This document convers the design of gutters and downspouts/downpipes for roof drainage. The design includes sizing of the gutters and downspouts. The rain data used is as per National Building Code of Canada, 2010.

1. Design of Roof Drainage System By: Ayaz Malik
Length of the Roof Panel, L = 292 ft
Plan Width of the Roof Panel, W = 124 ft
Pitch of the Roof, x = 1 in 12
Design Area for Pitched Roofs
When a roof is pitched, its plan area is less than its true area. However, using the true area in the
calculations has typically resulted in oversized gutters, downspouts and drains. Table below shows
the factors that should be used to determine the design area for pitched roofs. The plan roof area
should be multiplied by this factor. The result is the design roof area that is used to calculate the
required sizes of downspouts.
from upto
0 3 1.00
4 5 1.05
6 8 1.10
9 11 1.20
12 > 12 1.30
Roof-pitch factor, F = 1.00
Down Spout and Gutter Design
Roof drainage systems are designed to carry off rainwater from the most intense rainfall that is likely
to occur. A certain amount of time is required for the rainwater to flow across and down the roof
before it enters the gutter or drainage system. This resulst in the smoothing out of the most rapid
changes in rainfall intensity. The drainage system, therefore, need only cope with the flow of
rainwater produced by the average rainfall intensity over a period of few minutes, which can be
called the concentration time.
As per NBC 2010, Division B, Appendix C, it has been customoary in Canada to use the 15-minute
rainfall that will probably be exceeded on an average of once in 10 years.
Area
Factor
Pitch, in. /12 in.
x

2. Assumption:
· Rainfall intensity is measured over a 15-minute period. It is recorded in mm/hour as the resulting
accumulation, as if the intensity remained constant for a full hour.
· It is assumed that during a rainfall with an intensity of 1 in. per hour, each in 2
of downspout can
drain 1200 ft2
of roof. If the intensity is doubled, the downspout capacity is halved, or 600 ft 2
, if it is
tripled the capacity is one-third, and so on.
Downspout Design:
Length of gutter per downspout = 40 ft
(Note: each downspout should drain a maximum of 50 ft. of gutter)
Plan Area of Roof, A = 4960 ft2
Roof Area to be drained, A x F = 4960 ft2
As per NBC 2010, Division B, Appendix C, Table C-2, for Fort McMurray,
Rainfall intensity = 13.00 mm/15 min.
= 52.00 mm/ hour
= 2.05 in./hour
For rainfall intensity of 2.05 in./hour, we have
Area drained per in2
of downspout = 586.15 ft2
/in2
Required area of each downspout = 8.46 in2
(Governs)
Minimum recommended area of each downspout = 7.00 in2
From the table provided in the end, we have following design choices:
1- 4 in. Dia. Plain Round
2- 4 in. Corrugated Round
3- 4 in. Plain Rectangular
4- 4 in. Corrugated Rectangular
Gutter Design
Ratio of Gutter depth to width, M = 0.50
Area to be drained, A = 4960.00 ft2
Rainfall intensity,I = 2.05 in./hour
Length of the Gutter, L = 40 ft
I x A = 10154 ft2
. in./hour
In the chart provided in the end, find the vertical line representing L, proceed
vertically along this line to its intersection with the oblique line representing M, then move horizontally
to intersect the vertical line representing IA. The point of intersection gives the required width of
the gutter.
Width of the gutter (from chart) = 8 in.
Depth of the gutter = 4 in.

4. Dimensions of Standard Downspouts