Roof Drainage

Roofs are constructed in a variety of shapes, from a simple pitched arrangement with external gutters, to a more complex multi-span construction with valleys, hips, parapets or boundary wall gutters.


Gutter Types and Location

Gutter layout and roof drainage requires careful consideration at the building design stage to guarantee reliable performance.
Eaves gutters are outside the building envelope and any failure or leakage would not normally mean water entering the building.
Failure of valley, hip, parapet and boundary wall gutters, which are an integral part of the roof, results in water pouring into the building damaging both the fabric of the building and its contents.

Therefore, correct gutter design, construction and in use reliability form a vital part of the project team’s considerations. The design details for roof drainage are based on the recommendations in EN 12056-3:2000 Gravity drainage systems inside buildings – Part 3: Roof drainage, layout and calculation.

Kingspan design and manufacture a range of standard and customised internal and external gutter systems, see chapter Accessories.

Rainfall Rate

Rainfall rates have been recorded in many countries over the years, and this information has been used in EN 12056-3 to indicate where and how frequently particular rainfall rates are likely to occur. A rate of 75 mm/hr is suggested as the normal basis for calculation. This rate is generally suitable for eaves gutters.

Most gutter and drainage systems have to be able to deal with short periods of excess rainfall, provided they are correctly designed and maintained. Higher rainfall rates such as 150 mm/hr can be used if required to reduce overflow risks further, e.g. for valley, hip, parapet or boundary wall gutters.

Design – General

To establish the correct gutter design and size it is necessary to calculate the rainwater discharge rate from the roof. This involves assessing the rainfall rate and the effective catchment area - Ae.

Effective Catchment Area, Ae

The water drainage from a roof includes rain falling directly onto the roof and also wind driven rain running off adjacent roofs, walls and parapets which has to be taken into account.

The total effective area therefore is:

  • the shaded roof plate area
  • the shaded vertical elevation area


The effective catchment area Ae for slope A is:

The gutter and downpipe arrangement has to be designed to provide sufficient capacity for the predicted discharge rate.

Gutter design is normally based on the following assumptions:

  • Slope of the gutter is less than 1 in 350.
  • The gutter has a uniform cross section.
  • The outlets are large enough to ensure the gutter discharges freely.
  • The dimension from a stop end to outlet should be less than 50 maximum water depth
  • The dimension between outlets should be less than 100 maximum water depth.

The depth of water in the gutter will vary from a maximum at the upstream end, to a minimum ‘critical depth’ at the outlet, depending on gutter shape. For rectangular section gutters the maximum water depth equals twice the depth at the outlet.
Valley, hip, parapet and boundary wall gutters should include an allowance for freeboard to allow for splashing and waves below the spill over level. EN 12056-3 recommends minimum freeboard depth between 25 mm and 0.3 total gutter depth up to maximum of 75 mm. A minimum 50 mm freeboard is often considered good practice.


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