Guttering Calculations : Classical Rainwater provide tables of gutters and pipes capacities to show how much guttering your house will require.

Guttering Calculations : Classical Rainwater : Cast Iron Rainwater Gutter Systems

To select the size of the Classical rainwater system the following method should be used.

1. Calculate the effective catchment area (x) to be drained from the following table.

Table 1 - Calculation of area drained
Type of Surface	Effective design area (m²)
1 flat roof	plan area of relevant portion
2 pitched roof @ 30° pitched roof @ 45° pitched roof @ 60°	plan area of portion x 1.29 plan area of portion x 1.50 plan area of portion x 1.87
3 pitched roof over 70° or any wall	elevational area x 0.5

2. Let the rainfall rate (r) = 75mm per hour.

3. Your run off rate (q) = rx/3600 This gives the maximum litres of rainfall per second expected.

4. Calculate the maximum run rate required by the gutter taking into consideration the position of the outlet in the run. For example:

5. Find the capacity of the gutter from Table 2 below to match the run rate.

run rates

Table 2 - Gutter Capacities
	Gutter Type and Size
Capacity l/sec	Half Round	Beaded Half Round	Deep Half Round	Express	Ogee	Moulded No.46	Notts Ogee	Box
1.75	-	-	125 x 75	-	-	-	-	-
1.50	-	-	-	125	-	-	-	-
1.42	-	-	-	-	125 x 100	-	-	-
1.40	150	-	-	-	-	-	-	-
1.39	-	-	-	-	-	-	-	100 x 75
1.24	-	-	100 x 75	-	-	-	-	-
1.07	-	-	-	-	-	-	115	-
0.97	-	125	-	-	-	-	-	-
0.94	125	-	-	-	-	-	-	-
0.79	115	115	-	-	-	-	-	-
0.71	-	-	-	-	125	-	-	-
0.67	-	-	-	-	-	100 x 75	-	-
0.59	-	100	-	-	115	-	-	-
0.53	100	-	-	-	-	-	-	-
0.41	-	-	-	-	100	-	-	-

If you require your calculations checked, please contact our Technical Department.

6. This figure may be reduced due to the length of the gutter run. To Find the true capacity of the gutter use the following calculations.
a) Divide the length of the gutter to the outlet in mm (I) by the depth of the gutter in mm (d).
b) Consult table 3 to get the next highest reduction factor.
c) Multiply the reduction factor by the gutters stated capacity to get the true capacity.

Table 3 - Reduction Factors
l/d	Reduction Factor
50	1.00
100	0.93
150	0.86
200	0.80

7. When the true capacity of the gutter has been calculated Find the suitable size from table 2.

8. The capacities of the rainwater pipes can be found in table 4.

Table 4 - Pipe Diameters
Pipe Diameter (mm)	Capacity Litres/Second
65	2.00
75	3.00
100	7.00

Capacity Testing

Sizes of the standard half round gutters have undergone hydraulic testing to confirm actual capacity capabilities and performance (other gutter capacities are calculated values). Tests were carried out independently at H.R. Wallingford.

The new rainwater sockets have also been assessed in conjunction with the gutters.


	Installation Guide

	Technical Calculations

	Product Markings

	Standard Specification

	Technical Advisory Service

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Endorsed by British Standard BS EN ISO 9001:2000, BS460 Product Standards and BBA Approval.
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