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Treat crawlspace damp.
Sub-floor void with damp, mould or rot — air bricks, vapour barrier or tanking to stop it spreading.
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Costs, timelines, pitfalls and the right questions to ask before any damp proofer starts on crawlspace & Sub-floor Damp.
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Air bricks blocked or paved over
Most common cause. Drives, paths, gardens cover air bricks. Clear and add more.
Rotten joists
Wet joists for years rot. Sister-rafter splice repairs without removing entire floor.
Bare earth ground
Soil emits moisture. Visqueen sheet over the earth dramatically cuts humidity.
Old chemical treatment ineffective
Surface spray won't fix structural rot. Identify and replace affected timber.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and what affects price.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Air brick replacement (each)Clay or plastic
£40–£120
New air brick installHole cut in wall
£80–£250
Sub-floor membrane per m²Visqueen, DPM
£5–£20/m²
Sub-floor extract fanMechanical extract
£250–£600
Joist repair (per joist)Sister-rafter splice
£80–£300
Floor board replacement per m²If rotten
£40–£100/m²
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
Infographic · VideoThe Damp Proofer briefing.
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Damp Proofer explained
Damp under suspended floors causes timber rot, musty odours, and condensation rising into the room above. Expect to pay £400–£3,500 for UK crawlspace/sub-floor damp treatment depending on cause and scope.
What Does the Work Invovle?
Inspection: lift floor boards or access via floor hatch. Assess ventilation (air bricks), floor joists (rot, woodworm), ground (waterproof membrane), drainage. Remediation: clear air bricks (often blocked or paved over), add more air bricks if needed, install plastic sheet (Visqueen) over soil, install sub-floor extract fan, repair rotten timbers, treat wood with preservative.
Typical Costs
| Item | Low (£) | High (£) | Notes |
|---|---|---|---|
| Air brick replacement (each) | 40 | 120 | Clay or plastic |
| New air brick install | 80 | 250 | Hole cut in wall |
| Sub-floor membrane per m² | 5 | 20 | Visqueen, DPM |
| Sub-floor extract fan | 250 | 600 | Mechanical extract |
| Joist repair (per joist) | 80 | 300 | Sister-rafter splice |
| Floor board replacement per m² | 40 | 100 | If rotten |
| Full crawlspace remediation | 1,200 | 5,500 | Whole house |
| Survey + report | 120 | 400 | PCA timber & damp |
How Long Does It Take?
Air brick replacement and clearing: 1 day. Membrane install: 2–3 days. Full remediation with timber repairs: 1–2 weeks.
DIY or Professional?
Air brick clearing: DIY. Membrane install requires confined-space work — professional. Timber repairs need experienced builder. Extract fan needs Part P electrician.
Choosing the Right Tradesperson
PCA timber & damp specialist for diagnosis. Builder or carpenter for timber repairs. Mechanical engineer for sub-floor extract.
UK Regulations
Building Regs Part F (ventilation), Part C (moisture). Listed buildings need consent for air brick or sub-floor alterations.
Common Problems
- Air bricks blocked or paved over: Most common cause. Drives, paths, gardens cover air bricks. Clear and add more.
- Rotten joists: Wet joists for years rot. Sister-rafter splice repairs without removing entire floor.
- Bare earth ground: Soil emits moisture. Visqueen sheet over the earth dramatically cuts humidity.
- Old chemical treatment ineffective: Surface spray won't fix structural rot. Identify and replace affected timber.
Frequently Asked Questions
How do I know I have crawlspace damp?
Musty smell from floor, soft floorboards, visible mould on skirting or floor edge, damp patches on ground floor walls. Lift a floorboard to inspect.
How many air bricks do I need?
Building Regs guide: one air brick per 1.5m of external wall, evenly distributed. Most pre-1970 houses are under-vented by current standards.
Should I add a sub-floor extract fan?
For chronically damp crawlspaces with limited natural ventilation: yes. Active extract removes humid air. £250–£600 fitted including electrical.
Bare earth or membrane in crawlspace?
Bare earth emits significant moisture. Plastic membrane (Visqueen 1200-gauge) covers earth and cuts humidity 60–80%.
How long does crawlspace remediation last?
Air bricks: 50+ years. Membrane: 30+ years. Repaired timbers: indefinite if dry. Active extract fans: 8–15 years.
Will it solve damp in the ground floor?
Often yes — particularly if the floor was always damp without obvious cause. Sub-floor moisture is a hidden source of room-level damp.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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