Damp Proofing Costs UK 2024: Injection DPC & Tanking Rates

Quick Answer: Injection damp-proof course (DPC) retrofit costs £80–£140 per linear metre of wall treated. Tanking (cementitious slurry) for basements £80–£180 per m² wall. Cavity drain membrane systems £70–£150 per m². Always commission a damp survey first (£200–£450) — many "rising damp" diagnoses are actually condensation, leaking gutters, or bridged DPC. PCA (Property Care Association) Code of Practice governs treatment specifications and Insurance-Backed Guarantee terms.

Summary

Damp work is one of the most controversial areas of UK trade. Genuine rising damp affects perhaps 5–10% of UK homes — far fewer than damp companies historically diagnose. Many "rising damp" calls are actually condensation (poor ventilation), penetrating damp (gutter/render failure), bridged DPC (high external ground levels), or plumbing leaks. Misdiagnose and you sell a £3,000 injection DPC that doesn't fix the problem.

This guide covers the four main treatment types — chemical DPC injection, tanking, cavity drain membrane, and atmospheric drying — and when each is appropriate. It also covers the survey discipline that should precede ANY treatment quote. PCA-member surveyors who provide Insurance-Backed Guarantees (IBGs) accepted by lenders dominate the market; tradespeople not PCA-registered need to either join, partner, or stay clear.

The opportunity for trades: external damp causes (gutters, render, drainage) are often dismissed by damp specialists who profit from chemical treatments. A general builder who fixes the actual cause (£500–£2,000) saves the client £3,000–£8,000 versus injection.

Key Facts

Quick Reference Table

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Treatment Application Cost per m / m² Includes Plaster Indicative Total Small Job
Injection DPC Walls at ground level for rising damp £80–£140/m linear + £45–£90/m² plaster £2,200–£5,500 for 25m wall + 25m² plaster
Cementitious tanking Below-ground walls, basements £80–£180/m² + £45–£90/m² plaster £2,500–£7,000 for 20m² basement wall
Cavity drain membrane Below-ground, problem areas £70–£150/m² Often left as-is, channels behind £2,500–£10,000
Air brick install Suspended floor sub-floor £20–£40/brick installed n/a £180–£600 for 6 bricks
Vent install (mechanical) Condensation rooms £200–£500/fan n/a £400–£1,500
External waterproof render Penetrating damp source £45–£90/m² n/a £1,500–£6,000
Damp survey alone Diagnostic £200–£450 n/a £200–£450

Detailed Guidance

Survey before quote — non-negotiable

Never quote damp treatment without a survey. Survey cost should be quoted separately. Survey includes:

  1. External inspection — Ground levels, render condition, gutters/downpipes, drainage, vegetation
  2. Internal damp inspection — Visual signs, salt deposits, plaster integrity, ground floor type
  3. Moisture measurement — Electronic surface meter (top-line indicator) + calcium carbide test (true gravimetric, more accurate)
  4. Salt test — Salt analysis paper / chemical reagent for chloride/nitrate/sulphate (rising damp typically contains chlorides/nitrates from soil)
  5. Sub-floor ventilation — Number/condition of air bricks
  6. Conditions — Internal RH/temp, occupant behaviour
  7. Report — Diagnosis, causes, treatment options, indicative costs

A proper survey eliminates 50%+ of "rising damp" diagnoses — substitution with the real cause (e.g. blocked downpipe, missing render) often costs <£500 to fix instead of £3,500 injection.

Distinguishing damp types

True rising damp:

Penetrating damp:

Condensation:

Traumatic damp:

Treatment matching to cause

Cause Correct Treatment Wrong Treatment
Rising damp Injection DPC + replaster Just plaster (returns)
Penetrating damp Repair external (render, pointing, gutter) Injection DPC (waste of money)
Condensation Ventilation + heating improvement Injection DPC (waste of money)
Bridged DPC (raised ground) Lower external ground; remove bridging Injection (won't help if path remains)
Plumbing leak Fix leak Anything else (waste)
Sub-floor damp Air bricks, sub-floor ventilation Injection (doesn't address sub-floor)

Tradespeople should be specific in quotes: "I diagnose the cause as X; treatment is Y. Treatment Z is NOT recommended because [reason]." Document this.

Injection DPC — when correct

When rising damp confirmed via calcium carbide moisture + salt analysis:

  1. Strip plaster — 1m above visible damp line, full wall to studs (or to brick)
  2. Drill injection holes — 12mm at ~110mm spacing along agreed line (typically 100–150mm above ground)
  3. Inject silicone cream (Dryzone, Ultracure, Permagard) — fully expressed into wall
  4. Apply re-plaster system — Salt-inhibiting render coat (Renderoc S or similar) over 1m height; finish skim plaster
  5. Dry-out period — 6–12 months for full equilibration before final decoration

Plaster replacement is 60–70% of the cost — never skip it. Old plaster contains salts that wick from below DPC level upward and recontaminate.

Tanking — for below-ground walls

Tanking applies to basements and sub-ground walls where preventing water ingress (vs DPC for capillary rise):

Cementitious slurry — Apply 2 coats of waterproof slurry directly to masonry. Stick-on plaster system or new plaster over.

Cavity drain membrane — Studded plastic membrane with air gap; water that gets through wall flows down channels to a sump and is pumped out. Modern preferred method for habitable basements.

Bituminous tanking — Older method, less common. Applied as thick paint.

Tanking is below-ground specialist work. Mistakes lead to ongoing dampness and major repairs. PCA-member contractors with relevant experience preferred.

Treating the cause externally

For external causes of damp, external remediation:

Always quote external treatment as option B against injection DPC quote A. Some clients won't believe injection isn't needed; provide written diagnosis.

Plaster replacement specification

After damp treatment, plaster replacement essential. Specification:

  1. Backing coat — Salt-inhibiting cement render (Renderoc S, Permagard Salt Inhibitor) 8–15mm thick. NOT standard sand-and-cement.
  2. Mesh reinforcement in chase if deep
  3. Skim coat — Bond, set, finish coat (1 day after backing)
  4. Drying period — 4–8 weeks before painting (cement curing)
  5. Decorating — Use breathable paint (mineral or silicate); avoid vinyl emulsion until fully dry

Costs typically £45–£90/m² for full reinstatement.

Selling against cowboys

Damp companies have a reputation problem because of historic over-diagnosis. To compete fairly:

  1. Always charge for survey separately — Shows independence
  2. Report findings in writing — Including what isn't damp
  3. Recommend external/cause-based fixes first when applicable
  4. Provide options — "Here's injection cost £X; here's the cause-based approach £Y"
  5. Reference PCA / RICS guidance — Builds credibility
  6. Offer IBG — Insurance-Backed Guarantee with treatment for client confidence

Trade-led damp work commands 25–35% margin and builds long-term reputation. Mis-sold injection work damages everyone's market.

Worked example — 25m ground floor flat affected by rising damp

Verified diagnosis: rising damp (tide marks, chlorides, calcium carbide 12% moisture)

For the alternative (cause-based external repair) where external bridge identified instead: £1,200–£2,000. Always present both options to client.

Frequently Asked Questions

How do I know if damp is really rising?

Three tests in combination: (1) calcium carbide moisture test — >5% WME on samples taken from wall core indicates rising damp; (2) salt analysis — chloride and nitrate ions present indicate ground water; (3) visual — tide mark with salt efflorescence at top, peeling paint, plaster damage 600–1200mm above ground. Surface moisture meter alone is unreliable — measures conductivity, not moisture.

Will an injection DPC work in solid wall houses?

Yes if rising damp is the genuine cause. Modern silicone cream products work in solid stone, brick, and lime mortar walls. Best practice: use specialist cream rated for the wall type. Some products work better in dense brick; others in porous stone. Manufacturer guidance critical.

Why do you not always recommend injection?

Because most "rising damp" diagnoses are actually other causes — penetrating, condensation, plumbing, bridged DPC. Injection doesn't fix those — and clients pay £3,000+ for a treatment that doesn't address the problem. Tradesperson's job is to diagnose accurately and treat the real cause.

Does tanking always need a sump pump?

Cementitious slurry tanking: no — water doesn't penetrate (if properly applied). Cavity drain membrane: yes — water gets through wall by design and is channeled to a sump and pumped out. Both work but cavity drain is more tolerant of water pressure (high water table situations).

Will damp treatment increase my house value?

Documented PCA-member treatment with 10-year IBG: protects sale value (no buyer concern). Untreated rising damp: typically -5 to -10% asking price. Untreated tanking failure in basement: -10 to -20%. Treatment is preventive, not value-adding — but un-treated damp is a major sale hindrance.

Regulations & Standards