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Guttering Calculations
Design basis
To assess the suitability of a gutter system to drain the roof to a building the following factors need to be taken into consideration:

1. The effective roof area to be drained
2. Rainfall intensity
3.The flow characteristics of the gutter system
4. The number and position of downpipes 		Roof Drainage Example 1

Effective roof area
The effective roof area can be determined by calculation in accordance with the following.

1. BS EN 12056-3:2000 Gravity drainage inside buildings
2. The Building Regulations 2002 Approved Document H, part H3.

The formula and examples shown opposite reflects the method used in the above standard to calculate effective roof area.

Multiplication factors
An alternative approach to that described above is the use of multiplication factors to establish effective roof area. From plan area the appropriate factor for the roof slope can be applied to determine the effective area. This method is similair to that shown in Approved Document H of the Building Regulations. The table below provides a wider range of factors to enable accurate assessment of effective roof area to be determined.

 

Multiplication factor
Roof pitch
Factor
Roof pitch
Factor
10°
1.088
30°
1.286
12.5°
1.111
32.5°
1.319
15°
1.134
35°
1.350
17.5°
1.158
37.5°
1.384
20°
1.182
40°
1.419
22.5°
1.207
42.5°
1.459
25°
1.233
45°
1.500
27.5°
1.260
47.5°
1.547

For roofs of 50 and above a factor of 1.600 may be used.

Vertical surfaces
Where pitched roofs abut vertical walls the catchment area is likely to be increased as a result of wind driven rain. To allow for this half the vertical surface area of the wall should be added to the effective area of the sloping roof. 		 

Flat roofs
For roofs with a pitch of less than 10% the effective area is taken as the plan area

 

Roof with 30 degree pitch


Rainfall intensity
The Building Regulations 2002 Approved Document H recommends a flat race rainfull intensity of a 021 litres/second per square metre of roof area be used for design calculation. This intensity is virtually the same as 75mm/hour that has been used for the design of eaves gutter systems since the publication of BS 5367 in 1983 which has now been withdrawn.

Where a flat rate rainfall intensity is not adopted for the purpose of design, reference should be made to BS EN 12056-3:2000 which provides detailed information on rainfall throughout the UK by geographical location and frequency of occurence.

For more information about roof drainage click here or visit the official websites of Marley, Hunter & Polypipe

Untitled Document
important info
Building Regulations
heating info
boilers
underfloor heating
plastic roofline info
roof drainage design
gutter flow capacity
gutter calculations
roof & balcony outlets
aluminium systems
overview
roof drainage design
gutter systems
roof outlets
downpipes
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Drakes Plumbing Supplies is an independent plumbers merchants with branches in Kent and Sussex at Tunbridge Wells, Uckfield, East Grinstead, Sevenoaks, Ashford, Gillingham and Sittingbourne. Drakes has been an independent plumbers merchants since 1983.

Drakes Plumbing Supplies. Your independent plumbers merchants.

Head Office: Unit 16, Spa Industrial Park, Longfield Road, Tunbridge Wells, Kent. TN2 3EN.
Tel: 01892 559 915 Fax: 01892 559 916
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