Frequently Asked Questions

What is the minimum concrete blinding thickness required before SikaProof A+ is laid and how should the blinding surface be prepared?

SikaProof A+ requires a minimum 75 mm thick plain concrete blinding (lean concrete base) as the substrate for membrane installation. The blinding provides a clean, firm, level base for the membrane that would otherwise wrinkle and pool if laid on compacted sub-base or earth, and prevents aggregate piercing the membrane from below during and after installation. The blinding surface preparation requirements for SikaProof A+ are: Level and smooth: the blinding surface must be reasonably level (within 10 mm under a 3-metre straight edge) and free of sharp protrusions, aggregate points, or formwork nails that could puncture the membrane from beneath. Any sharp protrusions should be knocked off or filled with mortar before membrane installation. Cured and non-dusting: the blinding must be sufficiently cured (minimum 7 days at 25 degrees C) to prevent the membrane adhesive from bonding to uncured blinding and being pulled away when the membrane is repositioned during installation. However, a light cement dust surface is acceptable. Dry or damp: a slightly damp blinding surface is acceptable — SikaProof A+ adhesive does not require a dry substrate, which is a significant practical advantage in monsoon-season basement construction in Delhi NCR where dewatering keeps the blinding damp. Do not install SikaProof A+ on blinding with standing water or wet surface — mop dry before installation. After SikaProof A+ is laid, the concrete raft slab reinforcement is placed directly on the membrane (with plastic bar chairs) and the concrete is poured immediately or within 24 to 48 hours of membrane completion.

How should SikaProof A+ overlaps and perimeter upstands be sealed and what is the minimum lap width at membrane sheet joints?

Correct sealing of SikaProof A+ overlaps and perimeter upstands is essential — every unsealed or poorly sealed joint is a potential water path through the waterproofing envelope. At sheet-to-sheet overlaps (where two adjacent SikaProof A+ sheets meet): the minimum lap width is 100 mm (10 cm overlap), with 150 mm recommended for critical applications and in areas where the concrete or base is uneven (greater lap gives more robustness against any local poor adhesion in the overlap zone). The overlap sealing method: peel back the release film from the adhesive layer of the top sheet in the overlap zone and press down firmly onto the bottom sheet using a roller to ensure intimate contact and removal of air bubbles in the overlap. Alternatively, use SikaProof System Tape — a double-sided self-adhesive tape applied to the bottom sheet edge before the upper sheet is lapped over and rolled down. At perimeter upstands and corners (where the horizontal membrane turns up the basement retaining wall to connect with the vertical wall waterproofing): the SikaProof A+ membrane must be turned up a minimum 300 mm above the top of the slab and connected to the SikaProof P membrane or other Sika-approved vertical wall waterproofing using SikaProof Corner Tape or hot-air welding. At pile heads and column stubs penetrating the raft slab: the SikaProof A+ membrane must be cut and sealed around each penetration using prefabricated SikaProof pile head boots or site-formed seals using SikaProof System Tape, ensuring the membrane is continuous around each penetration with no unsealed gaps.

What are the advantages of SikaProof A+ pre-applied membrane over a conventional post-applied torch-applied bitumen membrane for a basement raft slab in Delhi NCR?

SikaProof A+ pre-applied membrane and conventional torch-applied bitumen membrane both waterproof basement raft slabs but differ fundamentally in application sequence, bonding, and performance. Application sequence: SikaProof A+ is laid before the concrete slab is poured (pre-applied) — no hot works, torching, or flame on site, and installation can proceed immediately after blinding is cured. Torch-applied bitumen membrane is applied after the blinding concrete has reached adequate strength and is post-installed before the raft slab is poured (for basement slabs) or below a protection screed (for roof slabs). Bonding: SikaProof A+ is fully bonded to the concrete poured over it — groundwater infiltrating a puncture cannot travel laterally. Torch-applied bitumen is typically partially bonded or non-bonded to the blinding — water entering through a defect can travel freely under the membrane across large areas. On large raft slab basements (500 to 2000 m2), this lateral migration characteristic makes leak detection and repair of post-construction bitumen membrane failures very difficult and expensive. Fire safety: SikaProof A+ installation requires no open flame or hot works permit — a major advantage on busy urban construction sites in Delhi NCR where hot work permits have lengthy approval processes and fire safety risks from LPG torches on congested city sites are a genuine concern. Service life: SikaProof A+ HDPE carrier film has an extremely long service life (above 50 years) without the thermal degradation and embrittlement that can occur in bitumen membranes exposed to high soil temperatures. For quality-conscious basement waterproofing in Delhi NCR high-rise buildings, SikaProof A+ represents the current state-of-the-art specification.