training is required to install most |
| Thermal Movement | Low | High |
Specifying Valley Troughs – Industrial Roofing Checklist
This checklist is designed to guide you through the key decision-making factors, ensuring your specification is watertight.
Phase 1: Project Analysis and Design
- Roof pitch and water flow rate – Calculate the expected volume of water. Steeper pitches with high flow rates may benefit from the smoother surface of a GRP trough.
- Valley length and geometry – Are the runs long and straight, or are there complex angles and junctions? Consider the shape and type of valley trough you need.
- Environmental and exposure conditions – Is the project coastal (high salt) or in an area with high UV exposure?
- Building use and criticality – What is inside the building? A server room, archival storage, or manufacturing equipment has a much lower tolerance for water ingress.
Phase 2: Material and Product Specification
- Material selection – Based on the analysis in phase 1, choose your material. Justify your choice in the specification documents.
- Dimensions – Specify the valley trough width required to handle the water volume.
- Fixing and support method – Specify how the valley trough will be fixed to the roof.
- Interface detailing – Provide clear details for how the valley trough interfaces with the roof membrane, the gutter system and any roof penetrations or other abutments.
Phase 3: Installation and Workmanship
- Pre-installation storage – Specify that materials should be stored off the ground, under cover and protected from damage and weather.
- Substrate preparation – The supporting roof structure must be straightand secure before valley trough installation begins.
- Installation – A skilled roofer can handle GRP valley trough installation. Follow the manufacturer’s instructions and tips for easier installation.
- Weather conditions – Do not install in adverse conditions that could compromise the installation.
- Post-installation protection – Specify that the installed trough must be protected from subsequent tradesmen working on the site to avoid damage from foot traffic or dropped tools.
Specifying the right valley trough is an engineering decision based on a clear analysis of the project’s unique requirements. Investing in a correctly specified and installed valley system is excellent insurance against potentially devastating water damage.
While traditional metals have a long and proven history, GRP has firmly established itself over the last 40 years as an excellent alternative for many modern applications.
By methodically working through our checklist above, you can move beyond guesswork and create a robust, defensible roofing specification that ensures the valley on your project is a source of confidence and not a liability.
Want to specify GRP valley troughs for your next project? Speak to one of our experts today.
Talk to us about GRP valley troughs
FAQs
What is the most important factor when specifying a valley trough?
There isn’t a single most important factor, but rather a combination. The key is to analyse the project’s specific requirements. The volume of water (based on roof pitch and size), the environmental conditions and the roof’s structural capacity to support weight are all critical starting points. Our checklist above guides you through prioritising these factors.
Why is GRP becoming a popular alternative to traditional lead valleys?
GRP (Glass Reinforced Polyester) offers several advantages for modern construction – it’s extremely lightweight, eliminating the need for additional structural support, it’s completely impervious to corrosion and it’s manufactured in long, seamless lengths, drastically reducing the risk of leaks at joints. It also offers zero scrap value, so is far less likely to be stolen.
What maintenance does a GRP valley require compared to a metal one?
GRP valleys are virtually maintenance-free. A GRP valley trough doesn’t require painting or protective coatings after installation. Filon V-Flow GRP Roof Valleys are coated with Filon Protect, a factory-bonded UV-protective film, ensuring excellent protection against weathering, discolouration and fungal growth.