COP v3.0:roof-drainage; rainfall-intensity

5.3 Rainfall Intensity 

When calculating roof drainage where significant inconvenience or injury to people or damage to property, including building contents, is unlikely—e.g., due to an overflow of external eaves gutters— the Average Recurrence Interval (ARI) used must be 10 years.

When calculating roof drainage where significant inconvenience or injury to people or damage to property, including building contents, is likely—e.g., due to an overflow of internal gutters—the Average Recurrence Interval (ARI) used must be 50 years.

A higher level of rainfall should be allowed when designing for higher risk situations.
The rainfall intensity in New Zealand is determined by the rainfall during a ten minute period, measured in millimetres per hour.

Although a rainfall of (for example) 17 mm in ten minutes (that equals 100 mm/hr) is unlikely, the equivalent of 3 mm in two minutes is likely and is, therefore, used as a basis to avoid flooding.

The MRM COP publishes 10 min rainfall for instensity maps for both islands as a quick reference. Rainfall Intensity information for a specific site can be obtained using the High Intensity Rainfall Design System (HIRDS) tool from NIWA at: https://hirds.niwa.co.nz/

 

 

 

 

The 5.3A New Zealand Rainfall Intensity Map shows areas with a 10% probability that rainfall will exceed the specified amount over a ten-minute duration for an ARI of fifty years.

During rainstorms, long periods of steady rainfall are interspersed with heavy downpours for short periods and the roof-drainage system should be capable of handling the peak intensities without flooding.

On large low pitched roofs, there is a considerable time-delay between the onset of rain and when the water drains at the downpipe. This time lag changes the rate of flow capacity needed for the gutter and downpipe to drain without overflow.

Gutter overflow is acceptable on eaves-gutters or freely discharging downpipes if they are designed to do so, but it is unacceptable from internal gutters or downpipes.

When the site rainfall intensity shown on the 5.3A New Zealand Rainfall Intensity Map is greater than 100 mm/h, the gutter and downpipe cross-section areas must be proportionally increased as prescribed in 5.3.2 Capacity Calculations.

5.3.1 Catchment Area 

The rain catchment area for a roof, or roof and wall, is determined by the direction of wind-driven rain; it depends on the fall angle of the rain and walls next to the roof.

The effect of wind on rainfall needs to be calculated for all roofs with a pitch greater than 10°, using a slope of 2:1 for the calculation.

5.3.1A Wind-effect on Rain

(optional caption

The roof catchment area needs to be adjusted because rain is usually accompanied by wind, which can increase the catchment area. There are various formulae and slope factors that can be used to determine the specific wind drift effect for each building, some of which are shown in AS/NZS 3500.
The formulae quoted in this C.O.P. assumes the worst scenario and provides a conservative answer because shielding would decrease the catchment area when the wind is in the opposite direction,

The sloped roof catchment area for all sloped roofs which has a pitch of more than 10° and is freely exposed to the wind, must be increased by 10% to allow for the wind drift effect.

For multiple roofs, some allowance can be made for shielding, but in the interests of simplicity—and because of the permutations of roof pitch and length—the sum of the sloped roof areas will give a conservative figure the internal gutter capacity calculation.

There are three ways to calculate the catchment area for roofs with a vertical wall next to the roof slope.

  •  Where the roof pitch is less than 10°, the catchment area is the sum of the sloped surface area and 1/2 of the vertical wall surface area.  See 5.3.1D Roof with a Wall.
  •  Where the roof pitch is more than 10°, the catchment area is the sum of 1.1 multiplied by the sloped surface area and 1/2 of the vertical wall surface area. See 5.3.1D Roof with a Wall.
  •  Where there are vertical walls at right angles to each other, the catchment area is the sum of the sloped surface area and 1/2 of both the vertical wall surface areas. See 5.3.1E Roof with Two Walls.

The catchment area for high vertical walls, such as a multi-storey building, may be considerably less than half its surface area.

 

 

 

 

When a high vertical wall added next to an existing gutter, it is likely that the gutter capacity would be overloaded. In this case either the increased catchment area should be drained separately or the gutter should be redesigned.

 

5.3.1F Catchment Area Calculation

All roofs freely exposed to the wind<10°=sloped roof area
Sloped roofs freely exposed to the wind>10°=sloped roof area x 1.1
Vertical wall/adjacent to the roof slope<10=sloped roof area + 1/2 wall area
Vertical wall adjacent to the roof slope>10°-sloped roof area + 1/2 wall area x 1.1

After the catchment area has been determined by using Table 8.2.1, the capacity of the gutters and downpipes can be calculated by two simplified methods:

  •     Method 1 Graph for pitches <10°. (See graph 8.2.2.)
  •     Method 2 Calculations for any pitch.

 

5.3.1G Single Sloping Roof

Catchment: A=Ah + 0.5 Av

 

 

5.3.1H Vertical Wall with Sloping Roof

Catchment A = Ah+ 0.5(Av2-Av1)

5.3.2 Capacity Calculations 

Because the roof pitch, length of run, gutter and downpipe size, shape and fall are all interrelated in the determination of the capacity of each other, the calculations in sizing these components can be complicated.

Assumptions can be made to provide a conservative and simple assessment of the capacity of spouting, gutter and downpipe for roof drainage by two methods.

 

 

 

5.3.2.1 Method 1: Gutter and Downpipe Capacity Determined by Graph 

When using the simplified capacity calculation graphs, the following assumptions have been made.

  • roof pitches are 3° – 10° (for greater pitch see 5.2.2.3A Roof Pitch Adjustments);
  • the roof area is 50m² – 300m²;
  • the minimum cross-sectional area of gutter is 4000mm²;
  • flat gutter or spouting (for design purposes only);
  • there are no restrictions — no spouting, gutter or downpipe angles;
  • free discharge — weir into a sump or rainwater head with overflow;
  • the rainfall intensity is 100mm/h (for greater rainfall see 5.3.2.3A Cross-sectional Area per m2 for rainfall of 100 mm/h); and
  • external vertical downpipes.

Given these assumptions, the design capacity of gutters and downpipes is given in the following graphs.

 

 

 


 

5.3.2.2 Roof Pitch 

5.3.2.2A Roof Pitch Factors

10° - 25°multiply by a factor1.1
25° - 35°multiply by a factor1.2
35° - 45°multiply by a factor1.3
45° - 55°multiply by a factor1.4

5.3.2.2B Example Calculation (Finding Capacities Using the Simplified method)

GivenFreely exposed mono-slope roof
Tauranga
Sloping rafter length 5.9 m
Length of building 10 m
Roof pitch 24°
Step 1:From Rainfall Intensity Maprainfall = 150 mm/hr
Step 2:From Table 2factor = 1.5
Step 3:Roof pitch from table 3factor = 1.1
Step 4To find the factorised catchment area from Catchment Area Calculation Table
 59 m² x 1.1= 65 m²
 65 x 1.5 x 1.1= 107 m²
Step 5External gutter= 10 0 mm² x 107= 10,700 mm²
Internal gutter= 200 mm² x 107= 21,400 mm²
Vertical downpipe= 50 mm² x 107= 5,350 mm²
Horizontal downpipe <15°= 100 mm² x 107= 10,700 mm²
Step 6From Table 8.2.2.3. Find suitable spouting and downpipe.

5.3.2.3 Method 2: Gutter and Downpipe Capacity Determined by Calculations 

After the catchment area has been determined by 5.3.1F Catchment Area Calculation, the capacity of the gutters and downpipes can be determined by using the tables below.

 
external gutter or spouting
= 100 mm²
internal gutter
= 200 mm²
vertical external downpipe
= 50 mm²
horizontal downpipe <15°
= 100 mm²
For rainfall of more than 100 mm/h the catchment area must be factorised to allow for the increased rainfall in line with the Rainfall Intensity Map.
80mm/hr
multiply by a factor
0.8
100mm/hr
multiply by a factor
1.0
150mm/hr
multiply by a factor
1.5
200mm/hr
multiply by a factor
2.0
 
 
10° – 25°
Multiply by a factor
1.1
25° – 35°
Multiply by a factor
1.2
35° – 45°
Multiply by a factor
1.3
45° – 55°
Multiply by a factor
1.4
 

5.3.2.3D Example Calculation (Finding Capacities Using the Simplified method)

GivenFreely exposed mono-slope roof
Tauranga
Sloping rafter length 5.9 m
Length of building 10 m
Roof pitch 24°
Step 1:From 5.3A North Island 10 min Rainfall Intensity: 50 year ARIrainfall = 150 mm/hr
 From 5.3.2.3B Rainfallfactor = 1.5
Step 2:From 5.3.2.3C Pitchesfactor = 1.1
Step 3To find the factorised catchment area from Catchment Area Calculation Table
 59 m² x 1.1= 65 m²
Calculation
65 x 1.5 x 1.1= 107 m²
External gutter= 10 0 mm² x 107= 10,700 mm²
Internal gutter= 200 mm² x 107= 21,400 mm²
Vertical downpipe= 50 mm² x 107= 5,350 mm²
Horizontal downpipe <15°= 100 mm² x 107= 10,700 mm²
Step 4From Table 8.2.2.3. Find suitable spouting and downpipe.
External Gutter or Spouting= standard 175 rectangularOK
One vertical downpipe= 100mm round or 100mm x 75OK
OR1/4 round with two downpipes(one at either end see drawing 8.2.2.)OK
Two vertical downpipes= 63mm roundOK
Step 5Internal gutter= custom-made = 220 x 100 = 22,000OK

 

5.3.2.3E Standard Gutter Capacity

Sizemm²
125mm 1/4 round5 000
125mm x 75mm rectangular9 375
175mm x 125mm rectangular21 875
300mm x 125mm rectangular37 500

 

5.3.2.3F Standard Down Pipe Capacity

RoundSize (mm2)Rectangular/squareSize (mm2)
65mm3 318100 x 50 mm4 500
80mm5 027100 x 75 mm6 750
100mm7 854100 x 100 mm9 000
125mm12 272  
150mm17 671N.B. square and rectangular downpipe capacities have been depreciated by 10%)

 

An internal gutter normally requires 20 mm freeboard but using this simplified method to calculate catchment for in areas with a rainfall of more than100 mm/hr, and having a catchment basis of 200 mm/hr, an internal gutter would be over-designed if additional freeboard was added.

Similarly, internal downpipes, having no overflow to the exterior of the building, could be under-designed and their capacity should be increased by 25%.

N.B. Flooding is usually the result of a faulty drain, rather than the downpipe.

For domestic and small commercial buildings standard spouting, gutters and downpipes are the most economical way to comply with the capacity requirements, but if large gutters are neededthey should be custom made.

The position of the outlet can make a significant difference to the size of the gutters and downpipes

 

As can be seen 5.3.2.3G Gutter Capacity in , when the catchment area is identical, the gutter capacity at A can be reduced by half at B and to one quarter at C. The downpipe capacity required at both B and C is half that of A.

The preferred proportion for an internal gutter is 2:1, i.e., the sole of the gutter should be twice the height. The minimum height of an internal gutter should be 70mm, but the recommended height is 1/60 of the length.

The recommended maximum length of a coated steel gutter, without an expansion provision, should be 12 m. However, 6 m can be the maximum length if the outlets are spaced at 12 m as shown in 5.3.2.3G Gutter Capacity. Non-ferrous metal gutters have length restrictions on their length based on their thickness.

When an external spouting has a dropper outlet or an external angle, the capacity of the spouting should be lowered by 10% for each outlet or angle. Outlets should be placed within 2 m of an angle.

Dropper outlets must not be used on internal gutters.
Sumps or rainwater heads must be used to drain all internal gutters and must, also, be placed at gutter angles.

 

5.3.3 Roof Cladding Capacity 

The capacity of metal cladding profiles is determined by their geometry, the roof pitch and rainfall.

The height of the lap is the determining factor for overflow and water ingress. Therefore, the lap should be sealed if it is below minimum pitch, as is required for curved roofs and bull-nosed verandahs.

All New Zealand manufactured roof cladding profiles, except corrugate, have adequate free discharge capacity for a rainfall of 100 mm/hr. Corrugate should be restricted to a maximum length of 40 m or a 30 m² catchment area for a rainfall of 100 mm/hr, or decreased or increased dependent on the rainfall.
(see section 8.2. catchment area)

Where the rainfall is greater than 100 mm/hr, sheets are longer than 40 m or multiple roofs are designed, it is recommended to increase the minimum pitch by 1° per 10  m over 40 m.

A step in the roof, or any penetration, will require an increase in the drainage capacity of the profile required.

Where multiple roofs are drained directly or indirectly onto a lower roof, the total catchment area is the sum of both roof areas.

When penetrations concentrate the run-off into one or more corrugations or pans, the capacity must be calculated as described in 8.2.6 Discharge Capacity.