Rainwater goods
Rainwater goods, consisting of gutters, outlets and downpipes, work in the fundamental role of drainage. The design process and selection of correct materials can be complex, and it is recommended that a company specialising in the design and manufacture of roof drainage systems is consulted for more detailed guidance.
The design and construction of parapet, boundary wall and valley gutters is critical to prevent leaking into the building, as they are positioned within the building line. Eaves gutters are normally positioned outside of the building line, but nonetheless correct design, construction and positioning can reduce the occasional overflow caused by wind-driven rain.
The principal factors to consider in designing a rainwater system are:
- Rainfall intensity
- Effective drainage area including parapets, adjacent walls etc, and taking roof slopes into account.
- Design flow which is the multiple of rainfall intensity and effective drainage area.
- Gutter size which is dictated by the design flow.
- The positioning and spacing of the outlet.
- A reduction in outlet capacity due to the installation of strainers should be taken into account when designing outlet capacity.
Guidance on each of these can be found in BS EN 12056 and in the relevant Building Regulations, and further assistance can be sought from rainwater goods manufacturers.
Eaves gutters have a major impact on the aesthetics of the building, so it is common to use a painted product which either matches or contrasts harmoniously with the main cladding. A product such as Colorcoat HPS200® Ultra double-sided can be used for this purpose.
Internal gutters in particular are difficult to replace, so good material selection at the outset is paramount. Durability is key to material selection and such items tend to have little or no aesthetic function. Since internal gutters form a part of the building envelope, they must contribute to the thermal efficiency of the building. Insulation for the gutter should be specified, whether as a part of a built-up system or as a factory fabricated insulated gutter.
Even when using insulated gutter systems, the minimisation of cold bridging in particular should be considered. Gutters are subject to standing water and debris, which will result in a corrosive environment. Since gutters tend to form an integral part of the building envelope, it is vital that the longevity of the system be considered. Galvanised gutters should be lined with a bituminous coating, and sheet-ends should be extended so that run-off is into the valley of the gutter.
