What Is A Damp Proof Membrane And Why Is It Important Under Screed?
When it comes to floor screeding, much of the focus tends to fall on the screed itself, the mix, the depth, the drying time and the finish. However, one of the most important elements of a well-executed floor build-up is something that sits entirely out of sight once the job is done: the damp proof membrane.
Getting this right is fundamental to the long-term performance of the floor, and skipping or cutting corners on it can lead to serious and costly problems further down the line.
What Is A Damp Proof Membrane?
A damp proof membrane, commonly referred to as a DPM, is a sheet of heavy-duty polythene that is laid across the floor structure before insulation and screed are installed. Its primary function is to act as a physical barrier between the subfloor or ground below and the layers above, preventing moisture from rising up through the floor construction.
DPMs are typically manufactured from 1200-gauge or 500-gauge polythene, with the appropriate specification depending on the level of moisture protection required for the specific project. The membrane is laid in sheets across the entire floor area, with overlapping joints that are taped to create a continuous, sealed barrier.
Why Is Moisture A Problem Under Screed?
Moisture rising from the ground is one of the most persistent and damaging issues in floor construction. Even where a concrete slab has been poured, ground moisture can pass through it over time. If this moisture reaches the screed layer without any barrier in place, several problems can follow.
Screed Damage & Delamination
Moisture working its way into a screed can weaken its structure, causing it to become soft, friable or prone to cracking. In more advanced cases, it can cause the screed to delaminate, lifting away from the subfloor beneath and creating an unstable surface that is unsuitable for any floor covering.
Damage To Floor Coverings
Moisture that passes through the screed and reaches the finished floor covering can cause significant damage. Timber and engineered wood floors are particularly vulnerable, as excess moisture causes boards to swell, cup or bow. Adhesive-fixed floor coverings such as vinyl or carpet tiles may also lose their bond, resulting in lifting, bubbling or failure of the installation.
Underfloor Heating Performance
Where an underfloor heating system is installed beneath the screed, uncontrolled moisture in the floor construction can affect the efficiency of the system and, over time, contribute to deterioration of the pipework and screed. A correctly installed DPM protects the entire floor build-up, keeping the system performing as intended for years to come.
How Is A Damp Proof Membrane Installed?
Preparation
Before the membrane is laid, the subfloor must be reasonably clean and free from sharp protrusions that could puncture the polythene. Any significant voids or irregularities in the subfloor should be addressed at this stage.
Laying The Membrane
The polythene sheets are laid across the entire floor area, with each sheet overlapping the adjacent one by a minimum of 150mm to 300mm depending on specification. All joints are taped using a compatible adhesive tape to create a continuous, sealed layer with no gaps through which moisture could pass.
Upstands To Walls
The membrane is taken up at the edges of the room to form an upstand against the walls, typically to a height that takes it above the finished floor level. This prevents moisture from bypassing the membrane at the perimeter and is a detail that is sometimes overlooked on poorly executed installations.
Floor Insulation & Perimeter Expansion Strips
Once the membrane is correctly laid and sealed, insulation boards are placed on top, followed by the perimeter expansion strip and then the screed itself. In projects incorporating underfloor heating, the pipework is installed at this stage before the screed is poured.
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