COP v3.0:roof-drainage; valleys

5.4 Valleys 

A valley is a gutter at the internal intersection of two sloping panes of roof cladding.

5.4.1 Valley Fixing 

Valleys should not be positively fixed, except at the head, because that would inhibit expansion and can produce noise.

Alternative means of securing the valley gutter to the substrate include:

  • A clip system allows for thermal movement and security.
  • A compatible washered nail or screw or a galvanised nail, provided they do not penetrate the sole of the gutter.

5.4.2 Valley Design 

Valley gutters must discharge into a rainwater head, sump, or an eaves gutter. The discharge point must be within 2 m of a downpipe if the catchment area exceeds 50 m².

When the roof pitch is less than 12°, the valley should be made in one piece or the joints must be sealed. To ensure snug fitting, the valley angle should be matched to the pitch of the valley support. Having the valley too open will result in a diminished capacity, and too sharp an angle will make installation difficult.

5.4.2B Internal Valley Angle

Roof PitchInternal Angle
176°
173°
10°166°
15°159°
20°152°
25°145°
30°139°
35°132°
40°126°
45°120°
50°114°
60°104°

5.4.2C Maximum Valley Catchment in m² for Areas Having a 50-year Rainfall Intensity <150 mm/h

Roof Pitch10°12.5°1520°25°30°
A 3-fold  1218294170106146
B standard  2534476399140184
C Deep6086152180215251321389452
D Tile    1722334557
Free Board: 15 mm for pitches 8° and above
                      20 mm for pitches below 8°

For other pitches, rainfall intensity, and valley shapes refer to the 5.4.7 Valley Capacity Calculator tool.

For information about internal corners, refer to 5.3.3 Internal Corners.

5.4.3 Bifurcated Valleys 

The maximum recommended catchment area for a bifurcated valley is 10 m². 

 

5.4.5 Changing Angles in Valleys 

A change of roof pitch in a valley run will usually result in the change of angle in plan view. The change is acceptable, but the freeboard of the lower valley must be at least 20 mm to allow for turbulence.

5.4.6 Asymmetrical Valleys 

Where opposing roofs of different pitches discharge into a valley an asymmetrical valley is required. They may be designed so the side under the flatter roof is at the same height as the steeper side and 20 mm freeboard is required. When this is impractical, the valley must be sized as an internal gutter in accordance with E1/AS1 Surface Water.

5.4.7 Valley Capacity Calculator 

The values for  5.4.2C Maximum Valley Catchment in m² for Areas Having a 50-year Rainfall Intensity <150 mm/h can be found in this PDF document. A responsive online tool for calculating valley capacity is available at 5.4.7 Valley Capacity Calculator.

Before using this calculator, please read 5.2 Roof Drainage Design.

To calculate valley capacity, insert the required values in the designated fields. All valleys require freeboard.

For an explanation of each element, please click on the corresponding question mark.

For rainfall intensities, refer to NIWA’s HIRDS tool or the 5.2.2 Rainfall Intensity.

Note that this site address is used only for convenience if printing calculations to attach to documentation.
This address is not factored into calculations - you must determine intensity from Rainfall Intensity Maps or NIWA's HIRDS tool.
The address is not recorded or shared with any other parties.
Select the appropriate Intensity from the Rainfall Intensity Maps, or use the Hirds-tool from NIWA.
 mm/hr
 
Select relevant options, which will determine the minimum Short-Term Intensity Multiplication Factor
 
 
The minimium Short-Term Intensity Multiplication Factor determined by the application type.
You can increase this manually for critical applications.
 
Enter 1:X or mm per metre- the calculator will automatically convert
Minimum Fall 1:500, Maximum Fall 1:100
 
1: =  mm per metre
  rads
 bends
 
 m
 
Minimum 1°, Maximum 60°
 
 °
  rads
Secondary pitch only needs to be entered manually if it is different to the main Roof Pitch
 
 °
  rads
 m
 
Select whether runoff will drain on both sides of penetration or just 1;
 
 m
 
 each
 

For rectangular gutters you can supply custom dimensions, or use pre-supplied manufacturer data
 
 
 
You can select Standard Corrugate, input profile dimensions for Trapezoidal, or use pre-supplied manufacturer data
Illustration is for explanatory purposes only.
 
B=159°D=20Width=130Freeboard=20
 mm
 
 mm
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm²
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm
 
 °
  rads
 °
  rads
 °
  rads
 
 mm
 
 
 mm
 
Must be less than the upstand, D
 mm
 
 
 °
  rads
= max ( RS , RS2 )
 °
  rads
= min ( RS , RS2 )
Using Martindales Formula:
 °
  rads
= atan ( tan ( A1 ) / tan ( A2 ) )
 °
  rads
= asin ( cos ( A1 ) * cos ( A2 ) ) + pi()/2
 
= cos ( A2 ) * cos ( A1 )
 °
  rads
= asin ( sC7 )
 
= tan ( A2 ) * sin ( aD )
 °
  rads
= atan ( tR1 )
 
= tan ( aD ) * csc ( R1 )
 °
  rads
= atan ( tC6 )
 
= tan ( pi()/2 - aD ) * csc ( R1 )
 °
  rads
= atan ( tC6' )
 °
  rads
= pi()/2 - C6'
 °
  rads
= pi() - C6 - C6' - C5'
 °
  rads
= C6 + C6'
Using WSP Sketch:
 
 
=W * sin ( C5' )
 
=D * cos ( C5' ) - FB
 
=IF ( ( h1max + h3 ) < h1max , h1max + h3, h1max )
 
=W * sin ( C5' )
 
=IF ( ( h1max + h3 ) < h2c,h1max + h3,h2max )
 
=IF ( ( h1max + h3 ) < h2max,0,h1max + h3 - h2max )
 
=0.5 * h1 * tan ( PI()/2 - C5 ) * h1
 
=0.5 * h2 * tan ( Beta - PI()/2 + C5; ) * h2
 
=IF ( ( h3 > 0) , ( W * cos ( C5; ) - 0.5 * h3 * tan ( C5; ) ) * h3 , 0 )
 
=( W * cos ( C5' ) - 0.5 * h4 * tan ( C5' ) ) * h4
 
=A1 + A2 + A3 + A4
 
=h1 / sin ( C5 )
 
=h2 / sin ( C5' )
 
=IF ( ( h3 > 0 ) , h3 / cos ( C5 ) , 0 )
 
=h4 / cos ( C5' )
 
=WP1 + WP2 + WP3 + WP4
 
=h2 * tan ( PI()/2 - C5 ) - IF ( ( h3 > 0 ), h3 * tan ( C5 ) , 0 )
 
=h2 * tan ( Beta - PI()/2 + C5 ) - h4 * tan ( C5')
 
=FWSW13 + FWSW24
 mm
 
 x   mm
 
 mm
 
Select Manufacturer (if applicable) and Profile
 
Pitch, or centre-to-centre measurement. Can also be calculated by (Effective Cover Data) ÷ (Number of Pans).
 mm
 
Width of the pan.
 mm
 
Calculated result from (Pitch) - (Crest).
 mm
 
Width of the crest (top of rib).
 mm
 
Total depth of profile.
 mm
 
Depth of profile from the pan to the height of the capillary tube.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm²
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 

 
 
 
 
 
 
 m
 
 m
 
 mm
 
 m
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 
 
 
 
 
 
 
 
 m/s
 
 m³/s
mm
 
 
 
 
 
 
 
 
 
 
This result is the maximum capacity that can be drained by an element of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 
This result is the maximum length of roof that can be drained by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 m
This result is the maximum area that can be drained above a penetration by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
This result is the maximum area that an upper roof area can drain using a spreader of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 

Conditions and assumptions for flat gutters:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions.
  2. Equations valid for gutters with min gradient 1:500, max gradient 1:100.
  3. Bends are accounted for by local loss coefficients (0.5 for each 90° bend).

Conditions and assumptions for downpipes:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Any grates must not restrict flow or site-specific design is to be completed - typically double the number of outlets
  3. Gutters must have fall for downpipe sizing to be valid
  4. Calculations consider weir, orifice and friction effects
  5. Orifice discharge coefficient of 0.61 assumed
  6. Weir coefficient of 0.65 and 75% of outlet perimeter assumed available for weir flow
  7. Minimum pipe gradient of 20% assumed for friction conditions

Conditions and assumptions for valleys:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Minimum height of Type A valley returns to be 16 mm
  3. Minimum freeboard of 20mm mm for valleys below 8°
  4. Minimum freeboard of 15mm for valleys 8° and steeper

Conditions and assumptions for maximum run:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)

Conditions and assumptions for penetrations:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Where Both Sides selected, assumes an even split of flow to either side of penetration

Conditions and assumptions for level spreaders:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Corrugate Profiles
    1. No discharge to lap row
    2. One discharge hole per second trough
    3. Assumes flow to top of profile (no freeboard)
  4. Trapezoidal or Trough Profiles
    1. May discharge to lap row
    2. One discharge hole per trough
    3. Assumes flow to capillary groove of profile