Basement Waterproofing — Your Questions, Answered

Below is a practical Q&A you can publish as-is. I’ve tailored it to how we (Amtaar GC) actually design and deliver basements in London.

What is basement waterproofing, in plain English?

It’s the set of design details and materials that keep water out of your basement. In London you’re dealing with groundwater, rain-laden soils, and hydrostatic pressure pushing on slabs and walls. Good waterproofing is a system, not a single product: structure + membranes/waterstops + drainage + QA.

Which system do I need—Type A, B or C?

• Type A (tanking): A barrier (sheet or liquid) applied to the inside or outside of walls/slab to resist water.
• Type B (integral): The concrete structure itself is the barrier (e.g., waterproof concrete with joint detailing).
• Type C (drained cavity): A managed cavity with membranes and channels that collect water to a sump and pump.

On most London basements we combine systems (e.g., Type B + Type C) to achieve reliability and compliance with BS 8102 design philosophy (defence in depth).

How is waterproofing sequenced on a new build basement?

Typical high-reliability sequence:

1. Substrate prep & blinding: Level, clean, and suitable for membranes.
2. Under-slab barrier: Pre-applied sheet membrane or integral system.
3. Critical joints: Waterstops, junction tapes, pile-head encapsulation, day-joint details.
4. Vertical walls: External tanking or internal drained cavity (or both), with protection boards.
5. Penetrations: Pipe and service entries wrapped/terminated with tapes and hydrophilic waterstops.
6. Perimeter drainage & sump: Channels direct any ingress to a pump set with alarms and backups.
7. QA/ITP: Inspection, adhesion checks, lap integrity, pull-off where applicable, photo records.

Our in-house method statements show this in detail—for example using Grace Preprufe 300R/160R for slabs and walls, Bituthene LM/8000 HC for liquid detailing, and Adcor 500S waterstops; with pile-head treatment and penetration drawings (e.g., SK-0112, SK-0119, SK-0123) to manage laps, corners and joints. 

What about basements with a high water table?

Design for hydrostatic pressure from day one. That means:

• Pre-applied membranes beneath the slab (so concrete bonds to the membrane).
• Hydrophilic waterstops at all construction joints.
• Robust corner/step details and protection boards.
• A Type C pump system sized for inflows, with dual pumps, battery backup and high-level alarm.

How are penetrations and awkward details made watertight?

Penetrations are where most failures start. We wrap services with compatible tapes, form terminations with liquid membrane, and add hydrophilic seals where pipes pass through walls. Our standard details include lap widths, termination bars, and minimum coverage requirements—backed by photos in the QA pack. 

Can I waterproof an existing damp basement?

Yes—retrofitting is usually a Type C drained cavity on the inside, because external access is limited. We add internal membranes, perimeter channels, inspection ports, and a sump/pump. Where the structure allows, we also improve joints and apply local tanking.

Do I always need a sump and pump?

If you adopt a drained cavity (Type C), yes. Even with external tanking, a belt-and-braces design often includes internal drainage as a failsafe. We specify dual pumps, independent circuits, battery backup and telemetry if the risk profile justifies it.

How long does a good system last?

With correct design, installation, and maintenance, decades. The weak points aren’t the products—they’re workmanship and detailing. That’s why we run an ITP (Inspection & Test Plan) with hold points for substrate prep, lap integrity, corner and penetration detailing, backfilling, and first-fix M&E coordination. 

What are the most common mistakes you see in London?

• Membranes applied over dusty, uneven substrates—poor adhesion.
• Inadequate laps and corners; no protection boards before backfill.
• Penetrations left to “site fix” without standard details.
• No redundancy (Type A only) where water table risk is high.
• Pumps without alarms, or maintenance neglected.

Will waterproofing stop condensation and mould?

It stops water ingress. For internal air quality you still need ventilation, thermal bridges addressed, and in some cases dehumidification. We design structure and insulation together to minimise cold spots.

How do you price a basement waterproofing package?

We price from drawings and risk. Key drivers: footprint, depth, water table, ground conditions, number of penetrations, step levels, finishing strategy, and whether we’re combining systems. We won’t guess—we issue an engineered waterproofing design and a line-item BOQ so you can see exactly what you’re paying for.

What do I get from Amtaar GC?

• A BS-aligned waterproofing design (defence-in-depth).
• Product-specific details for laps, corners, pile heads, and penetrations.
• A QA pack with photos and ITP sign-offs.
• Pump schedule, maintenance plan, and warranties coordinated with suppliers. 

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Call us if you:

• have a damp or leaking basement,
• are planning a new basement in a high-risk water table, or
• need an independent design/QA review before you pour.