Site Drainage: Keeping Water Away from the House

The difference between surface and subsurface drainage in a garden, when you need it, how pipe, gravel and geotextile systems are built and where the water should end up.

However well a house’s walls and foundations are waterproofed, if water stands around it for long periods it will sooner or later find a weak point. Puddles that linger in the yard for days after a downpour, damp basement walls, rotting plant roots and soft, squelching lawn are different faces of the same problem: water has nowhere to go. Drainage creates that route.

When drainage is needed

Not every yard needs an elaborate system. The decision depends on site conditions, and drainage deserves serious thought when:

• the soil is clay or heavy and water soaks in very slowly;
• the house sits low on a slope and water runs down towards it;
• the groundwater level rises seasonally;
• the house has a basement or semi-basement;
• much of the plot is covered with concrete, stone or paving, leaving water nowhere to soak in.

Before designing anything, establish the soil type, where water gathers in different seasons and the lowest point of the plot. A geotechnical report, a simple trial pit and a careful look at the yard after heavy rain all provide useful information.

Surface drainage: catching water before it enters the ground

The cheapest and most reliable place to manage water is at the surface. Ground, paths and terraces around the house are shaped to fall away from the building, and an apron is laid along the foundations. On long paved areas, driveways and garage entrances, channel drains are fitted along or across the fall: narrow gratings that collect water and lead it to an outlet with a silt trap.

Point gullies are used in hollows in the yard, at the bottom of steps or in the low corner of a terrace. Downpipes can also connect to this system, but pipe sizes must allow for the volume they deliver.

Subsurface drainage: collecting water in the soil

For moisture that has soaked in from the surface or comes from groundwater, subsurface drainage is installed. Its core is a perforated drain pipe. A typical build-up is:

• the trench bottom is graded to fall in the direction of flow;
• the trench is lined with geotextile, its edges turned up the sides;
• washed gravel is laid as a bed and the pipe placed on it;
• the pipe is surrounded and covered with more gravel;
• the geotextile edges are folded over the gravel, then topsoil or turf goes on top.

The geotextile stops soil particles washing into the gravel and pipe and clogging the system. In clay soils, pipes with a factory-fitted filter wrap may also be chosen.

It is important to distinguish foundation drainage from field drainage. Foundation drainage runs along the foundations, slightly away from them, and protects the basement. Field drainage is laid under lawns and garden areas in a grid or herringbone pattern and lowers overall soil moisture.

Where the water should go

The key question for any drainage system is the outfall. Collected water is directed to a surface-water sewer beyond the plot, a ditch, or a soakaway dug on site. A soakaway only works in free-draining ground; one dug into a clay layer simply fills up and stays full. Sometimes water is stored in a tank for garden irrigation, with an overflow line for the excess.

Clean water from surface drains and downpipes is often carried in separate pipes from subsurface drains, joining only at the final chamber. The reason is simple: in a downpour, the large volume from the surface can back up into the perforated pipe and turn it into a system that wets the soil instead.

Inspection chambers and maintenance

Drainage is hidden underground, so access for maintenance must be designed in. Inspection chambers are placed where pipes change direction, at junctions and along long straight runs. They let you check that water is flowing and flush the pipes with a hose when necessary.

Throughout the year, check:

• whether leaves and sand have built up in channel drains and silt traps;
• whether water flows or stands in inspection chambers after heavy rain;
• whether the soakaway stays full for a long time;
• whether trees or shrubs have been planted over drain runs, where roots can get into the pipe.

Planning together with the landscape

Drainage works best when it is designed at the same time as the landscape plan. The positions of paths, terraces and flower beds, and the levels to which soil is laid, all change how water flows. A trench dug later ruins finished lawn, irrigation lines and paving. When levels, pipe runs and chamber positions are shown on one plan, both the construction sequence and any future repairs become simpler.

In Zaferoğlu İnşaat's projects, the terrain and soil data a drainage scheme needs come from the topographic and geological maps of the site. If water pools in your yard after rain, we can work out with you where and how the drainage should run.