Page 541 - 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

ACTIVITY

Calculation of effective roof area using pitch factors
Using the pitch factors given above, calculate the following effective roof areas.
1 A roof has a length of 12 m, a width of 7 m and pitch of 45°.
2 A roof has a length of 8 m, a width of 8 m and pitch of 60°.
3 A roof has a length of 10 m, a width of 8 m and pitch of 30°.

ACTIVITY
IMPROVE YOUR MATHS
We can now calculate the amount of rainwater to be expected on any given Calculation of rainfall
rate in litres per second
roof area in a sudden storm deluge of 75 mm of rainfall per hour. To convert the
area to litres per second (l/s), multiply the roof area (m ) by 0.0208. Using the effective areas
2
Example 3 calculated in the above
The area of the roof in Example 1 is 75 m 2 . What is the expected rainfall in l/s? activity box, calculate the
rainfall rate in litres per
75 × 0.0208 = 1.56 l/s second.

Running outlet position
Figure 8.7 shows a running outlet. It is the connection between the guttering
and the rainwater pipe.

The position of the running outlets is usually based upon the position of the
gullies for the surface water sewer/drain to the property. These can be found on
the building layout drawing (Figure 8.8).

p Figure 8.7 A running outlet
Rainwater outlet

Rainwater outlet

Rainwater outlet

Inspection chamber
Surface water sewer in the road

p Figure 8.8 Building layout drawing

The more outlets there are on a gutter system, the shorter the distance the
water has to travel and the more effective the system is at discharging the
rainwater. Consider Figure 8.9.

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