Drainage & rainwater goods
It may seem like little more than a finishing touch, but a lot of detail goes into specifying the right rainwater management setup. Tim Doherty sets out what you need to know
Specify the right water management setup for your project with Tim Doherty's practical guide
Rainwater goods not only protect your home against water damage, they form part of the main aesthetic of the house. So whether you’re building from scratch or renovating an existing property, it’s important to consider this aspect at the design stage, so that you can accommodate all the installation details and pick materials and finishes that meet your expectations for style and performance. The setup will also need to be hooked up to a suitably-planned drainage system. It’s not rocket science, but it does need to be properly thought through.
Selecting materials
Unless you specify otherwise, most builders will default to plastic rainwater goods. These are affordable, value-for-money and easy to install – especially on buildings that have clear vertical fascias, standard roof overhangs and brickwork facades. Fitting downpipes becomes a bit more complicated when the external walls have more than one finish (such as tile hanging at first floor, interrupted by a corbe-led brick band at intermediate floor level), or where open rafter feet dictate the use of nonstandard metal brackets fixed to the side of the rafters to support gutters. But in general, plastic rainwater goods are simple to clip together, make waterproof and assemble.
Plenty of other options are available, including galvanised steel, aluminium, stainless steel, copper, GRP (glass-reinforced plastic) and cast iron. These all cost more than standard plastic, and the fixing details for some can be quite complex. But as a rule they will look smarter, wear better and outperform the lighter weight plastic options.
Galvanised steel products will either be supplied with a protective plastic coating or painted on site (otherwise the thin finish will eventually allow corrosion to set in). Aluminium is usually powder-coated and can be selected to match or contrast with the colours and profiles of your windows, doors and cladding materials.
Stainless steel is a good choice for coastal locations, as it can resist the caustic effects of salt water spray. Copper will develop its own special patina (through oxidisation) that provides long-term protection; while cast iron, as long as it is properly maintained, usually looks and feels the most solid and robust. GRP options can mimic the chunky profiles and sections of cast iron, making it a very credible long lasting, low maintenance alternative. Some older properties may have timber gutters using hardwood which has been lined with lead. On the right property, these will look fabulous and thoroughly authentic.
Gutter design & installation
The roof slope areas must be carefully calculated so you can be sure about where to position the downpipes, gutters and outlets. Sometimes this is obvious, as the roof design dictates suitable locations – for instance in between dormer windows. But large expanses of tiled surfaces may require a bigger sectional size of gutter to manage a larger runoff of rainfall.
To establish the required capacity, you need to calculate what’s known as the effective area of the roof (ERA) and multiply this by the litres of hard rain generated per second. The area of the roof depends on its pitch, but nominally you’d take the plan dimensions of the roof slope (ie length of roof multiplied by the flat dimension from the eaves to the ridge line) and multiply this by half of the ridge height from the eaves.
Hard rain is defined as 75mm of rainfall in one hour or 0.021mm in one second. So, if the effective roof area to be drained is 60m2, then the flow rate of your gutters will need to be able to handle 1.26 litres per second (being the ERA of 60 multiplied by 0.021). All manufacturers should be able to tell you their guttering products’ flow rates so that you can be sure about capacity.
Gutters should normally be laid with a fall (slight drop) of circa 1:80 from entry high point to the downpipe outlet. In practice this means a fall of around 125mm in the gutter height over a 10m run, which is quite a lot given that many fascia boards will only be 150mm deep in total. However, leaving the gutter level (flat) or with a back-fall would allow standing water to accumulate in the base. This would cause faster weathering, erosion, joint fatigue and leaks – especially when water freezes during the winter months.
Discharge points
Rainwater (surface water) must not contaminate the foul drains, so the two disposal systems must be kept entirely separate. The guttering could therefore be directed to a surface water sewer, if one is available. Alternatively it could connect to one or more soakaways and/or an irrigation system in the garden that will ultimately discharge to an established water course.
The surface water sewer route is really only available in urban settings, and you’ll have to apply to make the connection (there’s no guarantee of approval). For other sites, water courses and soakaways are the only options.
Due to increased flash flooding at unexpected times of the year, you may also need to consider attenuation. This effectively means building-in storage capacity, so that pipes, manholes and soakaways aren’t overwhelmed when high volumes of rainfall suddenly arrive. Attenuation tanks can be of beach shingle, compacted to the required thickness under hard-standing surfaces and contained by driveway edgings. Or they could be plastic crates which, when bolted together, will provide enough cubic meter capacity to hold anticipated volumes of surface water.
The soakaways are usually variations on this theme, being large excavations full of crushed stone or a whole series of plastic crates wrapped in geotextile membranes. Sizes are calculated based on the porosity of the soils and sometimes soakaways are simply not an appropriate solution, no matter how big they become; such as with very wet, sticky clay. Local boreholes may be used to provide a shaft (or several) through the clay down to a more porous layer of bedrock or superficial soils.
Some folks may consider rainwater harvesting for garden irrigation. Others will seek to capture and recycle their grey water waste for flushing WCS. Both options have great merit but will be designed separately. Last but not least, sustainable urban drainage (SUDS) should be mentioned. This approach seeks to encourage the use of fewer hard materials for driveways etc and more permeable options, so that surface water run-off will not artificially overload urban sewers.
Drainage installation
Foul drainage runs stem from the base of the soil and vent pipes (SVPS), which are typically positioned inside new homes rather than on the outside face of the external walls. Modern SVPS are supplied in grey or black plastic. Where the swept bend travels underground, it will then meet all the external drainage pipes, which will usually be of the same diameter but supplied in brown plastic. Some builders still prefer using clay pipes, but the lighter plastic ones are easy to install and just as robust.
Drain runs should never be flat. They should be laid to a fall of between 1:60 and 1:80. Pipes should be completely surrounded with pea shingle and, prior to backfilling the trenches, a geotextile layer fitted to protect the shingle from contamination by the larger stones contained in general hardcore. The shingle is not there to act as a quasi French-drain but instead to protect the pipe from any compacted point loads that could crack or pierce the pipe wall.
Manholes or inspection chambers will ideally be inserted at every change of direction. Where long pipe runs may not have an obvious manhole break, a rodding eye can be inserted to ensure access to clear any blockages. Foul drains have to be tested to prove that there are no leaks, and this is done by placing a bung downstream, charging the system with air or water and then measuring any drop in pressure.
Mains sewer connections
In most situations, it’s possible to tap into a nearby main (foul) sewer. In urban environments, these are usually located in the nearest road. This will almost certainly be the most cost-effective option – even though the connection itself and any works in a qualifying highway will have to be carried out by approved, registered contractors.
So, you’ll need to seek a quote from the water company responsible for the sewers in your area. They will assess the complexity of the connection, the length of time that the works will take and any safety procedures required (visibility, traffic lights, road closures etc).
No access to mains?
If you’re out in the country and there are no mains sewer options, then a private discharge setup will be needed. This will have to comply with the Environment Agency’s requirements, which are easily accessible on the internet. Essentially, you don’t need to have a permit for this as long as you accept what are known as the ‘general binding rules’ that apply for private discharge to either the ground or to a water course.
There are checklists for compliance, and the property owner must ensure all the criteria are satisfied. A significant recent change is that septic tanks can no longer be used if you are discharging to a water course. Instead, compliant treatment tanks must be used as a minimum. Up to 5m3 of water can be discharged to a water course through a treatment tank. When discharging to ground, a significantly smaller volume of only 2m3 is permitted – and there are rules about how this can be designed depending on the porosity of the soil.