Last edited 03 Jun 2021

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All eyes up to plastic rainwater systems



[edit] Introduction

With price such a key factor for all building products, it is vital that all options are carefully considered against criteria such as performance, life cycle, environmental impact, maintenance and cost. For projects requiring a heritage style, there are numerous rainwater systems available, including cast iron, aluminium and plastic. When specified correctly, each has its own advantages, but it is plastic, cast-iron style that is helping designers, developers and housebuilders meet the requirements of aesthetics, performance and cost, without compromising architectural intent.

It is crucial that every UK building includes a high quality rainwater system that will provide it with the maximum protection from days of heavy and intense rainfall, which are becoming more frequent in our changing climate. When specified correctly, this vital building component will prevent rainwater from pooling around the base of the property, saturating the foundations and causing damage to the building fabric, as condensation and damp will severely compromise its strength, durability and longevity.

A necessary part of any building’s operability, guttering and drainage are required to efficiently carry away rainwater from the roof without over-spilling under Part H of the Building Regulations. The specifier or contractor should consider local weather conditions, the potential impact on the underground drainage layout, the catchment area of the roof, gutter capacity, expansion allowance and fall calculations in any efficient rainwater system design.

[edit] Location, location, location

It is important to consider the geographic location of the building, as some areas of the UK can experience higher volumes of rain and snow than others. Armed with the knowledge of the frequency of heavy downpours related to a geographic location, specifiers will then be able to figure out what capacity of gutter will be required. A useful guide to rainfall intensities for the specification of gutter and downpipes can be found within Building Regulations approved document H: Drainage and Waste Disposal.

Roof insulation regulations indirectly play their part in rainwater design in areas of heavy snow. Better insulation has resulted in less heat loss, which in turn means that snow does not melt as quickly. The accumulation of snow can build to a critical point, so rainwater systems need to accommodate the higher load with appropriate gutter spacings and incorporation of snowboards as required.

Hotter temperatures also influence guttering design, as they need to be installed to allow for thermal expansion. The rate of expansion will depend on what material the rainwater system is manufactured from, and it is recommended to refer to the manufacturer’s guidance on this before installing. For example, a 2m length of uPVC gutter or downpipe will expand by 2.4mm for a 20°C temperature rise. This expansion must be accommodated when installing any system.

The importance of specifying the correct gutter and downpipe size should not be understated. The catchment area of the roof should also be considered, as an inadequately planned system could lead to overflow and saturation at ground level. However, costs can escalate on a build if there are too many outlets and downpipes which in turn can result in too many underground connections.

[edit] Optimum performance

The flow and volume of water into a gutter depends on the area of the surface being drained and the angle of the roof’s pitch. Ascertaining the roof size in terms of the drained area is the next step to designing a successful and cost effective rainwater system.

Information on calculating the area of a complex roof can be found in BS EN 12056-3: 2000: Roof drainage layout and calculations. After calculating the effective area of the roof, this must relate to the draining capabilities of the rainwater systems specified to ensure the right size and number of components are purchased to achieve the optimum performance.

Gutter systems come in all shapes and sizes, so specifying the right size of gutter and adequate number of outlets will lead to an optimal balance between cost and performance of the system.

Another important factor is gutter support spacing which should normally not exceed 900mm; however, in the case of roofs with a pitch exceeding 35°, smooth surfaces or those which may be subject to heavy snow loading due to geographical location, support spacings should not exceed 600mm. In these areas and circumstances, the installation of snowboards offers the client a precaution against sliding snow.

Regardless of the rainwater system selected, it is critical to check it has been tested to the relevant industry standards, as this will ensure it is capable of withstanding the most rigorous performance assessments. A rainwater system’s long term performance capabilities are tested through assessments of impact and tensile strength, load bearing capacity, water tightness, deformation resistance and the impact of accelerated ageing, and any reputable manufacturer will ensure rainwater systems perform as such. For example, Brett Martin’s uPVC rainwater systems undergo testing in accordance with BS EN 607:2004 and BS EN 1462:2004.

[edit] Good looks

Aesthetics should also not be an afterthought, and when specified correctly, should complement the building’s style. Whether choosing PVC, aluminium or cast iron, there are a wide variety of colour options and gutter styles.

Specifiers should also be aware of the true cost of the rainwater system – in terms of materials, labour and maintenance over the lifetime of the building. For example, cast iron is good to look at but much more expensive in actual material and installation costs and requires a lot of maintenance to keep it looking good. Plastic alternatives, such as Brett Martin’s Cascade Cast Iron Style Rainwater System, replicate the detail of a cast iron rainwater system and are a much more cost effective option.

[edit] Simple-to-fit

With significant additional weight, cast iron drainage systems can be heavy and difficult to instal. Builders must also be extra vigilant regarding health and safety and will require more time to lift and fix the system – leading to increased costs and potential delays in the installation process.

Almost every job will require both gutter and pipe lengths to be cut onsite using specialist tools. To ensure long-lasting durability, they need to be fully finished on site using a primer, undercoat and topcoat to ward off corrosion.

Even if the system used is delivered pre-painted, any cut lengths or site damage to the finished surface must be protected to the same level by the installer. It should be noted that in some cases, if the level of protection applied on site is deemed insufficient, any guarantee offered may be, quite rightly, invalidated, leaving the installer liable.

The plastic ranges are so much lighter than cast iron. Installation at height is less hazardous, being effortless to lift, easy to manoeuvre, simple to cut and quick to fit.

The systems also benefit from external fixing lugs, enabling easy power tool access, which further speeds up installation. All gutter fitting joints have integral seals and flexible retaining clips, allowing the gutter lengths to simply click into place. No extra-cost fixing kits or nuts and bolts are required to make gutter joints as with most traditional systems.

[edit] Adaptability

To ensure each rainwater system can be designed to meet every client’s demands, there is an expansive choice of options and gutter profiles available for domestic builds and refurbishments to suit most requirements. From the domestic style of gutter to high capacity systems for larger scale commercial projects, plastic rainwater systems are also compatible with existing cast iron systems and have been carefully designed to replicate original features, dimensions and detailing.

Integral to the overall aesthetics and the performance of the building itself, rainwater systems should be considered as a key part of the design process. As budgets are increasingly stretched and tight deadlines dictate product specification, it is critical specifiers are aware of every product’s cost – in both materials and labour. By following these simple steps, the specifier will be able to determine the best solution for the roof and building in question.

This article originally appeared in the Architectural Technology Journal (at) issue 137 published by CIAT in spring 2021. It was written by Brett Martin Plumbing and Drainage.


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