How to Specify Plastic Building Products for Your Project
How to Specify Rainwater and Drainage?
To specify rainwater and drainage systems correctly, start by calculating the effective roof area draining to each gutter run, then select a gutter profile and size with sufficient flow capacity to handle the rainfall intensity specified in BS EN 12056-3 (gravity drainage systems — roof drainage). For underground drainage, determine the number and type of sanitary appliances discharging into the system, calculate the required discharge capacity, and select pipe sizes and gradients that meet Building Regulations Approved Document H. All products should comply with the relevant British Standard and carry appropriate third-party certification such as BSI Kitemark or BBA.
Getting the specification right at the design stage prevents problems during installation and throughout the building’s life. Under-specifying leads to overflowing gutters, blocked drains, and water damage. Over-specifying wastes money on unnecessarily large components. This guide walks through the specification process for the main plastic building product categories — rainwater, drainage, roofline, soil and waste, and cladding — with the key standards, sizing considerations, and certification requirements for each.
Rainwater Systems
Calculating Gutter Size
The starting point for specifying any rainwater system is determining the effective roof area that drains into each gutter length. The effective area accounts for both the plan area and the pitch of the roof, and is calculated differently depending on the roof geometry and wind exposure.
Once you know the effective roof area, you can determine the required gutter flow capacity. In the UK, the design rainfall intensity used for gutter sizing is typically 75mm/hour for normal conditions, increasing to 150mm/hour in areas of severe exposure. These values are specified in BS EN 12056-3.
The flow capacity of a gutter depends on its profile, size, and the position of the outlet:
| Gutter Profile | Nominal Size | Typical Flow Capacity (L/s) — Central Outlet |
|---|---|---|
| Half Round | 76mm | 0.5 |
| Half Round | 112mm | 1.0 |
| Square | 114mm | 1.2 |
| Ogee | 116mm | 1.4 |
| Deepflow | 170mm | 2.5 |
| Hi-Cap | 170mm | 3.0+ |
Key point: The outlet position significantly affects flow capacity. A central outlet can handle roughly double the flow of an end outlet on the same gutter. Always specify outlet positions on drawings to ensure the system performs as calculated.
Choosing a Gutter Profile
Gutter profile selection is driven by a combination of functional requirements and aesthetic preferences:
- Half Round — The most traditional profile, suitable for most residential applications. Clean, simple appearance that suits both period and modern properties
- Ogee — A decorative profile with a curved front face, commonly used on detached houses and properties where a more ornate appearance is desired
- Square — A contemporary profile with clean lines, popular on modern developments
- Deepflow — Higher-capacity profile for larger roof areas or areas of high rainfall intensity. Often specified on extensions, dormers, and commercial buildings
- Hi-Cap / Industrial — High-capacity systems for commercial and industrial buildings, schools, and large residential blocks
Specifying Downpipes
Downpipe sizing must be matched to the gutter system and the discharge requirement. Standard residential downpipe sizes are 65mm (round) and 68mm (square) for domestic systems, increasing to 110mm for deepflow and commercial applications.
The number of downpipes required depends on the gutter flow capacity and the length of gutter run. As a general rule, each downpipe should serve no more than 50 square metres of effective roof area at standard rainfall intensity.
Key Standard: BS EN 12200
PVC-U rainwater piping systems should comply with BS EN 12200 (Plastics rainwater piping systems for above-ground external use). This standard specifies requirements for materials, dimensions, mechanical properties, and performance. Kitemark certification to this standard provides independent verification of compliance.
Underground Drainage
Pipe Sizing
Underground drainage pipe sizing is determined by the discharge capacity required and the gradient at which the pipe is laid. The two standard PVC-U drainage pipe sizes for UK residential construction are:
- 110mm — The standard for domestic foul and surface water drainage serving individual dwellings and small developments
- 160mm — Used for larger developments, shared drainage runs serving multiple properties, and situations requiring higher discharge capacity
Gradient Requirements
Building Regulations Approved Document H specifies minimum and maximum gradients for drainage pipes:
| Pipe Diameter | Minimum Gradient (foul drainage) | Typical Gradient |
|---|---|---|
| 110mm | 1:80 (12.5mm per metre) for up to 1 WC | 1:40 to 1:80 |
| 160mm | 1:150 | 1:60 to 1:150 |
Steeper gradients increase flow velocity but may cause the liquid to run ahead of the solids, leading to blockages. Shallower gradients may not provide sufficient self-cleansing velocity. The recommended range provides a balance between adequate velocity and practical trench depth.
Ring Stiffness
Underground drainage pipe must have sufficient ring stiffness to resist the soil and traffic loads above it. PVC-U drainage pipe is available in two ring stiffness classes:
- SN4 — Standard ring stiffness, suitable for most domestic installations with standard cover depths
- SN8 — Higher ring stiffness, required for deeper installations, areas with heavy traffic loads, or poor soil conditions
Specify SN8 pipe for installations under roads, car parks, and areas subject to HGV traffic. SN4 is adequate for garden and footpath installations with standard cover depths.
Key Standard: BS EN 1401
PVC-U underground drainage pipe systems should comply with BS EN 1401 (Plastics piping systems for non-pressure underground drainage — PVC-U). This standard covers material requirements, dimensions, ring stiffness, impact resistance, and long-term performance.
Roofline Products
Fascia Boards
Fascia boards are specified by width (to match the depth of the rafter feet) and thickness (to provide adequate rigidity). Common fascia widths are:
- 150mm — Older properties with smaller rafters
- 175mm — Standard residential specification
- 200mm — Larger properties and some commercial applications
- 225mm and 250mm — Deep eaves on larger or period-style properties
Fascia boards are available in solid and hollow chamber (cellular) profiles. Hollow chamber fascia is lighter and provides better thermal insulation but may have slightly lower impact resistance than solid board. Both types are suitable for residential applications.
Soffit Boards
Soffit boards are specified by width to match the eaves projection. Standard widths range from 100mm to 600mm, with 200mm, 300mm, and 400mm being the most common for residential installations.
For ventilated eaves (required by Approved Document F to provide roof space ventilation), specify soffit boards with integral ventilation strips or use separate soffit ventilator strips. The required ventilation area is typically 10mm continuous equivalent for the full length of the eaves at low level, with additional ventilation at high level.
Key Consideration: Colour and Finish
PVC-U roofline products are available in white, black, rosewood, golden oak, anthracite grey, and other colours. Woodgrain foil finishes provide a realistic timber appearance without the maintenance requirements.
Colour consistency across a project requires all products to be sourced from the same manufacturer and ideally from the same production batch. Different manufacturers may offer nominally similar colours that do not match when installed side by side.
Soil and Waste Systems
Pipe Sizing
Soil and waste pipe sizing follows the discharge unit method specified in BS EN 12056-2 (Gravity drainage systems — Sanitary pipework):
- 110mm soil pipe — Carries discharge from WCs (and can also carry waste from basins, baths, and showers if combined)
- 40mm waste pipe — Standard for individual basin, bath, or shower connections
- 32mm waste pipe — Suitable for individual basin connections with lower discharge
- 50mm waste pipe — Used for kitchen sinks and washing machine connections, or where multiple appliances connect to a common waste pipe
Jointing Method
PVC-U soil and waste systems are available in two jointing methods:
- Solvent weld — Permanent joints made by applying solvent cement to the pipe and fitting surfaces. Provides the strongest joint and slimmest profile. Preferred for concealed or inaccessible installations
- Push-fit (ring seal) — Joints made using rubber O-ring seals. Allows for thermal movement and is easy to disassemble for maintenance. Preferred for accessible installations and situations where future modification may be required
Key Standard: BS EN 1329 / BS EN 1455
Soil pipe systems should comply with BS EN 1329 (PVC-U pipe systems for soil and waste discharge — solid wall pipe) or BS EN 1455 (PVC-U pipe systems for soil and waste discharge — structured wall pipe).
External Cladding and Wall Panelling
External Cladding
PVC-U external cladding is specified by profile (shiplap, weatherboard, or decorative) and colour/finish. Cladding must be installed with appropriate ventilation behind the boards to prevent moisture build-up in the wall structure, and fixed in accordance with the manufacturer’s instructions and Approved Document C (Site preparation and resistance to contaminants and moisture).
Internal Wall Panelling
PVC-U wall panelling for internal use is specified by panel width, thickness, and joint type (tongue and groove, click-fit, or overlap). Panelling provides a hygienic, moisture-resistant wall finish for bathrooms, kitchens, utility rooms, and commercial spaces.
General Specification Checklist
For any plastic building product specification, ensure the following are addressed:
- Product standard — Confirm the relevant BS EN standard and require compliance
- Third-party certification — Specify BSI Kitemark, BBA, or WRAS approval as appropriate
- Material — Specify PVC-U (for rainwater, roofline, drainage, soil/waste, cladding) or MDPE (for water supply pipe)
- Colour and finish — Specify precisely and require colour consistency across the project
- Sizing — Specify dimensions based on calculated requirements, not assumptions
- Accessories — Include all necessary fittings, fixings, sealants, and ancillary items
- Installation standard — Reference the manufacturer’s installation guide and relevant Approved Documents
- Manufacturer — Name the preferred manufacturer or state “approved equal” with specific criteria for equivalence
Frequently Asked Questions
Can I mix products from different manufacturers?
It is not recommended. While nominal dimensions may be similar, products from different manufacturers may not be fully compatible — gutter clips may not fit, pipe diameters may vary slightly, and colour matches will differ. Specifying a single manufacturer for each product system ensures compatibility and a consistent appearance.
What happens if I under-specify the gutter size?
Under-sized gutters will overflow during heavy rainfall, directing water onto walls, windows, and foundations. This can cause staining, damp penetration, and long-term structural damage. It is always better to specify slightly above the calculated minimum than to risk under-sizing.
Do I need separate specifications for foul and surface water drainage?
Yes. Foul drainage (from toilets, basins, baths, kitchens) and surface water drainage (rainwater) are typically separate systems in the UK. Each requires its own specification for pipe size, gradient, and connection points. Combined systems exist in some older areas but are not standard for new construction.
How do I use Kalsi’s online calculators?
Kalsi Plastics provides online calculators for rainwater, drainage, roofline, and wall panelling that help you determine the right products and quantities for your project. Enter your building dimensions and the calculators provide product recommendations and material lists.
Related Kalsi Products
- Rainwater Systems — Full range of gutter profiles and downpipe systems
- Underground Drainage — 110mm and 160mm pipe systems with full fittings range
- Roofline Products — Fascia, soffit, bargeboard, and accessories
- Soil and Waste — Solvent weld and push-fit systems
- External Cladding — Shiplap, weatherboard, and decorative cladding
- Technical Resources — Datasheets and specification support
