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Floor Drains Design Considerations

Back to Floor Drains

Factors including size, shape, appearance and anticipated drainage requirements need to be considered in planning effective drainage solutions. Design is usually the responsibility of consulting engineers familiar with the floor construction, floor finish, piping systems, operations and function intended for particular building areas.

Building Regulations / Standards

International regulations and local building codes must be observed for all sanitary and other drainage installations so it is recommended these are considered as the first step.

As a matter of good practice, drains should be considered for any location where water is supplied or where surface drainage must be accommodated.

To prevent foul air from entering a building, a trapped gully may be used, and various types are available Bell, Bottle, P and S.

In other cases, a non-trapped gulley may be used with a trap installed in the drain line.

Weight Bearing and Load Rating

As an integral part of the floor, 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.

Flow Rate

Flow rate is the maximum amount of water (litres/second) which a gulley will drain and is influenced by several factors such as grating free area (i.e. gaps in the grating), sump capacity, body free area, design of the gulley, outlet size, and additional features and accessories such as filter buckets and traps.

At low heads of water, the flow rate of a gully is mainly governed by the size of grating free area. At heads of water above 50mm, the flow rate is governed by the body free area or the grating free area, whichever is the smaller.

Selected top size (i.e. grate free area), gully outlet size and piping system size all 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 gullies. Frost drainage gullies can be connected to all pipework in general use.

Floor and Flooring

Floor construction type has a direct influence upon appropriate drainage selection and so should be established at the outset.

The depth of the structural slab and available space will influence the selection of gullies, while the selection of an extension may be necessary to bring the grating or cover to the finished floor level.

Installations with a drainage membrane will normally require the use of gullies with a membrane clamping collar.

Drains located on pedestrian surfaces or access routes must be disabled compliant.

Different floor finishes, such as concrete, epoxy resin, ceramic tiles or sheet vinyl, will influence the type of grating or cover to be selected.

Material and Appearance

Strength, corrosion resistance and appearance are important considerations when making drainage selections.

Finished floor areas
Gullies with attractive gratings or covers made of satin nickel bronze, polished nickel bronze or stainless steel are for use in areas where they contribute to an area’s aesthetic appeal.

Unfinished floor areas
While extremely durable and visually acceptable, ductile iron gratings are best suited in unfinished and/or industrial settings such as service areas or for restoration projects to match period architecture.

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.

Get in touch

Contact our helpful team for more information.