Valley-Gutter-Design
waiting for calculations

VALLEY GUTTER DESIGN
For any Catchment area, any Roof slope, and any Rainfall
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Rainfall: Either choose a Location
How to find the Intensity for other places.

TRIANGULAR VALLEY GUTTER
Unit Conversions.

Parameter Sym Unit Value
Roof Slope 1 INFO ?? RS degs
Building Angle A INFO ?? A degs
Trial DP dia INFO? (90,100,125,150,225,300) DP mm
Freeboard FB mm
Flow Q L/s
Gutter Slope GS degs
Gutter Side Angle Sang degs
CFD Depth d mm
Allowance for Wave Motion w mm
Depth Adjusted for Wave Motion Dw mm
Sheeting drop in excess of FB of 15mm Sd mm
Total Effective Depth =
(Dw + FB + Sd)		 De mm
Effective Width We mm
Required Sheet Width SW mm
Chosen Sheet Width (OPTIONAL_??) SWopt mm
Rain Water Head depth INFO RWH_dvg mm
Rain Water Head Lgth RWH_Lvg mm


Valley gutter building angle used in the programme
Valley gutter cross section with nomenclature used in the programme
Valley gutter configuration used in the programme
Minimum valley gutter width
Roof 2 slope INFO?? RS2 deg
Box gutter width INFO_? BG_w mm
Box gutter slope BG_s deg
Box gutter water depth incl wave BGwdT mm
Max sheeting slope Sang2 deg
Sheeting drop Allowance > FB Sd2 mm
Freeboard FB mm
Total depth BG incl FB +Sd using 350w BG INFO_? BG_d mm
Depth using the entered BG width INFO_? BG_d2 mm
Rain Water Head depth RWH_dbg mm
Rain Water Head Lgth RWH_Lbg mm
Valley gutter designed as a box gutter. roof framing

Notes, instructions, and limitations.

  • The triangular valley gutter is intended to be used for equal roof slopes only.
  • The triangular valley gutter discharges into the eaves gutter. (or rain water head).
    •   provided the gutter satisfies the requirements of a RWH.
  • Sometimes more than one DP may be required to make this work. Scroll down for suggestions.
  • The Box gutter is intended for unequal Roof Slopes, and where a valley gutter is unsuitable.
      This would mainly occur where the valley gutter is too wide or the roof slopes are not equal.
  • The Box gutter discharges into an eaves gutter designed as a rain water head (RWH).
  • The eaves gutter should be designed for the total required overflow from all sources.
  • Clicking on the green underlined text will pop up an important note.
  • When changing the default building angle from 90 deg, the roof slopes must be equal.
    •   A triangular, or box gutter configuration may be used.
  • The required rainfall intensity for valley gutters is 1% AEP (1:100) 5 minute duration.
  • If BG only appears in the calculation cells, it means that you have two different roof slopes,
       and must use a box gutter.
  • Building angles other than 90deg, with unequal roof slopes, cannot be done with this programme.

    • The Valley gutter design is reasonably straight forward, and is just an extension AS/NZS 3500.3 clause 3.6.
    Scroll down to the section called "The Method" to see how this was done using computational fluid dynamics (CFD). The CFD program used was FLOW-3D hydro.

    The Variables.

    Roof Slope
    Steepening the roof slope will result in a smaller width valley gutter, and shallower box gutter.
    It will also result in being able to carry more flow. And hence be able serve a greater catchment area.
    Also a steeper roof may increase the rainwater head length This is due to the increased velocity of discharge.

    Unequal roof slopes have some very interesting geometry that the designer or the builder will have to solve.
    There is also another variable introduced in this case, and that is the width of the eaves.
    Varying this can have a significant effect. For instance, to have the Valley gutter crossing the eaves at the point of intersection of the building walls would require unequal size eaves.

    NOTE:A triangular valley gutter does not necessarily require the angled infill piece in the eaves gutter, as shown below for a box gutter, The eaves gutters can meet at right angles.

    Plan view of a valley gutter with two equal roof slopes Plan view of a valley gutter with two unequal roof slopes

    The down pipe size affects the size of the RWH.
    Increasing the diameter will reduce the rainwater head depth, and length.
    Conversely reducing the downpipe diameter will increase the depth and length of the rainwater head.

    Rain Water Head
    Placing two or more downpipes is also possible. For example, to design this for two down pipes, assume half the catchment area/flow in each. This will result in a substantial reduction to the water depth required over each DP.
    in other words this will give a reduced depth, and length, of rainwater head required.
    Note: this will have no effect on the size of the box gutter.
    Therefore you will need to do two calcs, one for the gutter size with the full catchment area, and one for the RWH size, with 1/2 the catchment area.

    Sometimes it may be easier to use that angled section of eaves gutter, as one large rainwater head.
    It's even better to place stop ends at each corner and make it a high point.
    This will ensure that the RWH does not accept water from the adjacent eaves gutters, and visa versa.

    Box Gutter Width is also a variable. Approximately 350 mm wide is the minimum. Any smaller than this would require reducing the 100mm roof sheet overhang, and/or the 150mm clearance between the sheets.
    These are Code requirements.
    However increasing the width will result in a shallower box gutter, And a shorter RWH length.
    There is no change to the rain water head depth, as this is only dependent on the size of the downpipe, and the quantity of flow.

    The Method

    CFD depiction of a working valley gutter test

    This programme has been derived from hundreds of calculations with a computational fluid dynamics (CFD) programme. The programme used was FLOW-3D Hydro.

    A range of flows from 1 L/s to 100 L/s was used.
    And a range of slopes from 5 degrees to 35 degrees was also used. This program will not calculate anything outside this range.

    CFD depiction of a valley gutter
    The slopes used were 5, 10, 12.5, 15, 20, 23, 23.5, 23.7, 25, 25.1, 27.5, 30, 35 degrees
    The flows used for each slope were 1, 2, 5, 10, 15, 20, 25, 30, 40, 50, 100 L/s
    3D CAD dwgs were produced for each slope, and gutter shape.
    This gave a total of 143 separate CFD calculations, for both the triangular shape, and the box gutter shape.

    This was sufficient to plot charts for each slope Flow/Depth combination. A typical chart looks like fig1.
    An equation was then found that fitted the curve as in fig 2.
    The equation results were then checked for accuracy against the CFD results, as shown in fig3
    CFD graph, depth vs flow for 23.7 deg roof CFD graph, depth vs flow for 23.7 deg roof CFD Table, depth vs flow for 23.7 deg roof

    This huge number of CFD results was sufficient to plot charts and calculate equations for each case. The programme uses the resultant equations to calculate valley gutter sizes for anything within the above range of flow and roof slopes. If the desired roof slope does not fall on a calculated case the programme will interpolate.
    This applies for both Triangular and Rectangular Valley gutters .
    Roof overhang showing interference with water in a valley gutter



    NOTE: if the roof slope is such that the overhang protrudes into the running water, then the freeboard is increased by this penetration amount. This starts to occur for roof slopes greater than about 25deg.

    To prevent this obstructing the flow, an extra free board has been applied.
    The extra freeboard is the amount that is required to keep the roof sheet out of the water flow.
    Note: The roof sheet overhang in this instance can be in the expanded freeboard space.
    Increasing the freeboard will also increase the valley gutter sheet width.

    BUT ARE THE ANSWERS CORRECT

    As a check of the results, the model was calibrated with Table 3.6.2 in AS/NZS 3500.3-2025 where possible.
    Table 3.6.2 relates to only one roof slope, stated as 23.5 deg, and one side slope of 16.5 deg, and one catchment area of 20 sqm.
    However to get a side slope of 16.5deg requires a roof slope of 23.7deg. This figure has been taken as more accurate as it can be verified using trigonometry.
    However it makes no difference to the results.


    So you can check it out yourself:-
    Enter 20 as the catchment area, select 23.7 as the roof slope, and for rainfall, set the dropdown list to "I prefer to enter a known intensity"
    Then check with the larger of the two intensities shown in the intensity column in table 3.6.2
    AS/NZS 3500.3-2015 Table 3.6.2
    One More Check you can do yourself Comparison table comparing Mannings equation to CFD results
    Comparison Chart comparing Mannings equation to CFD results
    Check with Manning's equation.
    Manning's equation is used to find the "normal" depth of flow in an open channel. However Manning's equation requires the flow to be constant along the channel, and does not take into account any wave motion.
    Whereas the flow in a valley gutter is increasing all the way along the channel.
    And there is plenty of turbulance.

    But here is a quick check which you can do yourself by using Manning's equation.

    Take the maximum flow in the gutter (shown as 'Q' in the program) and use 0.015 for Manning's 'n'.

    Set out a table comparing Manning's result to the CFD result.

    Then plot a chart of the results as shown.

    The multiplying factor of 1.67 is what the Australian Standard uses to allow for turbulance in a rain water head.

    The average free board is 31 millimetres. This appears to be in the ballpark.
    The standard recommends a free board of 15mm.

    Comparing the average depths of CFD to Manning's equation
    It can be seen that at the lower flows, there is a good match.
    However the two results diverge at the higher flows. This can be explained by the fact that the Manning's equation does not allow for any wave motion or turbulence. Whereas the CFD results allow for this.
    (Click to enlarge image. Close window to return)
    Comparison chart for 350w box gutter designed as a valley gutter with CFD against Manningas eqn
    Snippets from CFD
    In this view the maximum depth is shown in red.
    It can be seen that this depth occurs all over the place and continually changes location.
    CFD depiction of flowing box gutter as a valley gutter
    Plot of Depths
    Ten depth Probes were placed along the centre line of the gutter, and 10 underneath the roof overhang in an effort to reduce any effects from directly falling water.
    The probes were about 150 mm apart clustered around the max depth location for each case.
    This is a depiction of depth against time for each probe.
    You can see the water level starting from zero on the left hand side. This is at time zero.
    After a few seconds, the flow converges to roughly an equilibrium, although the surface is still very chaotic.
    (Click to enlarge image. Close window to return)
    Depth of flow in a valley gutter at 10 different locations. A plot of depth against time for each location
    Plot of a single probe
    Plot of depth against time of a single probe.
    I have generally used the highest spike, unless it was well outside the plot of flow against depth for that roof slope, as shown above. (Click to enlarge image. Close window to return)
    Plot of depth against time of a single probe.
    Plot of the flow profile along a valley gutter designed as a box gutter.
    Plot of depth against distance along the gutter. Better known as the backwater curve.
    Zero length being at the gutter outfall.

    This program calculates the equation that best fits the line of the maximum peaks in the surface profile.
    (Click to enlarge image. Close window to return)
    Plot of water surface profile along a valley gutter designed as a box gutter.

    A PERFORMANCE SOLUTION According to the latest Australian standard AS/NZS 3500.3-2025
    A Computational Fluid Dynamics (CFD) program is acceptable as a performance solution.

    POSSIBLE BOX GUTTER CONFIGURATIONS
    Plot of water surface profile along a valley gutter designed as a box gutter.
    Plot of water surface profile along a valley gutter designed as a box gutter.
    Plot of water surface profile along a valley gutter designed as a box gutter.
    Plot of water surface profile along a valley gutter designed as a box gutter.
    Valley Gutter draining to an eaves gutter with 3 downpipes.
    Sometimes for a V shaped valley gutter
    adding more DP's will reduce
    the size of the required RWH.
    Try two calcs, one to size the valley gutter,
    and if using 3 DP's use 1/3 of the catchment
    area to size the RWH. Then check to see if
    this RWH depth and RWH length will fit in the
    eaves gutter. The Depth is the depth of the eaves
    gutter, and the length is the length of the 45 deg
    diagonal of the eaves gutter
    Valley Gutter draining to an eaves gutter with triangular sump.
    Sometimes for a V shaped valley gutter
    draining to a triangular RWH maybe a solution.
    Especially if the Eaves gutter is deep enough
    but not wide enough for the RWH length.
    DESIGNERS RESPONSIBILITY

    The designer must always take responsibility.

    The programme is just a tool to make the legwork a lot quicker.

    It is assumed that the user has sufficient knowledge and experience to do a few calculations to satisfy himself as to the suitability of this programme.

    This can be done by checking one or two results with another CFD program,
    or checking with Mannings equation for a comparison as shown above.
    The CFD depth should be at least 1.67 times the Manning depth.
    This is the factor applied to the water depth in a RWH to allow for turbulance etc.
    Refer to Fig H3 in AS/NZS 3500.3-2025.

    It is also the user’s responsibility to saisfy himself that the programme is suitable for the project in question, that the results are as expected, and the correct data is entered in the correct box.