How to Install Damp Proof Membrane and Damp Proof Course: A Trade Guide

20th Aug 2024 -

• What is Damp Proofing?
• What Are the Types of DPC and DPM?
• DPC vs DPM: What is the Difference?
• How to Install a DPC and DPM: Step-by-Step Trade Guide
• 10 Common Issues With the Installation of DPCs and DPMs
• Source Premium DPC and DPM Solutions Today

What is Damp Proofing?

Damp proofing is the process of protecting buildings from water ingress and rising damp. Two ways of doing this are through a damp proof course (DPC) or a damp proof membrane (DPM).

What is a damp proof course? (DPC)

A DPC is a horizontal barrier made from water-impermeable materials like plastic or bitumen inserted into the wall mortar bed to prevent moisture from rising through the structure. 

What is a damp proof membrane? (DPM)

DPMs are sheets of water-impermeable material (polythene or bitumen-based compounds) installed beneath concrete floor slabs or in crawl spaces to prevent moisture from passing from the ground into a building. 

What Are the Types of DPC and DPM?

Each DPC and DPM type has unique properties that make it suitable for particular applications. Learning these differences ensures effective moisture protection in diverse building scenarios. 

Materials that you’ll commonly find making up a damp proof course

• Bitumen: A flexible, traditional option made from bitumen-impregnated materials. Best for standard brick and block construction in low-risk areas.
• Plastic: Typically made from polyethene, offering excellent flexibility and chemical resistance. Suitable for most residential and light commercial applications.
• Metal: Usually lead or copper, providing high durability and resistance to movement. Ideal for buildings with high structural loads or in areas prone to settlement.
• Slate: A rigid option with excellent compressive strength. Best for traditional construction methods and heritage buildings.
• Engineering brick: A course of engineering bricks with very low porosity. Suitable for high-load-bearing walls and areas with severe exposure to moisture.
• Chemical: Injected silicone or silane-based solutions that create a water-repellent barrier. Excellent for retrofitting where physical DPC insertion is impractical.

Materials that you’ll commonly find making up a damp proof membrane

• Polyethene sheet: A common option for basic ground moisture protection. Suitable for most residential and light commercial floor slabs.
• Bitumen-based: Self-adhesive sheets offering good waterproofing properties. Ideal for basements, foundations and roofing applications.
• Ethylene propylene diene monomer (EPDM): Highly elastic and durable synthetic rubber membrane. Excellent for flat or low-slope roofs.
• PVC: Flexible and weldable, offering good chemical resistance. Suitable for basements, tunnels and areas with potential chemical exposure.
• Bentonite clay: Self-sealing, expandable clay sheets. Ideal for complex underground structures and areas with high hydrostatic pressure.
• Liquid-applied: Spray-on systems form a seamless barrier. Perfect for complex shapes, detailing around penetrations and retrofit applications.
• Gas-resistant: Multi-layer membranes designed to block both moisture and gases. Essential for construction on brownfield sites (previously developed land) or areas with methane risks.
• Crystalline waterproofing: A cementitious product that grows crystals to seal concrete pores. Ideal for waterproofing concrete structures from the inside out.

DPC vs DPM: What is the Difference?

While both DPCs and DPMs serve to protect buildings from moisture, they have distinct characteristics and applications. Understanding the differences between what is damp proofing with a DPC or DPM  is crucial for proper implementation.

Feature	DPC	DPM
Definition	Horizontal barrier for damp proofing walls	Sheet material to prevent ground moisture entry
Primary Function	Stops moisture rising through capillary action	Creates a barrier against ground moisture
Common Materials	Bitumen-coated hessian, lead, slate, polyethene and polymer-modified bitumen	Polyethene, PVC and multi-layered synthetics
Installation Location	In walls, typically 150mm above ground level	Under concrete slabs, in crawl spaces, on foundation walls
Thickness	Generally 0.5mm to 1mm	Usually 0.25mm to 1.2mm
Flexibility	Less flexible	More flexible
Orientation	Primarily horizontal	Both horizontal and vertical
Coverage Area	Specific to wall width	Large areas (entire floors; wall surfaces)
Retrofitting	Can be retrofitted into existing walls	Difficult to retrofit; usually requires major reconstruction
Pressure Resistance	Primarily deals with capillary action	Often designed for higher hydrostatic pressures

How to Install a DPC and DPM: Step-by-Step Trade Guide

Proper installation ensures effective moisture protection and longevity. While techniques vary, there are key principles that apply to both. Below, we outline the steps for how to install a damp proof course and how to lay a damp proof membrane. 

These steps include the use of the common materials of bitumen and polythene. 

Step 1: Preparation and tools

Start by gathering the necessary tools and materials. 

DPC installation

Damp proof course	Tape measure
Chalk line	Circular saw with masonry blade
Hammer	Chisel
Spirit level	Trowel
Utility knife	Safety goggles
Mortar	Gloves and dust mask

DPM installation

Damp proof membrane	Roller
Utility knife	Tape measure
Chalk line	Straight edge
Jointing tape	Safety goggles, gloves and dust mask
Primer	Screed

Clean the area thoroughly, removing all debris and repairing any substrate damage. 

Step 2: Measuring and marking

Measure and mark where a damp proof course could be installed, typically 150mm above ground level. When installing a DPM, measure the whole area to be covered. Use a chalk line to mark clear guidelines for installation. 

With DPMs on complex shapes, consider creating a template.

Step 3: Preparing the surface

If installing a DPC, use a circular saw to cut the marked line, creating a channel in the mortar joint. Clean this channel thoroughly with a hammer and chisel. For DPM installation, apply a suitable primer to the substrate if needed. 

Allow it to dry according to the manufacturer's instructions (usually 1-2 hours). 

Step 4: Cutting the material

Cut the DPC or DPM material to the required size using a utility knife. For DPCs, measure each section individually, allowing for overlaps at corners and joints. When cutting DPMs, allow for enough overlap between sheets (at least 150mm).

Step 5: Installation

For DPC installation, apply a bed of mortar to the bottom of the channel. After, carefully insert the DPC, ensuring it is flat and free from folds. Press it firmly into place with a trowel. When installing a DPM, roll it out across the area, starting from one corner. 

Ensure the membrane lies flat against the substrate, removing any air pockets.

Step 6: Securing and sealing

Secure the DPC by applying mortar on top and filling the channel. For DPMs, secure the edges temporarily if installed on vertical surfaces. Apply jointing tape to overlaps between DPM sheets, pressing firmly to ensure a good seal.

With both types, pay special attention to corners and joints, ensuring proper overlapping and sealing.

Step 7: Dealing with penetrations and details

For both DPCs and DPMs, carefully cut the material around any pipes or other penetrations. With DPMs, use preformed collars where possible. Seal thoroughly with the appropriate tape. 

Ensure continuous coverage for DPC around windows, doors and other openings.

Step 8: Finishing and protection

Remove any excess mortar and ensure the wall is left tidy after DPC installation. For DPMs, if the membrane will be exposed temporarily, protect it from damage with boards. Apply protection boards or screed as soon as possible for permanent installations. 

Allow all materials to cure or set as per the manufacturer's instructions before continuing construction.

10 Common Issues With the Installation of DPCs and DPMs

Even if you carefully plan your damp proofing job and do everything perfectly, you might still encounter problems. 

When you recognise and address these problems early, you can still ensure you install an effective damp-proofing solution for your customer. Let's look at some common issues and their solutions.

1. Bridging in DPC installation

The DPC fails to extend fully across the wall thickness, creating a path for moisture to bypass the barrier.

Solution: For new constructions, ensure the DPC extends to the full width of the wall and protrudes slightly on both sides. In existing structures, inject a chemical DPC at multiple points along the wall's thickness to create a continuous barrier. 

Use a drill with a long masonry bit to reach the centre of the wall, injecting the chemical DPC at intervals.

2. Punctures or tears in your DPC/DPM

Accidental damage to the membrane during installation or subsequent work.

Solution: With small punctures, apply a patch of the same membrane material. This should extend at least 150mm beyond the damaged area in all directions. Use a compatible adhesive or heat welding for a secure bond.

For larger tears, cut out the damaged section and install a new piece of membrane, ensuring proper overlap and sealing. In existing structures, carefully remove flooring to access the damaged area for repair. 

3. Poor sealing

Inadequate sealing at joints and penetrations allows water ingress at vulnerable points.

Solution: Use specialised tapes and sealants designed for the specific membrane material. For pipe penetrations, install proper collars or boots. In retrofit scenarios, remove surrounding materials to apply sealants, and reinstate the materials once finished.

Consider using expandable water stops for dynamic joints in concrete structures.

4. Inadequate lapping of damp proofing sheets

Insufficient overlap between membrane sheets compromises the waterproof barrier.

Solution: Ensure a minimum overlap of 150mm for horizontal surfaces and 300mm for vertical surfaces. In existing structures, carefully lift the edges of the membrane to create proper overlaps. 

If this is not workable, apply an added layer of membrane over the joint. The added layer should extend beyond the inadequate overlap on both sides.

5. Damp proof course bridging failure

The DPC fails to connect properly at the junction of internal and external walls.

Solution: Install preformed cavity trays at these junctions in new constructions. For existing buildings, inject a chemical DPC at the junction point, ensuring it connects with the main DPC. 

Also, apply a waterproof render to the internal wall surface up to 1 metre above ground level.

6. You discover damp rising in the wall

Moisture appears on walls above the level of the DPC in existing structures.

Solution: This often indicates that ground levels outside have risen above the DPC. Lower the external ground level if possible. If not, install a new chemical DPC above the current ground level. 

Additionally, apply a waterproof render to the affected area internally. Consider adding an external French drain system (perforated pipe surrounded by gravel) to divert water away from the foundation.

7. The damp proof membrane has detached

The DPM separates from the wall in basement tanking applications.

Solution: For new installations, ensure proper surface preparation and use a fully bonded system. In existing structures with damp proof membrane on walls, inject a resin-based grout between the membrane and the wall. This re-establishes the adhesion. 

If detachment is extensive, consider installing a new, internally applied cementitious tanking system over the existing one.

8. Insufficient protection from the elements

UV degradation or mechanical damage to exposed membrane edges.

Solution: Always terminate exposed edges of the membrane under a cover flashing (protective metal sheet). For retrofits, carefully cut a chase into the wall above the exposed membrane. Tuck the membrane into the chase and seal it with a suitable mastic. 

Protect horizontal surfaces with screed or other coverings as soon as possible after installation.

9. The right materials haven’t been used or material isn’t suitable any more

Chemical reactions or physical incompatibility cause degradation.

Solution: Always check material compatibility before installation. If incompatibility is discovered in an existing structure, build a barrier between the incompatible materials. You can create the barrier using a neutral separator sheet or coating. 

In severe cases, replacement of the affected sections may be necessary.

10. Inadequate ventilation

Lack of airflow above DPC in suspended timber floors leads to condensation and timber decay.

Solution: Ensure proper subfloor ventilation by installing added air bricks if necessary. In existing structures, consider retrofitting small electric fans in air bricks to improve air circulation. Also, check and clear any blocked air bricks or vents.