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Roof Drainage Design by David Roy

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From 1st April 2002, Drainage and Waste Disposal Approved Document H of the Building Regulations came into force. Section H3 relates to Rainwater Drainage.
The regulations give detailed advice on rainfall rates, gutter and outlet sizes for half round eaves gutters. For other types of gutters the regulations recommend that the design be carried out in accordance with BS EN 12056-3:2000 Gravity drainage systems inside buildings - Part 3: Roof drainage layout and calculation. This standard replaces BS6367:1983 Code of practice for drainage of roofs and paved areas.
Design Rainfall Rates
BS6367 gave rainfall rates in mm/hr whereas the new standard gives design rainfall rates in l/s per mē, to convert to mm/hr multiply by 3600.
Eaves Gutters and Flat Roofs
The main change in BS EN 12056-3 from the previous standard is the design rainfall rate, whereas previously a design rainfall rate of 75mm per hour was used for eaves gutters and flat roofs, now the standard recommends a rainfall intensity for a 2 min duration storm event with a return period of 1 year as the basis of design.
Depending whereabouts in the country you are, the rate will vary from 0.01 l/s per mē (35mm per hour) to 0.022 l/s per mē (80mm per hour)
Valley and Parapet and Boundary Wall Gutters
Generally two categories of design rate should be considered depending on the return period required:
1.5 x Building design life
4.5 x Building design life
For a building of say a design life of 30 years, this would give a return period of 45 years and 135 years respectively. The standard gives rainfall intensity maps for differing return periods 5, 50 and 500 years.
I would suggest that in the absence of more detailed information that for normal risk building use the return period 50-year map and for high risk building e.g. museums use the return period 500-year map.
Based on the a rainfall intensity map for a 2 min duration storm event with a return period of 50 years, the rainfall rate will vary from 0.02 l/s per mē (72mm per hour) to 0.056 l/s per mē (201mm per hour).
For the a rainfall intensity map for a 2 min duration storm event with a return period of 500 years, the rate will vary from 0.032 l/s per mē (115mm per hour) to 0.088 l/s per mē (317mm per hour).
Gutter Types
Eaves gutters are defined as gutters where any spillover will discharge outside the building, whereas with valley and parapet gutters, any overflow will discharge inside the building.
The width and depth of eaves gutters will depend on the area to be drained, outlet and rainwater pipe size and position.
For valley and parapet gutters it is recommended that the gutter should be large enough to enable a person to walk on them for maintenance and cleaning, for valley gutters the top width should not be less than 500mm and for parapet gutters 300mm. The depth of the gutter will again depend on the area to be drained, outlet and rainwater pipe size and position, although I would suggest a minimum depth of 150mm.

parapet gutter

Valley gutter

Design Procedure
The standard gives detailed advice and calculation procedure for the design of roof drainage systems
As a guide:
1. Calculate the run-off for each length of gutter for a given rainfall rate and rainwater pipe position.
2. Calculate the capacity of the gutter
3. Calculate the capacity of the outlet in the gutter
4. Calculate the capacity of the rainwater pipe
If the capacities in steps 2, 3 and 4 are greater than the run-off from step 1 then the system is satisfactory. Otherwise vary the positions of the rainwater pipe, or gutter and outlet sizes until the system is satisfactory. Often changing the outlet type from a straight drop outlet to a tapered outlet will prove effective.

Outlet types

For easy to use tables to determine gutter sizes and outlet positions - see the Drainage section
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