Damp Floor Diagnosis: DPC Failure, Ground Moisture & Subfloor Problems

Summary

Damp floors are less common than damp walls but harder to diagnose without investigation. The floor structure (solid concrete with screed vs suspended timber) determines the possible causes and the investigation method. Concrete floors typically fail due to DPM degradation or absence; suspended timber floors fail due to sub-floor ventilation failure, DPC problems at floor level, or direct contact between timber and ground.

For builders and damp surveyors, the distinction between rising moisture and surface condensation is critical — a solid concrete floor that looks wet in winter mornings may be condensing from warm humid air hitting the cold floor, rather than moisture rising from below. The polythene test and calcium carbide test are the most reliable diagnostic tools.

Key Facts

• DPM (damp proof membrane) — a polyethylene sheet (typically 1200 gauge, 300 micron) laid under a concrete floor slab to prevent ground moisture rising; should be continuous, lapped at joints (300mm minimum), and lapped up the walls to the DPC in the wall
• DPC (damp proof course) — horizontal layer in the wall preventing rising damp; must be at or above floor level; if the floor DPM does not connect to the wall DPC, moisture can track under the floor screed from the wall base
• DPM continuity — the DPM and DPC must be linked; if a new floor is laid without ensuring this link, moisture from the wall base gets under the floor screed; this is a common failure in extension floors where the DPM is installed but not integrated with the existing wall DPC
• Ground water — floor damp from ground water pressure is more serious; typically occurs in basements or ground floors in areas with high water tables; requires a Type A (tanking) or Type B (cavity drain) waterproofing system (BS 8102)
• Polythene test — the simplest field test for floor damp source; tape a 300mm × 300mm piece of polythene to the floor and seal all edges; leave 24–48 hours; if moisture appears under the polythene, it is rising from below; if moisture appears on top of the polythene, it is condensation
• Calcium carbide test — most accurate; drill a sample from the floor screed and test moisture content; combined with the polythene test, confirms the source
• Screed cracking — cracked or hollow screed allows moisture to accumulate under the floating layer; use a chain or walking rod test (drag across the floor listening for hollow sound) to identify debonded screed
• Sub-floor void (suspended timber floor) — the void between the ground and the timber floor must be ventilated; inadequate ventilation leads to high humidity in the void, condensation on the underside of flooring, and eventual rot; Building Regulations minimum: 1500mm² of ventilation per metre run of wall
• Airbrick blockage — airbricks are commonly blocked by: external ground raised above the airbrick; garden beds or soil against the wall; internal insulation or floor covers placed over the airbrick from inside; any blockage creates a damp sub-floor void
• Timber moisture content — sound timber: below 20%; timber at risk of rot: above 20% (wet rot can begin); structurally safe but at risk: 18–22%; sound and dry: below 18%; measure with a resistance moisture meter at floor joist level
• Floor insulation — adding insulation on top of a damp concrete floor or in a damp sub-floor void traps moisture; always resolve damp before adding insulation; polystyrene boards on a damp floor accelerate DPM failure and corrosion of any metal elements

Quick Reference Table

Polythene Sheet Test Interpretation

Detailed Guidance

Solid Concrete Floor Investigation

Step 1: Polythene test Tape 3–4 pieces of polythene (300mm × 300mm) in different locations across the floor (near walls, middle of room, suspected worst areas). Seal all four edges with tape. Leave 24–48 hours. Inspect for moisture location.

Step 2: Damp meter survey Take resistance damp meter readings at 500mm intervals across the floor surface. Map readings. If DPM is failing, readings will be more elevated where the DPM is deficient or missing; less elevated where intact.

Step 3: Core sample (calcium carbide) Drill a sample of screed (25–30mm diameter, 50mm deep) and test in a calcium carbide apparatus. This gives the actual moisture content percentage. Compare to baseline for dry concrete (typically <0.5% for a dry screed).

Step 4: DPM junction at walls Inspect the base of the walls around the perimeter. Lift skirting boards (carefully) and look for the DPM. Check whether it laps up the wall to the DPC level. If the DPM terminates at the edge of the screed without connecting to the wall DPC, moisture bridges from the wall base under the screed.

Step 5: Check external ground levels Look at the outside of the building. Is the external ground above the DPC (the horizontal line of engineering brick or bituminous sheet visible in the brickwork)? If so, the DPC may be bridged and moisture from the ground outside is tracking under the floor internally.

Remediation of Failed DPM — Solid Floor

Option 1: Waterproof screed overlay If the floor is sound and level, apply a surface DPM (liquid-applied epoxy or polyurethane membrane, e.g., Sika, Weber, Mapei products). Surface DPMs are applied as a liquid paint in 2–3 coats; each coat is allowed to cure before the next; then a fresh screed or tile adhesive is applied on top.

Requirements: the existing floor must be clean, sound, and free of residual standing water before application; surface DPMs bridge fine cracks and pinholes but are not appropriate for moving cracks or structural voids.

Option 2: Physical DPM relay For more serious cases, break up the screed (and possibly slab) and lay a new physical polyethylene DPM (1200 gauge) before relevelling with new screed. This is the most thorough solution but very disruptive.

Option 3: Cavity drain membrane Dimple-faced polyethylene membrane (Newton, Delta, Triton) fixed to the floor and walls; water entering through the floor travels behind the membrane to a channel drain and is pumped out (or flows to a drain). Used in basements and high water table situations. Does not prevent water entry — it manages it.

Suspended Timber Floor Investigation

Step 1: Open an access hatch (or lift a board) Inspect the sub-floor void visually. Look for: airbricks visible on all relevant walls; air flow (hold tissue near airbrick — it should flutter); accumulated moisture or standing water; rot visible on floor joists.

Step 2: Check airbrick positions Identify all airbricks from outside and check they are clear. External causes of blockage: soil or gravel against the wall; planting beds; rendering over the airbrick by mistake. Internal causes: insulation products filling the void; debris.

Step 3: Probe joist undersides and ends Use a bradawl to probe the underside of joists at mid-span and at the bearing points in walls. If the probe penetrates easily, decay is present. Check moisture content with a resistance meter directly in the timber.

Step 4: Assess moisture levels Timber moisture content >20% indicates a problem. Between 18–22%, the timber is at risk. Below 18%, the timber is within acceptable range (safe). Record readings across the sub-floor for comparison after remediation.

Remediation of suspended timber floor moisture:

1. Clear all airbrick blockages; increase ventilation if below 1500mm² per metre run
2. If DPC is absent or bridged (external ground above DPC): lower external ground; rebuild DPC if absent
3. Treat affected timber with preservative (boron-based, e.g., Boracol; permethrin-based; applied by brush or injection to existing timber)
4. Replace any timber with moisture content above 25% or showing structural decay
5. Monitor — revisit after one heating season to confirm moisture levels have dropped

Frequently Asked Questions

The floor tiles are lifting and the adhesive is failing. Is this definitely damp?

It can be. Moisture rising from below attacks cement-based adhesives, causing de-bonding. However, lifting tiles can also result from: insufficient adhesive coverage; differential movement between the tile and substrate; sulphate attack on the screed (the screed itself expands and cracks); or a failed screed that has become hollow. Test the floor with a chain to find hollow areas, then perform the polythene test to determine whether moisture is a factor.

My customer has a concrete ground floor and the skirting boards are rotting at the base. Is that floor damp?

This pattern — rot at the skirting base with concrete floor — is classic DPC/DPM junction failure. Moisture from the wall base, not bridged by a continuous DPM-to-DPC junction, is saturating the timber skirting at the point where it meets the floor. Investigate the DPM junction at the wall as described above. The fix is often a surface DPM applied at the wall-floor junction, taken up the wall face behind new skirting, and sealed to the existing DPC.

We checked the sub-floor and found standing water. What now?

Remove the standing water (a pump if necessary). Identify the source — is it an overflowing drain cleanout, a burst pipe, a drain failure, or groundwater? Address the source before any timber treatment. Dry out the sub-floor void (increased ventilation, a temporary dehumidifier in the void if accessible). Test timber moisture content once it has had a drying period and assess whether any structural members need replacement.

Regulations & Standards

• Approved Document C (2004) — Site preparation and resistance to contaminants and moisture; DPM requirements for ground floors

• BS 8102:2009 — Protection of below ground structures against water

• BS 5250:2011 — Management of moisture in buildings; condensation

• BS 8203:2017 — Code of practice for installation of resilient floor coverings; substrate requirements including moisture content

• BRE: Good Repair Guide 33 — Damp in Floors — Technical guidance on floor damp diagnosis

• Property Care Association — Damp survey and treatment guidance

• NHBC: Chapter 5.1 — Substructure — DPM and DPC requirements

• rising damp — Wall rising damp and DPC treatment

• wet wall — Wall damp diagnosis and differentiating causes

• bouncy floor — Structural diagnosis for suspended timber floors

• underpinning — Foundation work affecting floor moisture paths