Rain and storm water drainage

Make sure to tackle the issue of drainage around your house early on, it will save you time, money and heartache. We look at what solutions are available to you…

If you’re building new, don’t leave the drainage works to be done later otherwise you’ll have to pay again for bringing machinery back to dig trenches, paths and driveways, as well as workmen to lay paving and/or decking. For the finished surface, choose your paving contractor carefully; look at samples, photographs, testimonials, outline plans, costs, extras and a basic agreement. For the security conscious, cable needs to be buried for electric gates and lighting, as will drains and pipes for water features.

The wetter the soil the more in depth you will have to look at the drainage issue on your site. The soil conditions will have a direct impact on the house construction as well, with regards to foundations and your septic tank. Building on a flood prone area or on boggy land is not only a bad idea, the planners won’t give you planning permission to do so! 


Connect the dots

First off, you need to know about how drainage is dealt with by your local authority. In older constructions all of the discarded water (wastewater and stormwater) went into a single pipe and fed directly into the sewage treatment plant. In more modern times, the two are separated with stormwater led off into nearby waterways and sewage to the main sewer or an on site treatment plant. The reason for this change in design is twofold, population growth and the fact that rainfall has increased over the years.

Dublin’s Ringsend wastewater treatment plant is in a high growth area and is primarily connected to the old dual system (all of the water, including rainwater, goes to the plant) and on days of heavy rainfall the plant used to have to release raw sewage into Dublin Bay due to the overload! It sounds worse than it is as the sewage is extremely dilute due to the amount of rainwater. And you’ll be glad to know works on the plant were carried out to deal with this issue by increasing capacity.

What’s perhaps most alarming is the tendency of builders and plumbers to misconnect pipes. Washing machines in particular were connected to the stormwater drainage, leading to serious contamination of waterways. Have you ever noticed suds in the stream near your home? Now you know why! It’s crucial to get these connections right and it’s actually the homeowner’s responsibility (that is, you!) to make sure it is because Ireland’s environmental laws are based on the ‘polluter pays’ principle.


Where to?

Thankfully, when dealing with stormwater drainage there is little room for error as all of the drainage coming from your roof and paving will be redirected to the stormwater system.

That said, any drained water coming from your garden probably won’t be allowed to direct it to the council’s stormwater system so you’ll need to find somewhere on your site to redirect the water. That could be a ditch, a stream or a soakaway (deep hole filled with rubble up to the point where the soil/substrate allows water to drain).

If none of the above options are open to you, it may be time to go and knock on your neighbour’s door to see if they might have somewhere you could dump your excess water! Another, perhaps preferable, option would be to create an artificial pond or swale on your land.

Go with the flow

Your drainage system will have to be sized according to the amount of water it expects to receive from your house and its environs. This will in large part be determined by the amount of rainfall you get in your area, (which you can do little about), and by the type of roof covering and amount of hard paving you decide to use.

The best type of roof covering for water management is a green roof as the plants will hold some of the rainfall in its roots, considerably reducing the amount of water siphoned into the pipework. As for other roof coverings, the water is channelled directly into the gutters; a sharp angle on a pitched roof allows for good water deflection but also increases the water flow to the pipes.

The more hard paving you use the more complex your drainage system will be and as such it is necessary to get a specialised company to design and install your drainage system, or get an engineer to design it. More drains will have to be installed and a comprehensive design for the underground piping will have to be devised.

Nowadays it’s recognised to be better to adopt what’s referred to as a sustainable drainage system (SUDS). SUDS has become the norm on commercial projects and is gaining ground in individual houses too; it takes into account not just collecting and disposing of rainwater but also looks at water treatment and conservation. SUDS also deals with avoiding misconnections (see Connect the dots above), giving preference to green roofs, as well as installing rainwater tanks and greywater systems.

From a drainage point of view, SUDS primarily deals with minimising the amount of water discharged into the waterways and also where necessary treating it via natural means to allow the rainwater to be filtered through to the ground. Methods include permeable paving and soakaway devices (see Permeable paving below). For these systems you will typically rely on the specifications provided by the manufacturer as a kit or build them on a DIY basis.

The drainage design will be specified on your construction drawings including the gradient (fall) and where the inspection chambers are to be located.


As seen above the type of paving you choose will go a long way towards determining your drainage needs; here’s your options and what you need to consider.

Hard paving

Poured Concrete can be mixed with a tint to colour it, but the colour will fade over time. Spraying with a sealant every two years helps to maintain the shading, but leaves the surface shiny. Recently, patterned concrete has gained some market share; the process involves using a rubber mat to leave an impression on the poured concrete. Cracking can sometimes occur if a large area is covered and expansion joints not included. This option requires drains to be strategically placed to prevent flooding.

Concrete Block paving comes in a range of colours and finishes and can be made to resemble stone, brick etc. It is very durable but as with poured concrete requires an comprehensive piped drainage system as sand/cement mortars are used to bond the blocks together.

Brick in the form of clay bricks and pavers are extremely robust, resistant to damage by oil and petrol and less susceptible to permanent staining than concrete. They can also help to ‘tie in’ the garden to the house by using the same or a similar type of product. If this is your plan, it is best to buy all you will need at the one time to maintain uniformity of colour. Because of the different lighting and weathering conditions, and the different composition of the mortar used, brick paving often has a different appearance from walls built of the same brick. It’s also a good idea to mix the bricks from at least three packs before starting so that the effects of slight colour differences can be minimised. There’s a huge variety of both colour and texture available, both of which can be used to created patterns or mark particular areas. Bricks are virtually maintenance free, will mellow with age and range in colour from buff and golden through to shades of red, grey and blue. The surface texture can be either rough (drag faced) for a slip resistant finish, or smooth. Slip resistance can also be achieved by laying bricks on their edge, but this increases the cost because more are needed. Subjected to a constant cycle of freezing, thawing and wetting, it is important that you choose bricks and pavers that are fit for the purpose. Bricks suitable for external walls where they are protected from being saturated by design detailing, may not have adequate resistance to frost and the conditions found on a ground surface. Don’t assume that just because the brick is heavy or is described as having allow water absorption that it will be frost resistant; follow the manufacturer’s recommendations carefully. Reclaimed bricks can be something of an unknown quantity in this regard; they may be weakened if they have been exposed to frost and the surface flake under the impact of traffic, they may also be of varying thicknesses.

Stone and Slate will last forever but must be set in a fixed base of semi dry sand/cement mix otherwise weed growth, the moisture of the soil and chipping from footfall could affect the integrity of the slabs. Because the stones/slates are, unlike flags or new brick, all different shapes and sizes, the sand/cement mix makes it easier to get a reasonably level surface. To finish, they are pointed with wet cement to prevent weed growth. Power wash every twelve months to remove algae and dirt. Similar in price are artificial stone and concrete look alike products; they weather differently as the colour is either applied to the surface or tinted right through for a longer lasting finish, so check this before you buy.

Soft paving is used for foot traffic, and include options such as wood mulch or crushed shells. The ground underneath is usually left as is but with compaction you’re likely to spend a lot of time replacing it on a regular basis!



Mortar and joints

The joints on hard paving are usually made of a sand/cement mortar but could be made of turf or other water permeable material that allows for drainage. Proprietary mortars have been devised to be both permeable and highly resistant.

On sand/cement mortar consider frost resistance; it should have a low permeability and adequate cement content, that is, a designation (i) mortar (1:3 cement:sand or 1:1/4:3 cement:lime:sand). Using coarser sand also helps, and avoid plasticisers as they may affect the durability of the mortar.  



It can occur with all clay and concrete products. It is noticeable as white powdery patches on the surface of the bricks or paving and will gradually weather away. How long this takes depends upon the site exposure, the amount of traffic over the path/driveway and the composition of the bricks/blocks. In concrete products the efflorescence is caused by lime in the limestone (one of the components of concrete), and in bricks it from salts contained in the raw clay, both of which rise to the surface when the material is wetted. It is therefore good practice to keep the pavers as dry as possible before fixing into position. Although the weather will do the job for you, the white deposit can be removed with a stiff, non metallic (i.e. bristle) brush if you want a more immediate result. Washing with water is not recommended as this will soak into the pavers and the cycle will begin again. Acid based cleaners are available but follow the instructions with care and consult the manufacturer of the paving before using as acid attacks concrete and inappropriate application may alter the appearance of the paving. Neither is it advisable to use salt as a de-icer as this can affect both the durability and the looks of the paving.


Permeable paving and systems

This has become the preferred alternative to hard paving as it provides strength, enough to be used as a driveway and other lightly trafficked areas, as well as much more effective stormwater control. It consists of either porous asphalt, porous concrete, or modular paving (large gaps between impervious areas allows infiltration).

The water makes its way down to an underlying stone reservoir which is capable of removing pollutants before the water gets to the next layer of subsoil or to a watercourse. If the pollution expected from above is high you could drain the water off to the sewage system. The soil must be relatively porous (a moderate infiltration rate) for this system to work.

These systems need to be adequately designed, and usually incorporate a top layer of porous surface, then bedding gravel or porous bricks and an geotextile fabric. The depth and volume of the stone reservoir must relate to rainfall depth and the reservoir must be built at least 1.5m/5ft above the groundwater level.

There is some maintenance involved including vacuum brushing or jetting twice a year, in the spring and late autumn. The permeable elements will become clogged over time and will need to be cleaned out; in some cases the surface may need to be lifted and some or all of the elements replaced.

Infiltration trenches and soakaways are another two SUDS alternatives. Infiltration trenches are filled gravel or rock to allow water to filter through, slowing down the amount of stormwater discharged and providing some water filtration. A soakaway is the same thing except it’s a in the form of a pit or tank instead of a trench. As with permeable paving, these must be precisely designed to work effectively, for instance the underlying soil must be permeable (clay content of less than 20% and silt/clay content of less than 40%).

Gravel is another option as are grass paving blocks which are becoming more widely used as a natural, alternative option for the need to have a hard area. Again on driveways the sub ground must be properly prepared to support the weight of vehicles, with perforated blocks available in either concrete or plastic, through which the grass grows.


Flexible paving

Although pavements can be laid using either a rigid base or a flexible one, for reasons of costs mainly to do with the need for skilled labour and the extra time involved, nearly all pathways use a flexible construction. Flexible paving can also be put on top of a concrete base, is easy to adjust and, if you’re keen to do your bit to finish the house, is a not too difficult DIY job.

Beginning at the bottom, the thickness of the sub base will depend upon the quality of the sub soil something that can vary considerably. Each soil type has a rating on the California Bearing Ratio (CBR) with low CBR’s indicating poor, clay soils and high, sandy gravel. If the CBR is low then a compacted depth of 200mm of Type 1 Roadbase material (crushed rock, frost free) will be required, falling to 100mm for a high CBR. On top of this sand is compacted in to a depth of 50mm with a screed board drawn across it to leave a smooth and level surface ready for your artwork with the pavers. Remember to use a string line to help you to keep straight lines and use a diamond tipped saw when cutting to give a clean edge. A 2-5mm gap should be left between each paver.

The paving should be finished with very dry fine sand over the top, and the compacting process repeated several times.

As with driveways, edge restraints and good drainage should also be built in. The best laid pavements and paths are made, not in heaven, but by attention to detail and not being in too much of a hurry. Some points to note are:

1. Jointing sand should be free flowing and very dry.

2. Use jointing sand that does not stain the surface of the flags or pavers and so spoil their look.

3. Joint filling with sand should be done on the same day as laying or before bad weather.

For concrete paving where cars are used don’t under specify the base or depth of filling.



Your drainage strategy will have to be designed early on and will be site-specific; here we look at what you can expect to come across.

Types of drainage for your site

On your land you have two options, either you dig a ditch or you go for piped drainage. The options listed below range from dealing with the least problematical to the wettest sites.

A simple ditch will require some form of digger as you will need to go down about 3 feet (90cm) and slope the edges. Even on this scale make sure to consult health and safety guidelines about excavations as the soil could collapse, especially past the metre mark, hence the need for a digger. Also make sure there are no underground pipes or cables, and if there are get the map of their location to avoid cutting through them. Test for any that might be carrying an electrical current.

A French drain or ditch is the next step up and these are like simple ditches except they’re filled with gravel and covered with a permeable membrane or turf, all of which is then covered with topsoil.

A full-fledged drainage system typically involves burying porous pipes about 45cm/18inches deep. But that’s hard to do on a waterlogged site because it’s generally difficult to dig into wet soil! It’s therefore advisable to this when the ground is at its driest, from late summer to early winter.

After that, perforated plastic pipes may need to be laid onto a 5cm/2in deep bed of coarse gravel and filled with 10cm/4in of standard gravel or pea shingles; you could do it yourself if you’re looking at a small area but anything on a large scale would require specialist intervention as a design will have to be put together and installation will require some degree of experience to get it done right.

If you’re looking at doing it yourself consider where the water will go (see Where to? in the section below) and whether you’ll be able to dig deep enough to allow for enough of a fall to allow gravity to do its work. If not, you will have to think about installing an electric pump and float switches.

The pipes must be laid beneath your planting scheme, typically about 40-60cm or 16-24in deep. They should be spaced out, about 3m/10ft to 6m/20ft apart and the fall must be in the order of 1:40. A herringbone layout is commonly used, with each branch feeing the line at a 45 degree angle.

Finally, Ireland has an extensive network of underground springs so if you have one on a part of your site you intend to use, you will probably need to divert it with a pipe.


Special considerations when laying pipes

Underground drainage systems rely on inclining the pipes to a precise angle so that gravity can do its work. Problems arise when the site is sloped or hilly and therefore may not allow for the required fall, or provide too much of a fall. Pumps may be used in the first instance while a receiving chamber can be used in the second instance (for sewage they’re referred to as ‘tumbling bay junctions’).

If you need to lay the pipe within a metre of your or your neighbour’s foundations you will need to encase the pipe in concrete; same goes for pipes laid near trees to avoid the roots damaging the pipe.

If the pipes need to go through foundations, they need to be protected from the house settling; openings supported by lintels can be used or the pipe can be encased in circular plastic ducting. In both instances the openings will then need to be sealed to protect from rodents and other unwelcomed guests and to prevent gases such as radon filtering into the house.

Flexible pipes will also need to be protected and this is done by surrounding them with 100mm of gravel (the gravel should be no more than 40mm wide), then 300mm soil, and another 300mm of soil needs to be added before you can compact the soil. If there is going to be heavy loading from above, e.g. car park, the pipe is usually protected with a concrete casing.

Storage of the pipes on site needs to be given special attention to prevent accidental damage. Plastic pipes are very lightweight and can easily blow away while vitrified clay can be chipped!

Ducting materials

The cheapest but least desirable option from an environmental point of view is PVC, although some varieties are made to look like traditional cast iron for gutters, downpipes and hoppers. PVC emits volatile organic compounds during its lifetime and recycling is difficult to achieve; also when broken the product can’t be mended and has to be entirely replaced.

Vitrified clay for below ground pipework is considered to most environmental followed by concrete; above ground the more durable and environmental option is often considered to be polyethylene plastic.

Gravel used in drainage is pea shingle or pea gravel, they’re of a small diameter to allow water to flow through.


Around driveways

For driveways good drainage is essential in areas where water might be inclined to lie. Directing the water flow to a single point is difficult to do, especially if you’re using herringbone bond pattern. There are two options, either a dished channel formed from either the paving blocks or from concrete, or a channel which allows you to maintain the level surface if that is an important consideration.

The first option is usually the preferred one because you can see immediately when it’s blocked. Block and flag paving is almost impermeable, as are mortar joints once set. Sand filled joints develop water resistance within a few months. Weather jointing of the paving can also help drainage by giving rainwater the least amount of obstruction as possible and helping it to run off.

Your driveway will also be a source of pollution, not so much from the taramac or asphalt but due to your car. It’s therefore considered best practice to use a natural filtration device via a permeable paving system or equivalent.



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Written by Astrid Madsen

Astrid Madsen is the editor of the SelfBuild magazine. Email astrid.madsen@selfbuild.ie

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