How To Waterproof Below-Ground Structures (Lift Pits, Substructures and Underground Spaces)

Below-ground waterproofing is one of the hardest jobs in building work to get right and one of the most expensive to fix. Lift pits, basement slabs and underground plant rooms sit in soil and groundwater their whole life. They never dry out, and once it’s backfilled, you can’t get back to the outside face. When something goes wrong, the fix usually means digging up half the building. Water damage spreads, corrosion sets in, and remedial costs make the original waterproofing budget look like pocket change.

Getting it right comes down to selecting the right waterproofing systems for the site conditions, properly sorting joints and penetrations, and following the manufacturer’s specifications. The rest of this article covers, step by step, how to waterproof below-ground structures.

How to Waterproof Below-Ground Structures: Step by Step

1. Assess the structure and water risk

Different waterproofing systems suit different jobs, so the assessment tells you which one fits the project. Look at whether the building sits at or below the water table, the hydrostatic pressure you’re dealing with, the condition of the concrete, soil chemistry, and where every joint, penetration and movement point sits. The risk profile drives the spec.

2. Choose the right system

Below-ground waterproofing is a system, not a product. Australian designers often reference the BS 8102 framework, which uses three protection categories:

• Type A (barrier) uses a continuous external waterproofing membrane to keep water out
• Type B (structurally integral) uses water-resistant concrete with waterproofing admixtures
• Type C (drained cavity) lets any water that gets through run into an internal drainage layer and a sump

For high-consequence structures, a “belt and braces” combination of two types is the safer call than relying on one.

The main system options:

Pre-applied sheet membranes go down before the pour and bond to the back of the slab during the pour. They suit new builds where you’ll lose access to the outside face. SikaProof A and ARDEX WPM 1500 are both pre-applied waterproofing membranes suited to substructures, retaining walls, lift shafts and tunnels. A pre-applied waterproofing membrane is often the recommended option for critical below-ground spaces because it bonds directly with the concrete during curing and prevents water infiltration at the interface.

Post-applied liquid waterproofing membranes are applied to formed concrete after striking. They suit new builds and remedial jobs where you can get to the surface. Continuity of film thickness is the priority.

Cementitious systems bond directly to concrete and masonry. They suit water tanks and basements where movement is limited, but are generally semi-rigid, so don’t reach for them where you expect cracking.

Bentonite systems are self-sealing waterproofing membranes that swell on contact with water. They suit below-grade foundations in moist soil and blind-side applications.

Waterstops and swellable joint sealants aren’t a membrane substitute. You need them at every construction joint and pour interface to prevent water infiltration along the pour line.

3. Detail the critical junctions

Junction detailing is what separates good below-ground waterproofing from the kind that causes structural damage to the building two years later. The flat field of a wall is rarely where the leak starts. The critical points are listed in detail further down. For now, the rule is that every junction, joint, penetration and termination needs its own treatment, not just the field membrane carried across it.

4. Manage hydrostatic pressure with drainage

Where the water table is high or rainfall is heavy, drainage takes pressure off the building structure and reduces moisture against the wall. That usually means agricultural pipes installed at the lowest point of the foundation, wrapped in geofabric to stop them clogging, and graded to a sump or stormwater connection. Engineered drainage mats sit between the membrane and the soil on bigger jobs, directing groundwater away from the foundation walls. Even with Type A waterproofing as the primary defence, drainage protection is what extends its working life.

5. Prepare the substrate

Mechanically clean the surface to remove laitance, contamination and old coatings. Repair honeycombing and surface defects with a suitable repair mortar. Treat any cracks wider than 1mm. Prime if the manufacturer calls for it. Check moisture content against manufacturer specifications, since below-ground concrete is usually damp, and some products won’t bond to a damp substrate.

6. Install to the system requirements

Follow the manufacturer’s technical data sheet. Apply reinforcement bandage at all internal angles before the field coat. Hit the manufacturer’s overlap recommendation at every joint. For sheet membranes, finish welding and taping at the seams before moving on. For liquid waterproofing membranes, ensure continuity of film thickness across the surface. Place and protect waterstops before the pour.

7. Inspect, test and protect

Walk the full membrane area and check overlaps, terminations, corners and penetrations for any sign of leaks or damage. For high-consequence projects, flood test or do moisture mapping before covering. Get a protection board over the membrane before the backfill goes in. The protection board stops the membrane from being punctured during backfill.

What Counts as a Below-Ground Structure?

A below-ground structure is anything you’ve built where soil sits against the wall or slab: lift pits (usually 500mm to 2,500mm deep), basement slabs and walls, retaining walls forming an enclosed space, underground plant rooms, tunnels, foundation walls, and basement car parks.

Different structures, same problem. Water wants to get in through the concrete slab, the joints, and every penetration you’ve cut. Below-ground spaces face constant hydrostatic pressure, groundwater movement and chemical exposure from the soil. Standard reinforced concrete is porous, and that pressure pushes water through hairline cracks if it isn’t addressed. Water ingress also corrodes reinforcing steel, causes moisture damage to finishes, takes out electrical and mechanical equipment in basements and plant rooms, and leads to mould.

Why Below-Ground Waterproofing Fails

Wrong waterproofing system for the job. Plenty of products are fine for wet areas or balconies, but aren’t rated for hydrostatic pressure or sustained water infiltration. Pick the system based on site conditions and project risk.

Bad substrate prep. Honeycombing, voids and contamination cause the waterproofing membrane to debond or bridge. The membrane does only half the work; the substrate does the other half.

Weak joint and penetration detailing. This is where most failures actually start. The flat field of a wall holds up fine. It’s the wall-to-slab junction, the cold joints and the pipe penetrations where leaks show up.

No allowance for movement. Concrete cracks. A rigid system on a moving substrate will crack with it.

Skipping the design phase. Drainage, joint placement and sequencing need to be sorted on paper before the slab goes down. Communication among architects, designers, and waterproofing contractors helps avoid the most costly oversights.

Which Waterproofing System is Best for Lift Pits, Substructures and Underground Spaces?

There’s no single answer. The right system depends on the structure, the water conditions and whether it’s a new build or a remedial job. The table below sums up where each option earns its place.

System	Best for	Why it works
Sheet membranes (pre-applied or PVC/HDPE)	Critical below-ground barrier protection in new builds	Uniform film thickness, heat-weldable seams, fully bonded to concrete
Cementitious membranes	Water tanks, basements, masonry substructures	Bonds directly to concrete and masonry; positive or negative-side capable
Waterstops and swellable joint sealants	Construction joints, penetrations and pour interfaces	Designed for joint waterproofing where the membrane alone can’t bridge
Combined systems	Complex projects (lift pits, plant rooms, multi-joint basements)	Membrane plus dedicated joint and penetration treatment outperforms any single product

Sheet membranes hold up especially well in challenging underground conditions, while cementitious systems are well-suited for subterranean and immersed applications. SikaProof A is built specifically for below-ground concrete structures. ARDEX WPM 1500 is used for substructures, retaining walls, lift shafts and tunnels. Waterstops prevent water ingress at concrete joints, including where the structure sits below the water table.

You can view our full range of waterproofing supplies here.

Key Details That Matter Most in Below-Ground Waterproofing

The system you spec matters, but the details matter more. These are the spots that decide whether the job holds up:

Cold joints and construction joints. Every interface between two pours is a weak point. Design the waterstop in before the first pour. Retrofitting one in later is harder and less reliable.

Movement joints. If the structure is designed to move, the waterproofing has to flex with it. Rigid systems crack at these points.

Penetrations and pipe entries. Each gets its own detail. Use flanged or mechanical seals where you can with a compatible sealant around the perimeter. The field membrane doesn’t bridge these.

Corners, fillets and changes in direction. Inside corners need a covered fillet and reinforcement bandage before the field coat. They concentrate stress, and they’re where bridging shows up.

Terminations. The top edge of the membrane gets mechanically fixed and sealed. Leave it open and water tracks in behind it from above.

Protection from damage after application. Get a protection board on before any other trade sets foot near the surface.

Coordination with concrete pours and reinforcement. This is sequencing as much as application, especially for pre-applied membranes and waterstops.

Common Mistakes to Avoid

Buying on price alone. The cost difference between right and wrong is small compared to a remedial job, and the risk to the building is much higher when you cut corners.

Mixing incompatible products. Membranes, primers, and sealants from different ranges don’t always play nicely together. Check compatibility with the manufacturer.

Skipping the joint strategy. A membrane over flat concrete with no joint treatment isn’t a system.

Ignoring drainage. Even the best membrane works harder than it should if water sits against it permanently.

Treating it like wet-area waterproofing. Below-ground waterproofing is different from waterproofing a bathroom or laundry. Lift pits, basements and underground spaces are exposed to ground moisture and water pressure, so the system needs to suit the site conditions, drainage, joints and penetrations.

When to Get Expert Advice for an Illawarra Project

Get advice when the project involves a lift pit, plant room, or basement at or below the water table; when it’s a remedial job, and you don’t know what’s in the existing building; when the geometry’s complex; or when you’re not sure which waterproofing system suits the conditions.

Illawarra Industrial Supplies works with builders, contractors and trades across the Illawarra region, including Wollongong’s northern suburbs, the Southern Highlands, Goulburn and Eaton. Get in touch if you’ve got a below-ground project coming up.

FAQs

What’s the best waterproofing system for a lift pit?

On a new build, a pre-applied sheet membrane bonded to the outside of the pour is generally the way to go. SikaProof A and ARDEX WPM 1500 are both designed for this. On a remedial pit, a flexible liquid membrane combined with waterstops at the joints and dedicated penetration seals is typically the right approach.

Can cementitious waterproofing be used below ground?

Yes. Cementitious systems work well for water tanks and basements where movement is limited. They’re generally semi-rigid, so not the right pick where you expect cracking.

Do I need waterstops in below-ground concrete joints?

Yes. Any construction joint or pour interface that needs to stay dry should have a waterstop. PVC waterstops are readily available in a range of sizes and lengths. Swellable types are a cost-effective option for joints with no movement.

Is below-ground waterproofing covered by an Australian Standard?

No, not specifically. AS 3740:2021 covers waterproofing for domestic wet areas, while AS 4654.1 and AS 4654.2 cover external above-ground waterproofing membrane systems. For substructures and lift pits, waterproofing is usually based on evidence of product or system suitability, manufacturer specifications and project-specific design, often as a Performance Solution under the NCC.

What’s the difference between pre-applied and post-applied waterproofing?

Pre-applied membranes go in before the pour and bond to the back of the structure during the pour. Use them when the outside face will be inaccessible once you backfill. Post-applied membranes go on after the concrete has cured, used on new builds and remedials.