Page 543 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 8 Rainwater systems

The fall of the gutter

BS EN 12056–3:2000, Section 7.2.1 and NE.2.1 states that:

1 Gutters should be laid to a nominal gradient of between 1 mm/m and
3 mm/m where practicable.
2 The gradient of an eaves gutter shall not be so steep that the gutter drops
below the level of the roof to such an extent that water discharging from the
roof will pass over the front edge of the gutter.

In most cases, manufacturers interpret these two points as a slight fall of 1:600
(25 mm in 15 m). Laying a gutter with a fall greatly increases the flow capacity
and, therefore, the area of roof that can be drained. It also ensures that silting
of the gutter does not occur. However, manufacturers design guttering systems
in such a way that the performance of the gutter is not compromised if it is laid
level, with little or no fall. A fall of 1:600 ensures that the gutter will not fall so
low as to be below the discharge point of the roof.

INDUSTRY TIP

Gutter falls

Not all manufacturers recommend a fall of 1:600. Some manufacturers advocate a fall of
1:350. This increases the amount of fall, thereby increasing the flow rate of the gutter. It
also, however, lessens the length of the run of gutter before the gutter will dip below the
discharge point of the roof. For instance:
l a fall of 1:600 is the equivalent of a 25 mm fall in 15 m
l a fall of 1:350 is the equivalent of a 25 mm fall in 8 m.
A fall of 1:600 therefore ensures that the rainwater will clear the gutter effectively and
cause no problems with discharge from the roof.
Before installing a guttering system, check the manufacturer’s installation instructions for
the fall gradient that is recommended.

Changes of direction in the gutter run
In most domestic gutter systems, changes of direction cannot be avoided.
Where changes in direction greater than 10° occur within a guttering system,
they restrict the flow of water through the system. A 90° gutter angle reduces
the effectiveness of the run of gutter where the angle is situated by 15 per
cent, effectively reducing the roof area that the gutter can usefully serve. Each
subsequent change of direction reduces the gutter’s effectiveness still further.
A gutter angle that is placed near an outlet will also reduce the effectiveness of
the outlet.

Thermal expansion of PVCu gutters and
fittings

One of the problems with PVCu gutters is the large expansion rate. This can
cause the gutters to creak as they are warmed by the Sun and, in extreme cases,
it can cause joint failure.

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