Moisture Barriers & DPM Installation

Moisture Barriers & DPM Installation – Floor Build-Up Layers

Understanding the correct sequence of layers in a ground-bearing concrete floor assembly is the starting point for effective moisture barriers and DPM installation. Each layer serves a specific protective or structural function — and each must be installed in the correct order. The diagram below shows the standard floor build-up for a ground-bearing reinforced concrete slab from subgrade to finished floor level, with DPM positioned in its correct location.

The position of the DPM within the build-up — whether above or below the blinding layer, and whether above or below the insulation — is a critical design decision that affects both moisture performance and the structural integrity of the slab. This is discussed in detail in the sections below. The backfilling around concrete foundations guide on ConcreteMetric covers the sub-base and fill layer requirements that form the foundation of this build-up.

DPM Types and Grades – Moisture Barrier Selection Guide 2026

Selecting the correct moisture barrier and DPM type for the specific application, ground conditions, and floor finish is one of the most important decisions in the floor build-up design. The minimum acceptable DPM for residential ground-bearing slabs in Australia (NCC 2026) and the UK (Building Regulations Approved Document C) is 300 micron (1200 gauge) polythene sheet to BS EN 13967 or equivalent. Higher-risk applications — aggressive ground contamination, high water table, or sensitive floor finishes — require heavier or more chemically resistant membranes.

DPM Specification Reference – Thickness, Standards, and Applications 2026

The table below provides a ready reference for moisture barrier and DPM specification covering the minimum thickness, applicable standard, vapour transmission performance, and appropriate applications for each DPM type. Use this table to confirm that your DPM specification meets the minimum requirements for your project's exposure class and floor finish sensitivity before ordering materials.

DPM Above or Below Insulation – Which Position is Correct?

The most frequently debated question in moisture barrier and DPM installation practice is whether the DPM should be placed above or below the insulation layer. Both positions are used in practice and both can be correct — the right choice depends on the design intent, the type of insulation, and the floor finish. Understanding the implications of each position is essential for correct specification.

Moisture Barriers & DPM Installation – Step-by-Step Process

The following step-by-step procedure covers the complete installation of a polythene sheet DPM (300–500 micron) beneath a ground-bearing reinforced concrete slab — the most common moisture barrier and DPM installation scenario in residential and commercial construction in 2026. Follow each step in sequence — post-pour remediation of a failed DPM is one of the most expensive and disruptive floor defect rectification scenarios possible.

1. Step 1 – Prepare the Sub-Base and Blinding: Compact the granular sub-base to the specified depth (minimum 150 mm Type 1 granular sub-base per AS 2870 / BS 8204). Apply a 50 mm sand blinding layer or 50 mm lean concrete blinding over the sub-base surface to create a smooth, even bed free from sharp aggregate that could puncture the DPM. Sand blinding must be carefully levelled — any high points or embedded stones will create puncture risk. Allow lean concrete blinding to achieve sufficient strength (minimum 3 MPa) before laying the DPM.
2. Step 2 – Lay the DPM Sheet in Correct Position: Unroll the DPM sheet across the prepared blinding surface. Position the membrane so it is large enough to extend up all surrounding walls to at least 150 mm above finished floor level (FFL) to connect with the wall DPC. Work from the centre of the slab outwards, smoothing the membrane flat without stretching. Avoid dragging the sheet across sharp aggregate or tools. In windy conditions, weight the edges temporarily to prevent displacement before the concrete is poured.
3. Step 3 – Lap All Sheet Joints by Minimum 300 mm: Where multiple sheets are required to cover the full slab area, all joints must overlap by a minimum of 300 mm per BS 8102 and AS 2870. Seal every lap joint with a proprietary DPM jointing tape rated to the same WVT standard as the membrane — do not rely on simple overlap alone as moisture will track between sheets at unsealed laps. For radon-resistant DPMs, all laps must be double-sided taped and the tape pressed down firmly along the full length of every joint.
4. Step 4 – Form All Wall Upstands: Turn the DPM up all perimeter walls and internal walls passing through the slab to a minimum height of 150 mm above FFL — this connects the DPM to the wall DPC and prevents moisture from tracking around the membrane edges. Fix the upstand temporarily to the wall formwork using DPM tape or staples. Fold the membrane neatly at internal and external corners — do not cut the membrane at corners. At external corners, form a triangular pleat to maintain a continuous membrane without cuts or tears.
5. Step 5 – Seal All Service Penetrations: Every service penetration through the DPM — drainage pipes, electrical conduits, gas pipes, and structural bolt-downs — is a potential moisture pathway. Seal all penetrations using purpose-made pipe collar seals or by wrapping the penetrating element with a minimum 300 mm wide strip of DPM and taping it fully to the membrane on both sides. For pipes with complex geometry, use a liquid-applied DPM flashing compound around the penetration base before taping the sheet overlay. Never simply cut a hole in the DPM and tuck the sheet around the pipe.
6. Step 6 – Protect the DPM During Reinforcement Placement: Polythene DPMs are highly vulnerable to puncture and tearing during reinforcement placement, especially from bar chairs, wire ties, and workers walking across the membrane. Where possible, place a protection layer — either a second 300 micron polythene sheet, a 3–6 mm sand-bed, or a purpose-made protection fleece — over the DPM before placing reinforcement. Inspect the DPM immediately before concrete placement and repair any visible punctures, tears, or dislodged tape joints with DPM tape before the pour begins.
7. Step 7 – Final Inspection Before Concrete Pour: Walk the entire DPM area immediately before the concrete pour begins — this is the last opportunity to identify and rectify any damage. Check: all lap joints are sealed and tape is adhered for the full length; all wall upstands are in place and at the correct height; all penetrations are sealed; the DPM has not been displaced or folded under by foot traffic; and no pooled water is sitting on the membrane surface (if present, pump out and allow to drain before pouring). Record the inspection with photographs as part of the quality assurance documentation for the project.

Common DPM Installation Failures and How to Avoid Them

The majority of rising damp and moisture-related floor finish failures traced back to the DPM are caused by a small number of predictable, avoidable installation errors. Understanding these failures before work begins is the most effective quality control measure available for moisture barrier and DPM installation in 2026. Rectifying a failed DPM after a floor finish has been installed typically costs 10–30× the cost of the original DPM installation.