Interior vs exterior basement waterproofing — which is better?

Exterior (applied outside the structure before backfill) is technically superior — it stops water before it contacts concrete. Interior tanking (applied inside) is practical for existing structures where excavation isn't possible. Both work long-term when applied correctly. Exterior gives 20–30 year life; interior gives 15–20 years.

My basement has water running through the floor slab. Can CM300 fix this?

First stop the active flow with FASTFIX WP100 hydraulic cement or HARDPROOF INJ PU injection. Once water ingress is stopped and surface is damp (not flowing), CM300 can be applied. CM300 withstands negative hydrostatic pressure — water pushing from outside in — after full cure.

How do I waterproof the wall-floor junction in a basement?

This is the highest failure point. Cut a V-groove 20mm x 20mm at the junction. Fill with FASTFIX WP100 or hydraulic mortar. Then apply 3 extra coats of CM300 across the junction with a brush, covering 300mm on wall and 300mm on floor. This zone gets more coverage than any other area.

Will basement waterproofing work if the water table is high?

Yes — CM300 is rated for negative hydrostatic pressure of up to 3 bar (30 metres water head). High water table basements need interior tanking done correctly with full wall and floor coverage, sealed construction joints, and no gaps at penetrations. Contact Hardex for high water table specification.

How do I waterproof pipe and conduit penetrations through basement walls?

Seal around pipe penetrations with HARDPROOF PU350 polyurethane sealant before applying CM300. The sealant bridges the pipe-concrete gap flexibly. Then apply extra CM300 over the sealant. Never rely on CM300 alone to bridge a gap — it's a coating, not a sealant.

How long before I can use the basement after waterproofing?

Walk-in access after 3 days. If applying tiles or screed over CM300, wait 7 days minimum. Do not introduce standing water or backfill before 7 days full cure.

Is CM200 or CM300 for basements?

CM300 is the correct product for basements and below-grade structures under hydrostatic pressure. CM200 is flexible but designed for wet areas above ground (bathrooms, balconies). CM300 is a heavier two-component system specifically rated for hydrostatic resistance.

Basement Waterproofing & Tanking

The Problem: Basements Face Pressure, Not Just Moisture

Water Under Pressure Is a Different Problem Entirely

A terrace leaks because rain gets in. A basement floods because groundwater is actively being pushed through the concrete by hydrostatic pressure — the weight of saturated soil and water table above. Surface treatments and damp-proof paints fail immediately. You need a system rated for pressure.

Why Basements Fail

Construction joint failure — The joint between the base slab and walls is the primary entry point in 70% of basement failures. Cast at different times, they never fully bond.
Hydrostatic pressure — Monsoon season raises the water table. Saturated soil pushes against the basement wall. Pressure forces water through any weakness — cracks, joints, pipe penetrations.
Rebar corrosion — Once water reaches reinforcement, it corrodes. Expanding rust spalls the concrete from the inside. The structure degrades invisibly until failure.
Wrong product choice — Damp-proof paint (bitumen or acrylic) has zero pressure resistance. It peels off as soon as hydrostatic pressure develops behind it.

Two Systems: Interior or Exterior Tanking

Interior tanking stops water from entering the habitable space. Exterior tanking stops water before it contacts the structure. Choose based on access and stage of construction.

Basement Waterproofing Decision

INTERIOR TANKING — Existing Structures
CM300 applied inside · No excavation · 3–5 day installation · Resists negative pressure
✓ Cost-effective ✓ Fast ✓ No excavation ⚠ Requires active leaks stopped first

— or —

EXTERIOR TANKING — New Construction / Full Excavation
CM300 + bituminous membrane outside · Applied before backfill · Maximum protection
✓ 20–30 year life ✓ Stops water at source ⚠ Requires excavation

Parameter	Interior Tanking	Exterior Tanking	Damp-Proof Paint
Pressure resistance	✓ Negative hydrostatic	✓ Positive hydrostatic	✗ None
Excavation needed	✗ No	✓ Yes	✗ No
System life	15–20 years	20–30 years	2–3 years
Cost per m²	₹300–500	₹500–800	₹80–150
Best for	Existing buildings	New construction	✗ Not recommended

Product Specifications

HARDPROOF CM300 — Two-Component Flexible Cementitious Waterproofing

1.5–2.0

kg/m² for 2 coats

3 bar

negative pressure rated

7

days before backfill/flooding

2-part

powder + liquid mix

Key property: Resists negative hydrostatic pressure — water pushing from outside trying to force through from the wet side to the dry side. This is what differentiates CM300 from regular waterproofing coatings.

HARDPROOF INJ PU — Polyurethane Injection Grout (active leaks)

Expands

on contact with water

Sets in

60–90 seconds

Injected

by low-pressure pump

Use first

before CM300 coating

Use for: Active water flow from cracks and construction joints. Expanding PU foam blocks the water channel permanently. Apply CM300 over it after cure.

Cost Breakdown — 200 m² Basement (Interior Tanking)

Material Costs

INJ PU grout — construction joint injection (allow ₹5,000)₹3,000–6,000

WP100 hydraulic cement — active leak plugging₹1,000–2,000

SBR-50 primer (200 m² × 0.13 kg)₹4,000–5,000

CM300 (200 m² × 1.8 kg, 2 coats = 360 kg)₹54,000–72,000

Total — Materials₹62,000–85,000

Labour

Hacking, cleaning, crack repair, joint cutting₹15,000–20,000

INJ PU injection + CM300 application₹20,000–30,000

Total — Labour₹35,000–50,000

All-in: ₹97,000–1,35,000 for 200 m² (₹485–675/m²)

15–20 year protection. Prevents rebar corrosion and structural degradation. Stops mould growth and damp smell permanently.

Application Timeline — Interior Tanking

Day 1

Active Leak Control

Inject INJ PU into flowing cracks and joints. Plug running holes with WP100 hydraulic cement. These must be completely stopped before coating. A sealed crack = structural injection done. A damp crack = apply CM300 directly.

⏱ Full day depending on severity

Day 2

Surface Preparation

Hack and clean all surfaces. Cut V-grooves at wall-floor junction and construction joints (to receive extra coating). Remove efflorescence.

⏱ Full day · Damp (not wet) surface required

Day 2–3

SBR-50 Primer + CM300 Coat 1

Apply SBR-50 primer while tacky, apply first CM300 coat. Special focus on wall-floor junction — three extra coats here by brush, 300mm each side.

⏱ 4–6 hrs · Cure: 24 hrs · Keep damp

Day 3–4

CM300 — Second Coat

Apply second coat perpendicular to first. Coverage now 1.5–2.0 kg/m². Inspect for pinholes — apply extra brushed coat on any voids.

⏱ 4–6 hrs · Moist cure 3 days

Day 4–7

Moist Curing

Spray lightly with water 2–3 times daily. Prevent premature drying — cementitious systems shrink and crack if dried too fast. Critical step.

⏱ 10 mins/day · Don't skip

Day 7+

✓ Inspect and Hand Over

Inspect all surfaces. Check junction areas. System complete — basement can now receive screed, tiles, or fit-out. Backfill can proceed for exterior systems.

Total: 7 days start to finish

Frequently Asked Questions

Interior vs exterior — which should I choose for my existing basement?

For existing structures without excavation access: interior tanking with CM300. It's designed for this — rated for negative hydrostatic pressure. Exterior is only practical for new construction or full renovation with excavation access.

My basement has water actively coming through the floor. Where do I start?

Start by stopping active water flow — FASTFIX WP100 hydraulic cement for holes and cracks, INJ PU injection for joints. Only when water flow is stopped can you apply CM300. Applying coating over active flowing water is not effective.

Can I tile over CM300 in the basement?

Yes. After 7 days full cure, CM300 is tile-ready. Use standard tile adhesive — CM300 surface is compatible. Do not use tiles before 7 days — premature tiling over CM300 causes adhesive failure.

Will CM300 work in a basement with a very high water table?

CM300 is rated for up to 3 bar negative hydrostatic pressure — equivalent to 30 metres of water head. Most residential and commercial basements are well within this range. For exceptional conditions (high-rise basement, coastal high water table), contact Hardex for a specific project specification.