If you are going to build on an existing house block you will probably have a planning permit condition that storm water must discharge to an approved point.
If there is no surface water system one option can be to discharge to the street.
This is normally by constructing a proper kerb outlet like the photo below:
But not like this dodgy installation on the right!
Or even discharging over people walking along the path like the one illustrated in this post: Overflow Fail
A kerb connection can be at a reasonable cost as long as the house is above the road.
If the house is below the road you will need either:
When most people think about rainfall they think about the amount of rainfall . . . . . . . . . but it’s just as important to understand how hard it can rain, the ‘Rainfall Intensity’.
The ‘Rainfall Intensity’ is important when thinking about things like; Roof Gutters, Down Pipes, Stormwater Pipes , Tank Overflows, and even how high your house is above ground level.
Photo courtesy of www.mirror.co.uk
Recurrence Interval
When talking about heavy rainfall you will frequently hear talk on the news about things like a ‘1 in a 100 year storm’. What that means is that a statistical calculation indicates that a storm of that strength is only expected to occur once in any 100 year period, the ‘Recurrence Interval’.
There are two rainfall intensities that the building codes normally consider. They are based on the peak 5 minutes during a storm for vaious ‘Recurrence Intervals’.
Intervals can be from: 1 in 20 years to: 1 in 100 years (See this link for more information: Roof Choices)
Intensity Calculation
Statistical intensities have been calculated for all locations in Australia and are available at the Bureau of Meteorology (BOM) Website.
Search for ‘Intensity Frequency Duration (IFD)’
To Use the IFD
Enter “your town name ” “coordinates” into Google and write down the coordinates
Click on the Create An IFD button
Enter the coordinates into the Decimal Coordinate box
Enter the town name in the Location Name box
Tick that you accept the Conditions of Use
Press the Submit button
Click on the Table tab
The following is the top line of the table for Werribee in Victoria.
Duration
1EY
1 year
50%
2 years
20%
5 years
10%
10 years
5%
20 years
2%
50 years
1%
100 years
5 mins
44.9
60.1
83.4
99.7
121
153
179
The units are mm of rain in 5 mins (Multiply by 12 to get an hourly flow rate)
Low Rainfall Intensity
Parts of the Plumbing and Building Codes refer to areas of “Low Rainfall Intensity”.
A location of “Low Rainfall Intensity” means the 5 minute rainfall intensity for an average recurrence interval of 20 years is not more than 125 mm/hour.
Basement, or underground garages are becoming more popular.
Its one way of getting a bigger house on a smaller inner suburban block.
Before you make a decision to go for this solution its worth thinking about the cost, and the risk.
Cost
They aren’t a cheap solution for a few reasons:
There is a lot of additional excavation with all excavated soil needing to be removed. Expect to pay over$100/cubic m
Supporting adjacent properties during excavation. You are probably going to be excavating below the foundation level of your neighbours, so its not just digging a big hole. Specialised techniques to do this safely will add to the cost
Not only are the basement floor and walls going to be strong they are going to have to be waterproof.
As the garage floor is probably going to be below the Storm Water Drains you will need a pumping to keep the garage dry.
Risk
The drainage company I work for has had to pump out several flooded basement garages over the past few years. The reasons for this are:
Even in Melbourne a heavy storm can easily result in over 25mm/hour.
For a driveway like the one in the photo above with a total area of around 100 m2 that means about 2500/hour litres flowing into the garage.
With that sort of rain you might also get water coming off the street, or the garden.
If the pump blocks, or there is a power cut the water can quickly build up.
Haven’t got room for a ramp? . . . Check out a different idea at this link: Spot the Garage!
Have you ever had problems with an underground garage?
Have you ever wondered what this is in your garden?
Perhaps you have seen ‘ORG’ on a drawing.
Well the answer is it’s called an Overflow Relief Gully. It needs to between the house and the connection to the main sewer.
An ORG is a vital part of protecting your house against Sewage blockages in the main causing an overflow inside your house.
The grate is set 150mm below the level of the lowest waste water fitting in your house. Normally the shower drain or Floor Drain. ( In cases where the 150mm minimum height cannot be achieved, a reflux valve should be installed in addition to the ORG)
In the event of a sewer blockage the sewage can flow out of the ORG. Not very nice!…….. but much better than flowing out across your floor.
To make sure it works when you need it:
DON’T landscape over it!
DON’T put a plant pot …… or anything else, on top of it!
When I started in the drainage business 40 years ago we called them Manholes.
Since then due to political correctness we have had; Person Holes, Access Points, Inspection Pits, but now we seem to be coming back to calling them Manholes.
We are not talking about the small inspection covers less than 300mm across for the individual house system but pits big enough for someone to climb down to inspect the public sewer or council storm drain.
Some manholes look like the photo, some have round concrete covers and some have square concrete covers.
But what does it mean to you?
Appearance
The initial issue is that the manhole may spoil the look of your driveway or paved area of the garden.
Maintenance
Hopefully with most new developments it will be quite a while before something goes wrong. Rebuilding in a long established area you may not be as lucky. For older sewers tree roots getting into the pipes are a big cause of problems, which might need action a couple of times a year.
Well if something goes wrong with the public system someone will want access to the manhole
Don’t bury the manhole or cover it with paving. You don’t want to delay things if there is a blocked pipe which affect you.
When you plan your house It’s going to be easier if you can give access to the manhole without workmen going through the house, or garage. If not you might have to take some time off work.
This diagram indicates a charged drainage system. These are sometimes called either a “wet” or a “pressure” system.
With this type of system a section of the pipe always remains full unlike a Conventional System.
As the pipes are under pressure it is essential all the joints in above ground and underground pipework are fully watertight
Advantages
A neater appearance than having pipes above ground.
Allows you to discharge water at a height above the ground level at the down pipe.
Disadvantages
It is more likely to block as the flow through the pipes can be fairly slow and the low points can collect silt. Silt will get deposited at flow rates below 0.6m/sec which will be the situation for most rainfall events.
It is also more difficult to unblock.
Can cost more with excavation and additional inspection fittings .
Potential for mosquito breeding in water unless appropriate screes are installed.
The vast majority of new houses will have a conventional (sometimes called a Gravity, or Open system) storm water drainage system discharging to either:
Public Surface Water Drain – Typically in Eastern States
Soakwells on Sandy Sites – Mainly in WA
With a conventional system like this the pipes are either vertical or at a slope towards the discharge point.
A feature of this system is that when there is no flow all the pipes are empty.
Advantages
Simple and inexpensive to design and construct.
If well designed, and constructed, the speed of flow in the pipes will prevents silting and subsequent blockage.
Disadvantages
This type of system can look very untidy when taking water to a Rainwater Tank that is some way from many of the downspouts (It results in lengths of pipes suspended in mid air)
Difficult to transfer water to a discharge point that is above the ground level of the building, although below the gutter level. A problem often encountered on demolition and rebuild projects and battleaxe blocks.
If you are planning a rainwater tank or are having problems with getting storm water to a suitable discharge point you could consider a Closed System
Something you might never have thought about when thinking about the sort of roof that you want on your new house is Risk.
Low Risk
A conventional roof which slopes to the outside can be considered to be a low risk roof.
In severe weather conditions heavy rain can exceed the capacity of the gutters and the downspouts causing an overflow. Blockages in the gutters and downspouts will also cause overflows
With a conventional roof the gutters will overflow to the outside of the building as shown in the diagram to the right avoiding damage to the inside of your home.
High Risk
A couple of other roof choices are of much higher risk.
These are:
Roof with Parapet
This sort of construction is used to give the effect of a Flat Roof, although there is actually a pitched roof behind the parapet wall.
As you can see from the diagram in severe rain, or a blockage, there is a greater risk of the overflow occurring inside the house.
Butterfly Roof
A butterfly roof is when two pitched roof panels fall to a central box gutter.
Again you can see the real risk of overflow into the house in case of problems.
What the Regulations Say
For a ‘High Risk Roof’ the regulations require the roof plumbing to be be designed for a heavier storm (1 in 100 years), rather than 1 in 20 years for a ‘Low Risk Roof’.
A ‘High Risk Roof’ drainage system also require special overflows to be installed.
This link will show you how to calculate the rate of rainfall used in design: Rainfall Intensity
In practice I hear of a number of non complying plumbing installations, and many of the overflows I see don’t look to have adequate capacity.
Did anyone check your roof drainage calculations? ……..If I chose a high risk roof it would be something I wanted to be checked thoroughly.