How Is a Cavity Drainage Membrane System Installed?

Summary

Cavity drainage membrane systems are the most widely used method for waterproofing existing domestic basements in the UK. Their appeal lies in their tolerance of structural imperfections and movement — they manage water that enters the structure rather than trying to prevent all ingress. This makes them far more forgiving than cementitious tanking systems on ageing masonry, where wall movement or substrate imperfections would break a tanking coating.

The core products — Newton 508 wall membrane, Platon P8, Oldroyd Xv, and equivalent products from other manufacturers — are all based on the same principle: an HDPE sheet moulded into a dimple profile that, when pressed against the wall, creates a network of drainage channels behind the surface. Water entering through the wall runs down these channels to the perimeter drainage channel at floor level, which directs it to a sump chamber where a pump discharges it to drainage.

This article covers the full installation sequence for a standard domestic basement conversion.

Key Facts

• Newton 508 — 8mm dimple depth; standard wall membrane; pressure rating to 40kPa; Newton 520 is the heavier floor variant
• Platon P8 — 8mm dimple depth; HDPE wall membrane; equivalent to Newton 508; used with Platon basedrain perimeter channel
• Oldroyd Xv — 20mm dimple depth on wall version; larger air gap allows more drainage capacity
• Dimple depth options — 5mm, 8mm, 10mm, 20mm; deeper dimples allow greater drainage flow but project further from the wall surface
• Pressure rating — wall membranes must be rated to withstand the hydrostatic pressure of the water head; BS 8102 requires the designer to calculate the pressure and specify an appropriate product
• Floor membrane — must withstand compressive loads from the slab above; Newton 520 (520g/m²) and Platon P20 are standard floor membranes; dimple face must be down (drainage between membrane and existing slab)
• Perimeter drain — a proprietary channel (Newton Basedrain, Platon, Triton) fixed at the wall/floor junction; collects water from both wall and floor membranes
• Sump chamber — minimum 300mm internal diameter; positioned at the lowest point; sized to hold pump and allow access for maintenance
• Pump — submersible; rated to discharge the anticipated inflow plus a safety margin; Grundfos Unilift, Stuart Turner, and similar brands commonly used
• Backup — BS 8102 requires consideration of pump failure; battery backup, secondary pump, or gravity overflow outlet should be specified
• Fixing plugs — dimple cap plugs (Newton plugs, Hi-Fix plugs) fix the membrane through the dimple into the masonry; standard spacing approximately 600mm centres on wall membranes
• CSSW design — required for BS 8102-compliant installation; specifies membrane grade, sump size, pump capacity, and backup provisions
• Guarantee — typically 10–25 year insurance-backed guarantee; requires CSSW design and approved installer

Quick Reference Table

Detailed Guidance

Pre-Installation: Design and Survey

Before any product is specified or ordered, a CSSW-qualified surveyor must assess the basement and produce a design document. This document should include:

• The intended grade of use (Grade 1–4 per BS 8102:2022)
• The hydrostatic conditions (water table level, seasonal variation)
• The membrane specification including pressure rating
• Sump chamber dimensions and location
• Pump selection and sizing calculations
• Backup provision specification
• Any structural or tanking works required in conjunction with the membrane system

The survey should also assess structural condition. Any significant cracking (particularly vertical or stepped cracking in masonry) must be investigated and stabilised before the membrane is installed. The drainage system does not stabilise a moving structure.

Sump Chamber Installation

The sump chamber is installed first, before the membrane. Its location is determined by the lowest point of the basement floor and the proximity of an accessible discharge point. Where the basement floor is already below the external drainage, a pump is mandatory — gravity drainage is not possible.

To install the sump:

1. Break out the existing floor slab at the sump location (typically with a disc cutter and breaker)
2. Excavate to the required sump depth — typically 600–900mm below finished floor level; base on firm ground
3. Compact the base and lay a 100mm concrete blinding layer
4. Set the precast concrete or GRP sump chamber on the blinding; check level
5. Connect the inlet pipe from the perimeter drain to the chamber at the specified level
6. Install the pump with float switch set well below finished floor level
7. Connect the discharge pipe to storm drainage; route through the wall above ground if possible
8. Install the lid and temporary protective cover until the floor is reinstated

The discharge pipe must have a non-return (check) valve fitted close to the pump outlet to prevent back-flooding when the pump stops.

Perimeter Drainage Channel

The perimeter channel collects water from both the wall membrane above and the floor membrane below and directs it to the sump. It is installed after the sump but before the floor membrane and wall membrane.

Fix the channel to the wall/floor junction with the proprietary adhesive or screws specified by the manufacturer. The channel must be positioned so that:

• Its top edge is at or slightly above the base of the wall membrane
• Its bottom edge allows the floor membrane to lap up behind the channel face
• It falls continuously toward the sump inlet (minimum 1:100 gradient)

Mitre cuts at corners are made with a fine-toothed saw. Proprietary corner pieces are available for most systems and are preferred over site-cut mitre joints which can leak. Seal all joints with compatible sealant.

Drill the sump inlet opening to match the channel outlet pipe diameter before the channel is fixed, and connect with the supplied coupler.

Floor Membrane Installation

The floor membrane (Newton 520, Platon P20) is laid over the existing slab with the dimples facing down, creating a drainage layer between the membrane and the slab. This allows any seepage through the slab to flow to the perimeter drain.

Lay the membrane from one wall outward, cutting to size at the opposite wall and at any protrusions. The membrane must lap under the perimeter drain channel (not over it) so that water runs down the wall, behind the drain channel, and into it. Overlap between adjacent membrane sheets should be at least 200mm, with the upper sheet overlapping the lower in the direction of drainage.

Where the floor membrane meets the wall, turn it up the wall approximately 50–100mm and fix temporarily with tape until the wall membrane is installed.

After the floor membrane is positioned, a new concrete slab or sand-cement screed is poured over it. Minimum slab thickness over the floor membrane is typically 75mm for screed; 100mm for concrete. The slab must not be so heavy as to exceed the compressive rating of the floor membrane — Newton 520 has a compressive resistance of 150 kN/m² which is more than adequate for domestic floors.

Wall Membrane Installation

Wall membranes are fixed from the floor level up to at least 150mm above the finished ground level outside (or as specified by the CSSW designer). The membrane is fixed with cap plugs through the dimples into the masonry behind.

Installation sequence:

1. Clean the wall of any loose mortar, efflorescence, and debris; fill any large voids with sand-cement mortar
2. Unroll the membrane (dimples facing the wall) and offer up to the wall
3. Mark fixing positions at approximately 600mm centres in a grid pattern
4. Drill through the membrane and into the masonry (8mm diameter, 65mm deep for standard fixing)
5. Insert the cap plug and drive the fixing through the cap into the masonry
6. Work up the wall in horizontal bands, overlapping sheets by at least 100mm with the upper sheet over the lower

At the perimeter drain, the base of the wall membrane must sit inside the back of the drain channel so that water running down the membrane face discharges into the channel rather than behind it.

At window reveals, door openings, and any penetrations, the membrane must be cut, sealed, and terminated carefully. Proprietary termination strips are available for most systems; alternatively, a stainless steel or plastic angle bead can be mechanically fixed and sealed.

At the top of the membrane (at ceiling or DPC level), a termination batten fixes the membrane and forms the top edge. This prevents air circulation behind the membrane and provides a tidy finish for plasterboard or other linings.

Finishes Over the Membrane

The most common finish is plasterboard on treated timber battens fixed through the membrane into the masonry. This creates a habitable surface without adding excessive load to the wall membrane. Battens should be at 400mm or 600mm centres and must be fixed into sound masonry, not just into the dimple membrane.

Mesh-faced wall membranes (Newton System 500 Mesh, Platon Mesh) can be directly plastered with a two-coat sand and cement system or proprietary renovating plaster. This is more expensive than plasterboard but eliminates the batten void and reduces the total wall thickness loss.

Thermal insulation should be incorporated within the wall lining to achieve the required U-value. PIR board (Celotex, Kingspan) between battens, or a proprietary insulated lining system, is typically used. The thermal insulation should be on the warm side of any vapour control plane.

Sump Pump Operation and Maintenance

The pump is activated by the float switch when water in the sump chamber reaches the trigger level. For a domestic system with low to moderate inflow, the pump may only activate occasionally during heavy rainfall. In high water table conditions, it may run almost continuously.

Annual maintenance should include:

• Lifting the sump lid and inspecting for debris
• Manually activating the pump to confirm operation
• Testing the float switch across its full range
• Checking the non-return valve
• Flushing the perimeter drainage channel with clean water
• Testing the battery backup unit (if fitted)

A record of maintenance should be kept. Pump failure in a habitable basement will result in flooding within hours if inflow is significant. This is why backup provision is critical for Grade 3 use.

Frequently Asked Questions

How much wall thickness does the cavity drainage membrane add?

An 8mm dimple wall membrane adds 8mm of drainage space plus the batten (typically 25–38mm) and plasterboard (12.5mm), totalling approximately 45–60mm loss from each wall. Deeper dimple products (20mm) add correspondingly more. In a small basement, this is a significant loss — factor it into the design before specifying the product.

Can I use a cavity drainage membrane in an area that floods regularly?

Yes, provided the pump is sized correctly and backup provision is adequate. The pump must be rated to discharge the peak inflow during flooding conditions. For locations in active flood plains, multiple pumps, battery backup, and a gravity overflow at high level are all advisable. Consult a CSSW designer who should carry out a proper hydrological assessment.

What happens if the pump fails and I'm not there?

Without backup, the sump fills, water overtops the perimeter drain, and the basement floods. This is why BS 8102:2022 requires the consequences of pump failure to be considered in design. Battery backup is the most common solution; it provides 24–72 hours of pumping depending on the inflow rate. A high-water alarm (audible and/or GSM remote alert) gives early warning. Gravity overflow at a higher level can limit damage if the sump is overwhelmed.

Does the system need warranty or insurance?

Yes, for any habitable basement. Most CSSW designers offer installation through approved contractors whose work is covered by the manufacturer's insurance-backed guarantee (IBG). This IBG is important for mortgage purposes and for building insurance. DIY installation will not carry an IBG.

How is the membrane system affected if I want to add underfloor heating later?

Warm water underfloor heating systems installed in a screed over the floor membrane are fully compatible. The screed poured over the membrane encases the UFH pipes in the usual way. Electric underfloor heating mats can similarly be embedded in a screed or self-levelling compound. The membrane system itself is unaffected. Heat output calculations should account for the resistance of the floor membrane and screed.

Regulations & Standards

• BS 8102:2022 — Code of practice for protection of below-ground structures against water from the ground; the primary design standard for all basement waterproofing

• BS EN 12050-1 [verify] — Wastewater lifting plants for buildings and sites; relevant to pump selection

• Building Regulations Approved Document C — Resistance to moisture; sets minimum performance requirements

• Building Regulations Approved Document L — U-value targets for basement walls and floors

• PCA Code of Practice for Structural Waterproofing — professional body guidance; CSSW qualification

• Newton Waterproofing Systems — Technical Design and Installation Guides — full product range and installation documentation

• Triton Systems — Cavity Drain Membrane Technical Guide — alternative product range with design guidance

• Property Care Association — CSSW Qualification and Member Directory — find a qualified waterproofing designer

• Platon — Technical Literature — membrane specifications and installation data sheets

• NHBC Standards Chapter 5.4 — new build below-ground waterproofing reference

• basement waterproofing — overview of BS 8102 types A, B, and C; system selection by grade

• damp proof membrane — above-ground floor DPM requirements

• timber rot treatment — treating structural timber in damp basement conditions