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Roof Drains Design Considerations

Back to Roof Drains

Flow Rate

The quantity and types (warm, cold, inverted or parapet) of roof outlet to be used in a given application must be related to the roof area and the anticipated volume of rainwater to be drained.

Flow rate is the maximum amount of water (litres/second) which a roof outlet will drain and is influenced by several factors: grating free area (i.e. gaps in the grating), sump size, body free area, design of the outlet, outlet size and additional accessories such as gravel guards and filter buckets. Flow rates of Frost roof outlets are shown as the analytical flow rate per outlet based on a head of water of 35mm.

The roof outlet size and piping system size relate to each other and should be within recognised parameters.


The type and size of pipework in an installation is fundamental to the correct selection of roof outlets.

Frost roof outlets can be connected to all pipework in general use.

Thermal Insulation

Local climatic conditions and regulations should be considered in connection with local insulation requirements.

Materials and Appearance

Strength, corrosion resistance and discreet appearance are important considerations when making roof outlet selections. For areas such as balconies or roof terraces products are available in nickel bronze, ductile iron, stainless steel or aluminium materials. Drains located on pedestrian surfaces or access routes must be disabled compliant.

Weight Bearing and Load Rating

As an integral part of the roof it is vital gratings and assemblies withstand all anticipated loads. This website shows the load rating class for each grating, cover or gully assembly based on BS EN 1253, as follows:

Loading K3 (Pedestrian Only)

Areas for foot traffic only, such as washrooms, shower areas, hotels, schools, museums, retail centres, swimming baths, leisure centres, balconies, terraces and roof gardens.

Loading L15 (Light Industrial)

Areas with light vehicular traffic such as cycleways and domestic drives.

Loading M125 (Heavy Industrial)

Areas with vehicular traffic, such as car parks, factories, warehouses and workshops.

Accessories and Variations

Standard optional items are available such as filter buckets to intercept debris, gravel guards, bearing pans to spread roof loadings and underdeck clamps to secure outlet bodies.

International Standards

Satin Nickel Bronze and Polished Nickel Bronze to BS1400

Stainless Steel to BS EN 10088-1:2005 Grade 1.4301 (304) or Grade 1.4401 (316) if specified

Ductile Iron to BS EN 1563 & 1564 Cast with the ductility of steel and more than double the tensile strength of cast iron.

Aluminium LM 6 to BS EN 1559-4: 1999

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