Damp Proof Course Replacement: Chemical Injection, Physical DPC & When You Need It

Quick Answer: A damp proof course (DPC) is a horizontal barrier in masonry that prevents ground moisture from rising. Most pre-1920s properties have no DPC, or one that has failed. The two main retrofit remedies are chemical injection (silane/siloxane cream or fluid injected into drill holes at 150mm centres) and physical DPC insertion (cutting out mortar courses and inserting a new physical barrier). Chemical injection is covered by BS 6576:2005 and is the standard modern approach. Physical DPC insertion is rarely used in domestic work due to the structural risk.

Summary

Damp proof courses have been a building requirement since the Public Health Act 1875. Properties built before then have no original DPC; properties built between roughly 1875 and 1920 may have slate, bituminous felt, or engineering brick DPCs that have failed or been bridged. Post-1920 properties typically have bituminous felt or polymer-modified bitumen DPCs that can fail if cracked, bridged, or disrupted.

The decision whether to replace or install a new DPC should only come after a thorough investigation rules out other causes of damp (see rising damp). Genuine hydraulic rising damp requiring DPC treatment is less common than the remediation industry suggests. Many cases of apparent rising damp are caused by: external ground levels bridging the existing DPC, failed or missing cavity trays, bridged wall ties in cavity walls, or hygroscopic salts in old plaster from a previous damp episode.

When DPC work is genuinely required, chemical injection has largely superseded all other methods because it's non-disruptive to the structure, can be applied to walls that are already wet, and is backed by 20+ year performance data. Physical DPC insertion is retained for specific situations — particularly thin walls, party walls, or where the chemistry of the masonry would not support effective injection (e.g., very porous soft brick or very dry masonry that won't draw in chemical).

Key Facts

• BS 6576:2005 — code of practice for diagnosis of rising damp and installation of chemical DPC; the primary standard for chemical injection work
• PCA (Property Care Association) — the industry body; members are bound by the PCA Code of Practice; PCA-backed guarantees are scheme-insured
• Chemical injection depth — holes drilled to within 25mm of the external face of the wall; depth depends on wall thickness
• Drill hole spacing — 150mm centres horizontally; typically one row per leaf of masonry
• Drill angle — 30–45° downward to encourage product flow into the masonry and to allow product to drain to the base of the hole
• Drill diameter — typically 10–12mm (check product datasheet; some products need 12mm for effective cartridge injection)
• Injection height — holes centred at 100–115mm above internal floor slab level (allows product to penetrate below floor level)
• Silane/siloxane products — active ingredient forms a hydrophobic layer in the masonry pores; reduces capillary absorption without fully blocking pores (allows masonry to breathe)
• Cream vs liquid injection — cream products (e.g., Dryzone, Wykamol Damp Course Cream) are preferred for DIY or one-row injection; liquid injection under pressure is used for dense stone or concrete
• Cavity walls — each leaf injected separately; cavity itself acts as a break
• Solid walls — single row of holes drilled from inside; or two rows from both faces if wall is over 325mm thick
• Curing time — typically 3–7 days before replastering; longer in cold or wet conditions
• Renovation plaster — always replaster with lime or renovation plaster after DPC treatment (never standard gypsum); see rising damp
• Minimum replastering height — 300mm above the visible tide mark, or 1.0m from floor level minimum

Quick Reference Table

Detailed Guidance

Chemical Injection — Step by Step

1. Prepare the area

• Remove skirting boards and plinths
• Hack off existing plaster from floor level to 300mm above the damp tide mark (or at minimum 1.0m from floor)
• Remove any debris from floor/wall junction
• Check that external ground levels are not bridging the DPC; if they are, reduce them first

2. Mark and drill holes

• Mark hole centres at 150mm horizontal spacing along the wall at 100–115mm above the floor slab
• Drill at 30–45° downward to the calculated depth (see table above)
• Clear drill dust from holes using a rubber bulb blower or compressed air

3. Inject the product

• For cream products: load the correct nozzle, push into the hole until resistance is felt, inject slowly until cream appears at the hole entrance; move methodically along the row
• For liquid pressure injection: fit proprietary injection port, connect pump, inject at manufacturer-specified pressure
• Fill all holes; capping with matching mortar after injection is good practice

4. Allow to cure

• Do not replaster for the cure period specified by the manufacturer (typically 3–7 days at 15°C+)
• In cold weather (<5°C) curing is significantly slower — allow additional time
• Keep the area ventilated during curing

5. Apply salt neutraliser

• Before replastering, apply a hygroscopic salt neutraliser or primer to the cleaned masonry
• This reduces the ability of residual nitrate/chloride salts to attract atmospheric moisture and cause false meter readings

6. Replaster

• Use renovation plaster (not standard gypsum) — typically a two-coat system of lime/hydraulic lime or proprietary renovation plaster
• Apply from floor level to 300mm above the original tide mark
• Allow each coat to dry before applying the next

Physical DPC Insertion

Cutting out mortar courses and inserting a new physical DPC is rarely done in modern practice due to the risk of structural damage, but it remains a valid option in certain circumstances:

Method: using a concrete saw or specialist DPC cutting machine, cut along a mortar course (typically the second course above floor level). Remove a short section of mortar. Insert new DPC material (typically polyethylene strip or bituminous felt cut to wall width). Reinstate mortar. Work in 1.0–1.5m alternating sections to avoid undermining the wall.

Suitable for: thin walls where chemical injection is ineffective; very dense stone where cream products won't penetrate; party wall agreements where both parties consent.

Not suitable for: rubble stone walls (no clear mortar courses); walls carrying significant loads (risk of settlement); very old lime mortar walls where cutting can cause widespread damage.

Electro-Osmotic Systems

Electro-osmotic DPC systems (active type: an electrical current reverses the charge in masonry pores, repelling moisture downward) have been marketed for many years. Evidence for their effectiveness is mixed and they are not covered by BS 6576. They may have a role where chemical injection is impractical, but should not be specified as a substitute for conventional chemical injection on the basis of the current evidence.

Passive electro-osmotic titanium mesh systems (no electrical supply needed) have even less supporting evidence. The PCA does not endorse them as a primary treatment.

DPC Bridging — The Most Common Cause of Failure

A DPC that is intact but bridged is effectively no DPC at all. Common bridges:

Cavity Trays

Cavity walls over openings (windows, doors) and at parapet level need cavity trays — horizontal DPC membranes across the cavity to direct any water that enters the cavity to weep holes in the outer leaf. Missing or failed cavity trays above openings are a very common cause of localised damp patches above windows and doors on cavity wall houses (typically 1930s–1990s build).

Retrofitting cavity trays is complex — it requires either cutting out bricks to access the cavity above the lintel, or using an injectable cavity tray system (e.g., Sovereign Cavity Tray Injection). This is specialist work. Ensure weep holes are clear (open perpend joints at DPC/tray level).

Frequently Asked Questions

How long does chemical injection take to work?

The hydrophobic cream needs time to migrate through the masonry pores and cure. The wall will then dry from the injection level upward over several months. Allow 3–6 months before expecting a significant reduction in moisture readings. The replastering conceals the wall during this period — which is fine if renovation plaster was used, as it will accommodate residual moisture.

Can I do chemical DPC injection myself?

DIY chemical DPC products (e.g., Dryzone Damp Course Cream) are available from builders' merchants and are the same products professionals use. The technique is straightforward. However, unless the diagnosis has been done properly (ruling out other causes), DIY injection on a misdiagnosed wall is a waste of money. Most DPC guarantees also require professionally installed, supervised work.

Does a chemical DPC injection guarantee come with the property?

PCA member scheme-backed guarantees are transferable to new owners and are insured independently of the installing company. This makes them valuable on sale and solicitors increasingly request them. Non-PCA guarantees are only as good as the company providing them. Always recommend PCA members to clients who want guarantee-backed work.

What's the difference between a DPC and a DPM?

A DPC (damp proof course) is horizontal, in a wall — it stops moisture rising. A DPM (damp proof membrane) is typically in the floor — a sheet of polythene laid under a floor slab or screed to prevent moisture rising from the ground. In a complete damp-proof system, they link — the floor DPM must be lapped up the walls and connected to the wall DPC to be effective.

Regulations & Standards

• BS 6576:2005 — code of practice for diagnosis of rising damp in walls and installation of chemical damp-proof course

• BS 8215 — code of practice for design and installation of DPCs

• Building Regulations Approved Document C — moisture resistance requirements

• Public Health Act 1875 — first statutory requirement for DPCs in new buildings

• PCA Code of Practice — member requirements for diagnosis, specification, and installation

• Property Care Association — BS 6576 guidance, member directory, technical resources

• BRE Digest 245 — rising damp in walls: diagnosis and treatment

• Wykamol — DPC Installation Guide — product-specific technical guidance

• Sovereign Chemicals Technical Data — cavity tray injection and DPC products

• rising damp — rising damp diagnosis and full treatment process

• tanking — basement waterproofing systems

• condensation — condensation and ventilation solutions

• garage conversions — DPM continuity in floor conversions