Damp Proof Injection: How Chemical DPC Works

What is damp proof injection?

Damp proof injection, also called chemical DPC injection or chemical damp course installation, is a method of creating a damp proof course (DPC) in existing walls without cutting or removing masonry.

It works by drilling a series of holes into the mortar bed at the height where a DPC should sit (typically 150mm above external ground level) and injecting a water-repellent chemical under pressure. The chemical diffuses through the porous masonry, forming a continuous water-resistant zone that blocks capillary rise.

Chemical injection is the most common method for retrofitting a DPC in buildings that lack one (pre-1875 properties) or where an existing DPC has failed (degraded bitumen felt or cracked slate).

How does chemical DPC injection work?

Porous masonry materials (brick, stone, mortar) contain tiny capillaries that draw groundwater upward by capillary action. A chemical DPC works by lining these capillaries with a water-repellent coating, breaking the capillary pathway.

The process has three stages:

1. Injection

Holes are drilled at 100-120mm intervals along the mortar bed, typically 12-15mm diameter and 100-150mm deep (or two-thirds of wall thickness for thick walls). The holes are angled slightly downward to prevent the chemical from running out.

A silicone or silane-based cream is injected into each hole using a hand-held applicator or pressure pump. Modern creams are thick, paste-like substances that stay in place rather than running through the masonry like older solvent-based liquids.

2. Diffusion

Over the next 2-6 weeks, the active ingredient (silane or siloxane polymer) diffuses laterally and vertically through the pores of the masonry. As it spreads, it bonds to the internal surfaces of the capillaries, creating a hydrophobic (water-repellent) lining.

The diffusion rate depends on:

• Wall porosity (faster in soft brick and lime mortar, slower in dense brick and cement mortar).
• Wall saturation (very wet walls diffuse slowly; partially dried walls diffuse faster).
• Temperature (diffusion slows below 5°C).

3. Curing

The chemical cures to form a permanent water-repellent barrier. Curing time varies by product but is typically 4-6 weeks. During this period, the wall should not be replastered or sealed, as this can trap moisture and prevent full curing.

Types of chemical DPC products

Silane-based creams (most common)

Silane and siloxane creams are the current industry standard. They are thick, paste-like substances that diffuse slowly and evenly through masonry. They work by forming a molecular bond with silica in the masonry, creating a long-lasting hydrophobic barrier.

Advantages:

• Effective in most masonry types (brick, stone, lime mortar, cement mortar).
• Do not run out of holes or migrate downward through the wall.
• Long service life (20-30 years, some products claim 40+).
• Breathable (allow water vapour to pass through, which is important for wall health).

Common brands: Wykamol, Dryzone, Stormdry, Permagard.

Solvent-based liquids (older technology)

Solvent-based products (aluminium stearate or metallic siliconate in white spirit or water) were the original chemical DPC technology in the 1960s-1980s. They are thinner and flow more easily through masonry but are less reliable than modern creams.

Disadvantages:

• Can run out of holes before diffusing, leaving gaps in coverage.
• Migrate downward under gravity, leaving the upper part of the injection zone under-treated.
• Shorter service life (15-25 years).
• Less effective in very porous or saturated walls.

Solvent-based products are rarely used now except in very dense masonry where cream diffusion is too slow.

Installation process step by step

1. Pre-treatment assessment

A specialist surveys the property to confirm rising damp (not condensation or a leak) and check for bridging. Bridging (external render below DPC level or raised ground) must be fixed before injection, or the treatment will fail.

Moisture meter readings are taken at multiple heights to establish the moisture profile and confirm that the problem is rising damp.

2. Drilling

Holes are drilled into the mortar bed (not through bricks) at 100-120mm intervals, 150mm above external ground level. For cavity walls, holes are drilled into both inner and outer leaves. For solid walls over 350mm thick, holes may be drilled from both sides to ensure full coverage.

3. Injection

Cream is injected into each hole using a hand pump or electric pressure gun. The volume injected depends on wall thickness and porosity but is typically 100-200ml per hole. The holes are left open to allow excess cream to overflow, confirming that the wall has absorbed the maximum amount.

4. Plugging holes

Once the cream has been absorbed, the holes are plugged with mortar to match the existing pointing. The plugs are flush with the wall surface and barely visible once dry.

5. Curing period

The wall is left to cure for 4-6 weeks. No plastering, painting, or sealing should be done during this period. The wall may continue to feel damp due to residual moisture and hygroscopic salts in the old plaster.

6. Replastering

After curing, affected plaster is hacked off to 300mm above the highest tide mark and replaced with salt-retardant renovating plaster. This is essential because hygroscopic salts in the old plaster will continue to absorb moisture from the air, making the wall appear damp even though capillary rise has stopped.

How much does damp proof injection cost?

Typical costs in 2026:

• Chemical injection only: £40-70 per linear metre.
• Replastering with renovating plaster: £80-120 per m².
• Full treatment (injection + replaster) for a 10m wall, 1.5m high: £1,600-2,800.

Prices vary depending on:

• Wall thickness (thick walls require more cream and deeper drilling).
• Wall accessibility (external work may require scaffolding).
• Extent of replastering (one wall vs full room vs multiple rooms).
• Guarantee length (20-year vs 30-year guarantee).

Most contractors include a written guarantee that is transferable to future owners, which adds value when selling the property.

Does damp proof injection actually work?

Independent testing by the Building Research Establishment (BRE) confirms that silane and silicone cream injections form an effective water-repellent barrier in most masonry types when installed correctly.

BRE Digest 245 (2007) states that chemical DPCs achieve water absorption reductions of 90-95% in laboratory tests and 70-85% in field conditions. The difference is due to installation variables: wall saturation, drilling accuracy, and post-treatment care.

Factors affecting effectiveness

Wall saturation

Very wet walls (above 25% moisture content) allow poor cream diffusion because the capillaries are already filled with water. Some contractors recommend pre-drying the wall with dehumidifiers or waiting for summer conditions before injection.

Mortar bed quality

Chemical DPCs work best when injected into the mortar bed, which is softer and more porous than brick. If the mortar has been replaced with hard cement mortar or has eroded, the chemical may not diffuse effectively. In such cases, drilling through bricks may be necessary.

Wall thickness

Solid walls over 450mm thick may require drilling from both sides to ensure complete barrier formation. Single-sided injection in very thick walls can leave untreated zones in the wall core.

Post-treatment replastering

Failing to replaster with renovating plaster is the most common reason for apparent treatment failure. Hygroscopic salts in the old plaster continue to draw moisture from the air, so the wall appears damp even though no water is rising from below.

BRE Good Repair Guide 6 (2005) emphasises that chemical DPC injection and replastering are a package treatment. One without the other is unlikely to achieve a dry wall.

How long does chemical DPC last?

Silane and silicone cream DPCs are typically guaranteed for 20-30 years, but the actual service life depends on product quality and installation conditions. Modern formulations are expected to last 40+ years.

Unlike physical DPCs (plastic sheeting or slate), which can be torn by structural movement, chemical DPCs are flexible and accommodate minor settlement without failure. However, they can degrade if the wall is repeatedly saturated by leaks or poor drainage.

Alternatives to chemical injection

Physical DPC insertion

A horizontal slot is cut through the wall at DPC height using a specialist saw, and a plastic or slate membrane is inserted. This creates a guaranteed physical barrier but is expensive (£80-150 per linear metre) and requires propping to prevent wall collapse during cutting.

Physical DPC insertion is used where chemical injection has failed, where walls are too dense or saturated for effective chemical diffusion, or where a structural guarantee is required for mortgage purposes.

Electro-osmotic systems

A low-voltage electrical current is applied to the wall using embedded electrodes, reversing the electrical charge that drives capillary rise. This is a controversial technology with limited independent evidence of effectiveness. It is rarely recommended by surveyors.

Can you DIY damp proof injection?

DIY kits are available for £150-300 (10m of wall coverage), including cream, drill bit, and applicator. However, success depends on correct diagnosis, drilling accuracy, and product choice.

Most manufacturers require professional installation to activate the guarantee. Without a guarantee, you have no recourse if the treatment fails.

If you attempt DIY:

• Confirm rising damp with a moisture meter. Do not inject if the problem is condensation or a leak.
• Check for and fix any bridging (raised ground, render below DPC) before injection.
• Use a reputable silane-based cream (Wykamol, Dryzone), not cheap solvent liquids.
• Drill into mortar beds, not through bricks.
• Allow 4-6 weeks curing before replastering.
• Hack off all contaminated plaster and replaster with renovating plaster, not standard gypsum.

Sources

• BRE Digest 245 (2007): Rising damp in walls - diagnosis and treatment
• BRE Good Repair Guide 6 (2005): Treating rising damp in houses
• Property Care Association: Code of Practice for Installation of Chemical Damp-proof Courses (2023)
• BS 6576:2005: Code of practice for diagnosis of rising damp in walls and installation of chemical damp-proof courses