How Are Basements Built? A Step-by-Step Construction Guide

Basements are built by retaining the ground around the footprint, excavating in stages, then constructing a reinforced concrete floor slab, walls and roof slab, with waterproofing and drainage installed as the structure goes up. Each step depends on the one before it, and the structural engineer sets the sequence.

This guide walks through that sequence in the order it happens on a typical Australian residential site with boundaries close by. Details vary with soil, depth and design, but the logic is the same on almost every project: hold the ground, dig, build from the bottom up, keep the water out.

Before Construction: Design and Approvals

Three pieces of work come before any digging.

• Geotechnical investigation: boreholes identify soil, rock and groundwater.
• Engineering and architectural design: drawings set the retention system, slab and wall sizes, reinforcement, waterproofing and drainage.
• Permits: a building permit is issued, and where neighbours could be affected the building surveyor may require protection work to adjoining properties.

Basement Construction at a Glance

Step 1: Site Establishment and Set-Out

The site is fenced, existing structures are demolished if required, and services are located and disconnected or protected. A surveyor then sets out the basement footprint, pier positions and levels from the approved drawings.

Step 2: Retention Piers and Capping Beam

Where a basement sits near a boundary, the ground has to be supported before it is dug. A piling rig drills a row of bored piers around the perimeter. Each hole receives a steel reinforcement cage and is filled with concrete.

A reinforced concrete capping beam is then cast across the tops of the piers. It ties them together so they act as a single retaining structure. Our piling and bored piers service covers this stage.

On open sites with room to spare, the excavation can sometimes be sloped back instead, and conventional walls are built in the open cut.

Step 3: Staged Excavation and Wall Infill

Excavation proceeds in lifts set by the engineer. After each lift, the soil face between the piers is trimmed, reinforcement mesh is fixed, and shotcrete is sprayed to form a wall panel. The next lift begins only when that panel has gained strength.

Working this way keeps the exposed soil height small. Deeper basements may also need temporary props or ground anchors until the floor and roof slabs brace the walls. We compare the two wall-building approaches in shotcrete vs poured concrete.

Step 4: Footings and Sub-Slab Drainage

At the final depth, footings, lift pits and sump pits are excavated. Depending on the design, drainage pipes in free-draining aggregate are laid beneath the slab and connected to a sump.

A thin blinding layer of concrete is often poured to give a clean, level surface for the membrane and steel.

Step 5: The Basement Floor Slab

The under-slab membrane is laid, reinforcement is fixed on bar chairs, and starter bars are placed where the walls or columns will connect. After a pre-pour inspection, concrete is pumped in, vibrated, screeded and finished, then cured.

Where the slab must resist water pressure from below, it is designed thicker and more heavily reinforced, and joints receive waterstops.

Step 6: The Basement Walls

The permanent walls are built in one of several ways.

In many residential basements, the shotcrete placed during excavation is designed as the permanent wall. In others, a second structural wall is built inside it. Our guide to in-situ concrete wall construction explains the formed option.

Step 7: Waterproofing and Wall Drainage

Waterproofing is where basements most often fail, and its position in the sequence depends on the wall type.

• Walls built in an open excavation are waterproofed on the outside face after the concrete cures, then protected with drainage sheets before backfilling.
• Walls built against a retained face cannot be reached from outside. A membrane or drainage layer is fixed to the retention before the wall is placed, or the concrete itself is designed to be watertight with internal systems as backup.

A drainage path behind or beneath the structure carries water to a sump, where pumps discharge it to the stormwater system. What happens when this stage goes wrong is covered in our article on concrete basement water damage.

Step 8: The Basement Roof Slab

The roof of the basement is a suspended concrete slab that becomes the ground floor of the building above. Formwork decking is erected on props, reinforcement or post-tensioning tendons are placed, and the slab is poured.

Once cured, this slab braces the tops of the basement walls. Temporary props or anchors can only be released when the engineer confirms the permanent structure is carrying the load. Our article on formwork for concrete slabs covers installation and stripping.

Step 9: Backfill, Services and Fit-Out

Any open excavation outside the walls is backfilled in compacted layers with free-draining material. Inside, pumps, ventilation, electrical and fire services are installed, followed by ramps, stairs and finishes. The rest of the building then continues above.

Frequently Asked Questions

1. What is the first step in building a basement?

Site investigation and design. A geotechnical report and engineering drawings are completed and permits are issued before construction. On site, the first physical step on most suburban blocks is installing the retention piers that will hold the ground during excavation.

2. Are basement walls built before or after excavation?

It depends on the site. On tight sites, retention piers go in first and the walls are formed in stages as the dig goes down. On open sites, the full excavation is completed with sloped sides, and the walls are built up from the floor slab.

3. What are basement walls made of?

Most modern basement walls are reinforced concrete, placed as shotcrete, poured into formwork or cast into permanent formwork panels. Reinforced core-filled blockwork is also used for shallower basements. The engineer chooses the type from the soil pressure, water conditions and access.

4. How is a basement waterproofed?

By combining a barrier, drainage and a way to remove water. Membranes are applied to the outside of the walls or installed behind and under the structure before concrete is placed. Drainage directs water to a sump, and pumps discharge it.

5. Can a basement be built in any soil?

Basements can be built in most ground, but the method and cost change significantly. Rock is slow to excavate, sand needs continuous support and high groundwater demands a more watertight design. A geotechnical investigation determines what is practical.

Building a Basement in Melbourne

A basement is built in a strict order: retain, excavate, pour the slab, complete the walls, waterproof and close it with the roof slab. Most defects trace back to a step that was rushed or done out of sequence.

Urban Pour delivers residential basement construction across Melbourne, from piers and excavation through to slabs and walls. Contact Urban Pour to discuss your drawings and site.