Building a House on a Slope: Retaining Walls and Drainage

What to watch for when building on sloping land: site investigation, a stepped plan, retaining walls, surface and subsurface drainage, and landslip risk.

A sloping plot offers great views, natural light and the chance to build a house on several levels. At the same time, questions that hardly arise on flat land come to the fore: could the ground slide, where will rainwater flow, and how will the back wall of the house resist soil pressure? If the design does not answer them, problems can surface years after construction.

Begin with a site investigation

On a slope, a topographic survey is especially important: the gradient, level differences and natural water paths only become clear with accurate measurement. A geological survey shows the soil layers, their bearing capacity and the groundwater level.

On a slope, look for particular signs: leaning trees, cracked old fences, wave-like bulges in the ground, and strips that stay wet long after rain. These can indicate that the ground is moving or that water flows beneath the surface. In such cases, decisions should only be made after a specialist’s assessment.

How to place the house on the slope

There are two main approaches. The first is to cut the slope into a flat platform and build a conventional house on it. This can require large-scale excavation, soil removal and tall retaining walls. The second is to fit the house to the shape of the slope: a stepped plan with one level set lower than another, and the lower floor partly set into the ground.

The second approach reduces earthworks and lets the house sit more naturally in the landscape. But walls that hold back soil act as retaining walls, and their waterproofing, drainage and structural design need separate attention. Which side of the house is set into the ground is decided together with sunlight, views and the access road.

The foundation is also adapted to the slope. If part of the building rests on natural ground and part on fill, settlement will be uneven and cracks will appear in the walls. The underside of the foundation is therefore stepped so that every part bears on the same reliable soil layer. If excavated soil is to be reused as fill in the garden, it must be compacted in layers.

Access needs its own thought. If the driveway is too steep, it becomes hard to use in rain and winter. Sometimes it makes more sense to put the garage on the upper level near the street and the living areas below.

Retaining walls

A retaining wall stops soil from sliding downhill and creates flat, usable levels in the garden. There are several types:

• reinforced concrete walls: for tall and critical locations, based on calculation;
• gravity walls of stone or concrete blocks: they work through their own weight;
• gabion walls: wire cages filled with stone that let water through;
• terracing: several low steps instead of one tall wall.

The most common mistake is building a retaining wall like an ordinary garden wall. Water collecting behind the wall increases soil pressure and makes the wall lean or collapse. That is why a drainage layer, a drainage pipe and weep holes for water to escape are provided behind it. Tall walls, especially near buildings or roads, must always be designed with structural calculations.

Drainage: the key issue on a slope

On a slope, water always runs downhill, and if a house is in its path, it hits the house. Drainage works on two levels. Surface drainage catches rainwater above the house and diverts it: swales, channels and graded surfaces around the building. Subsurface drainage collects water along the foundations and earth-retaining walls with perforated pipes in a gravel layer and carries it away.

Intercepting water on the ground above the house is particularly important. Collected water is led to a storm drain or a dedicated soakaway without harming the land downhill or the neighbour’s plot. Roof water is part of the same scheme: downpipes should not discharge straight onto the soil of the slope.

Care during construction

Excavation on a slope is a risky stage: freshly cut soil can collapse in rain. It is advisable to schedule this work for the dry season, temporarily stabilise cut faces and not leave excavations open for long. Once construction is finished, covering exposed soil with planting prevents erosion, because rooted ground cover holds the soil in place.

Zaferoğlu İnşaat carries out construction stages from preparing topographic and geological maps of the site to excavation work. If you have a topographic survey, we can compare stepping the house into the slope against using retaining walls on it.