Here Kevin Wallis, Category Manager for Eaves Solutions at Marley Alutec highlights the factors that need to be considered when designing an effective eaves drainage system, including rainfall intensity, effective roof area (ERA) and gutter flow capacity.
Along with increasingly common extreme weather events in the UK comes heavy rainfall. With this in mind, it is critical to design an eaves drainage system that can effectively cope with an ever-increasing peak rainfall intensity.
Rainfall Intensity
There are several key factors to consider when designing an eaves drainage system but let’s first consider rainfall intensity. This is known as the rate at which rain falls on a roof and is vital in assessing the rainwater drainage capacity.
The approved method used for calculating guttering and downpipe sizes is the pan-European design standard BS EN 12056-3:2000. This standard provides in-depth rainfall data throughout the UK on meteorological maps. It shows the geographical location and frequency of rainfall likely to fall in a two-minute storm for a given return period. The UK national highest rainfall intensity is 0.021 litres per second per square metre for eaves gutters. The guidance given in the standard recommends that eaves gutters are sized to ensure the calculated peak run-off does not exceed 90% of the gutter capacity. If overflowing occurs, water overspill will discharge clear of the building without risk of water ingress.
To help designers to fully comply with BS EN 12056-3, some leading manufacturers, such as Marley Alutec have produced an online rainwater design calculator. This tool provides step by step guidance allowing fast and simple roof drainage design and when complete, a full set of calculations are produced that confirms product suitability.
Effective Roof Area
The calculation of the roof area, as set out in BS EN 12056-3, must also be considered to ensure the correct drainage system is chosen. Rainfall doesn’t always fall vertically, which is why the standard assumes the angle to be 65 degrees. To allow for this, architects must take half the vertical area of the roof into account to give an ‘Effective Roof Area’ (ERA).
In addition, an often-overlooked aspect is run off from vertical surfaces, which is 50% of the area. This must be added to the effective roof area before continuing with specifying the drainage system.
Gutter Flow Capacity
One of the most obvious factors to consider when designing an eaves drainage system is the gutter capacity. Often gutters are installed level to improve appearance, but if it is installed at a fall of 1:600 the flow capacity will greatly improve. The flow rate will also vary depending on the profile and size of the gutter fitted.
If designing a drainage system based on a rainfall intensity of 0.021 litres per second per square metre as identified in BS EN12056-3, it’s important to consider the location of the gutter outlet. This should be determined from the onset, whether the outlet is positioned centrally or towards the end of the gutter run.
Façade design may also have an indirect influence. For example, designers seeking to achieve a symmetrical positioning of rainwater pipes or avoiding window openings may have to choose gutters with a larger carrying capacity.
To be certain of the most appropriate size and type of gutter and rainwater pipes, most leading manufacturers produce a flow capacities table. The nearest roof area per square metre in either the ‘central’ or ‘end outlet’ options can be found on this table to determine the correct size and type of gutter. If a different rainfall intensity design rate is required, then the alternative design rate is multiplied by the effective roof area to establish the required gutter capacity. The nearest gutter flow capacity can be found on the flow capacities table. It is important though that appropriate proportional allowances for central or end of gutter outlets are made.
Gutter Design
There are several other important considerations to be made regarding the design, length, and discharge of the gutter to achieve an effective eaves drainage system. If for example, the gutter run specified features one or more angles greater than 10 degrees, BS EN12056-3 recommends the gutter capacity should be reduced by a factor of 0.85 and the positioning of outlets adjacent to angles should be avoided.
Also, if very long gutter runs are specified frictional resistance may occur reducing the flow capacity. If this arises, BS EN12056-3 recommends ways to reduce this. It may also be worth specifying a corner hopper or bespoke gutter angle with a large catchment area if there is a long valley discharge. These products can manage a high volume of water during storms.
Architects are often asked to design less conventional roofs. If this is the case, to accommodate any unusual curves or angles where roofs intersect at different levels, some manufacturers provide bespoke gutter solutions. For example, radiused gutters can be made using a sand-casting technique. This involves making a wooden pattern of the gutter profile to a given radius from which sand moulds are made. However, this method can be costly if needed in small quantities. There is though, a more cost-effective alternative to sand-casting. This involves internally welding together segments of machine mitred gutter to achieve a given radius.
The final, yet no less important factor to consider when designing an eaves drainage system is the material. There are a range of materials to choose from such as PVC or cast iron, but aluminium is particularly well suited for a guttering system.
Aluminium has a functional lifespan of 50 years or more and requires minimal maintenance. There are several key aspects to consider during installation of the guttering. The most significant, yet most overlooked in aluminium rainwater system design is application, with many theoretically proven designs failing as application was not considered alongside performance. It is recommended that designers consult with their chosen supplier to avoid this and prevent any unnecessary contingency costs. With this in mind, most reputable suppliers do offer a complimentary pretender specification service which covers application.
In addition to this, a secure connection between each component is crucial, while long-term durability can be achieved if non-corrosive fixing components are specified. To avoid bi-metallic corrosion, ensure electrolytically incompatible materials do not come in direct contact with un-insulated plain aluminium surfaces. Recommended compatible screws and fixings should only be used.
If architects consider rainfall intensity, effective roof area, gutter flow capacity, and the appropriate gutter design and application, then an effective eaves drainage system that performs well, looks good and is long-lasting will be achieved.