Damp Proofing Floors: Causes, Signs & Treatment

Damp does not only affect walls. Moisture can also enter or become trapped within floors, damaging floor coverings, skirting boards, plaster and structural timber.

The correct method of damp proofing floors depends on the construction of the property and, crucially, where the moisture is coming from.

A solid concrete floor affected by ground moisture requires a very different approach from a suspended timber floor suffering because of poor ventilation or leaking pipework.

Before installing a new damp-proof membrane or replacing flooring, it is important to identify the underlying cause.

What Causes Damp in Floors?

Floor damp can develop for several reasons.

Possible causes include:

• A missing or defective damp-proof membrane
• Ground moisture passing through a concrete slab
• High external ground levels
• Water entering at the wall-to-floor junction
• Plumbing leaks
• Defective drainage
• Flooding or previous water damage
• Condensation beneath impermeable floor coverings
• Poor ventilation beneath suspended timber floors
• Damp walls affecting adjoining floor structures

More than one source of moisture may sometimes be involved.

This is why accurate diagnosis should come before remedial treatment. Our guide to why damp diagnosis matters explains why treating the visible symptom alone can lead to recurring problems.

Signs of Damp in a Floor

The symptoms depend on the type of floor and the severity of the moisture problem.

Common warning signs include:

• Damp or discoloured patches
• Musty smells
• Lifting or bubbling vinyl
• Loose floor tiles
• Damaged laminate or timber flooring
• Mould around skirting boards
• Crumbling plaster at low level
• White salt deposits
• Floor coverings feeling damp
• Timber becoming soft or decayed
• Floorboards becoming springy
• Persistent moisture around the edges of a room

Sometimes the first indication is simply that a new floor covering repeatedly fails.

Adhesives may lose their bond, wooden flooring may swell or laminate may begin to lift.

Rather than replacing the flooring again, the moisture source should be investigated.

Damp in Concrete Floors

Modern solid floors will normally incorporate a damp-proof membrane designed to prevent ground moisture reaching the internal surface.

Problems can occur if the membrane is:

• Missing
• Damaged
• Poorly installed
• Bridged at the edges
• No longer continuous with the damp proof course in the walls

Older properties may have solid floors constructed before modern damp-proof membranes became standard.

Moisture can then move through the slab and affect finishes above it.

What Is a Damp-Proof Membrane?

A damp-proof membrane, often referred to as a DPM, is a barrier designed to prevent moisture passing from the ground into the floor construction.

It may consist of a sheet membrane incorporated into the floor or, in certain remedial situations, a specialist surface-applied system.

The correct specification depends on factors including:

• Floor construction
• Moisture levels
• Condition of the substrate
• Intended floor finish
• Relationship with the wall damp proof course
• Whether water pressure is involved

Installing a membrane without understanding the source of the moisture can sometimes simply redirect the problem elsewhere.

Where a damp-proof course within the walls is also part of the issue, it is important to understand how the floor membrane and DPC should work together. Find out more about physical damp proof courses and how they protect a building against ground moisture.

Damp Around the Wall-to-Floor Junction

One area that deserves particular attention is where the floor meets the wall.

Moisture around the perimeter of a room may be caused by:

• A break between the floor membrane and wall DPC
• High ground levels externally
• Penetrating moisture
• A leaking pipe
• Defective plaster extending below the damp proof course
• Moisture moving through the floor
• A waterproofing defect in a below-ground room

Because floor and wall damp can interact, looking only at the floor may not reveal the complete problem.

This is another reason why a full inspection is preferable to simply applying a surface treatment.

Can Damp Come Through a Concrete Floor?

Yes.

Concrete itself is not necessarily a complete barrier to moisture.

Where there is no effective damp-proof membrane, ground moisture may migrate through the floor construction.

This may become particularly noticeable after an impermeable finish such as vinyl, rubber or certain floor coatings is installed.

The covering can reduce evaporation from the surface, resulting in moisture becoming trapped beneath it.

Possible consequences include:

• Flooring lifting
• Adhesive failure
• Mould
• Persistent odours
• Damage to timber finishes
• Deterioration around the perimeter of the room

Before replacing damaged flooring, moisture levels and floor construction should therefore be investigated.

Suspended Timber Floors Need a Different Approach

Not all floor damp is caused by moisture rising through the ground.

Traditional properties often have suspended timber floors consisting of floorboards and joists above a ventilated void.

In these properties, ventilation beneath the floor is extremely important.

Problems can develop where:

• Air bricks become blocked
• External ground levels cover ventilation openings
• Later extensions interrupt airflow
• Debris accumulates beneath the floor
• Plumbing leaks wet the joists
• Damp masonry remains in contact with timber
• Insulation is installed incorrectly
• Air movement is restricted

Persistent dampness can eventually lead to fungal timber decay.

Why Underfloor Ventilation Matters

Airflow helps moisture evaporate and keeps structural timber dry.

Where ventilation is inadequate, humidity within the floor void can increase and timber may remain damp for prolonged periods.

Improving ventilation may therefore form part of the solution in some suspended-floor problems.

However, installing additional ventilation will not fix a leaking pipe or significant water ingress, so the underlying cause still needs to be established first.

Find out more about Shield’s approach to ventilation and drainage and the role these can play in controlling moisture within a property.

Can Damp Floors Cause Wet Rot or Dry Rot?

Persistent moisture can create conditions in which timber-decay fungi become established.

Suspended floor joists, wall plates and floorboards can be vulnerable where they remain damp.

Warning signs might include:

• Timber becoming soft
• Cuboidal cracking
• Fungal growth
• Musty or mushroom-like smells
• Springy floorboards
• Localised floor movement
• Darkened timber
• Visible decay beneath floorboards

Wet rot tends to remain associated with the source of moisture, whereas dry rot can spread further through a building under suitable conditions.

If timber beneath a floor is showing signs of decay, Shield provides specialist wet rot treatment and wider timber preservation services.

Where structural timber has been weakened, further investigation may be required before repairs are carried out.

Can Damp Floors Affect Walls?

Yes.

Moisture within a floor can sometimes contribute to damp around the base of adjoining walls.

Likewise, damp within a wall can affect nearby skirting boards, floor edges and timber joists.

This can make diagnosis difficult because the most visible symptom may not be the original source of moisture.

For example, damp staining at skirting level might be blamed on rising damp when the actual problem is:

• A leaking floor-level pipe
• Moisture beneath the slab
• A defective wall-to-floor junction
• Water penetration from outside
• Damp timber beneath the floor

Looking at the relationship between walls, floors and external conditions is therefore important.

What Happens During a Damp Floor Survey?

A professional investigation may consider:

• The type of floor construction
• Moisture readings
• Existing floor coverings
• Condition of skirting and plaster
• External ground levels
• Damp proof course position
• Plumbing
• Drainage
• Wall-to-floor junctions
• Ventilation beneath timber floors
• Condition of joists and floorboards
• Evidence of fungal decay
• Previous damp-proofing work

Where necessary, some floor coverings or boards may need to be lifted to inspect concealed areas.

The surveyor should use the available evidence to determine the likely moisture source before recommending remedial work.

You can find out more about the general inspection process in our guide to what happens during a damp survey.

How Are Concrete Floors Damp Proofed?

The appropriate treatment will depend on the existing construction.

Possible remedial approaches may include:

Installing or Repairing a Damp-Proof Membrane

Where a floor is being replaced or substantially rebuilt, a suitable membrane may be incorporated into the construction.

The membrane needs to form a continuous barrier and should integrate correctly with damp-proofing measures in the surrounding walls.

Surface-Applied Damp-Proof Systems

Certain existing solid floors may be suitable for specialist surface-applied membranes or coatings.

The floor normally needs careful preparation, and the chosen product must be suitable for the moisture conditions and intended finish.

Repairing Defects

If damp is caused by a plumbing leak, drainage problem or localised defect, correcting that problem may be all that is required.

A new membrane should not be treated as a substitute for repairing an active leak.

What About Below-Ground Floors?

Basements and cellars require additional consideration because their walls and floors may be exposed to groundwater pressure.

A simple domestic damp-proof membrane may not provide an adequate solution where water pressure is acting against a below-ground structure.

Depending on the circumstances, a properly designed basement waterproofing system may be required.

These systems can include:

• Barrier waterproofing
• Cavity drain membranes
• Perimeter drainage channels
• Sump and pump systems
• Specialist detailing around wall-floor junctions

If the affected floor is in a basement or cellar, the problem should be assessed as part of the complete below-ground waterproofing design rather than treated in isolation.

Should You Put New Flooring Over a Damp Floor?

Generally, the moisture problem should be investigated before new flooring is installed.

Covering a damp surface can trap moisture and damage the new finish.

Potential problems include:

• Swollen laminate
• Warped timber
• Loose tiles
• Failed adhesive
• Mould beneath vinyl
• Musty odours
• Staining
• Damage to skirting boards

The floor may also require an appropriate drying period after the source of moisture has been corrected.

Installing an expensive floor covering too soon can result in unnecessary replacement costs.

Do Moisture Readings Matter?

Moisture testing can provide useful information, but results need to be interpreted correctly.

Different materials behave differently, and a single reading should not be used as the sole basis for recommending damp-proofing work.

The surveyor should consider moisture readings together with:

• Floor construction
• Property age
• Visible symptoms
• External conditions
• Plumbing
• Ventilation
• Wall construction
• Previous repairs

The objective is diagnosis, not simply finding a high reading.

Why DIY Damp-Proofing Products May Not Solve the Problem

There are numerous paints, sealers and membrane products marketed for damp floors.

Some may be suitable when used as part of the correct specification, but applying a product without diagnosing the source of moisture can cause further problems.

For example, a coating will not repair:

• A leaking water pipe
• Blocked underfloor ventilation
• A drainage defect
• Rotten timber
• High external ground levels
• Groundwater pressure in a basement

A successful repair should deal with the cause first and then provide the necessary protection for the floor construction.

When Can Floor Damp Become a Structural Problem?

Moisture itself does not necessarily mean that a building is structurally unsafe.

However, persistent damp can contribute to deterioration of structural materials.

This is particularly important where:

• Timber joists have decayed
• Floorboards have lost strength
• Supporting timbers are damaged
• Reinforced concrete has deteriorated
• Water ingress is associated with significant cracking

If a floor has begun to sag, move or feel noticeably unstable, professional investigation should not be delayed.

Shield also provides specialist structural repair services where moisture or decay has contributed to more significant building damage.

Preventing Damp Floor Problems

Good property maintenance can reduce the risk of many floor-moisture problems.

Useful measures include:

• Repairing plumbing leaks promptly
• Keeping external ground levels below the damp proof course
• Keeping air bricks clear
• Maintaining gutters and drainage
• Investigating unexplained damp smells
• Checking suspended floors when renovating
• Allowing new floors and screeds adequate drying time
• Avoiding impermeable coverings over unexplained damp
• Dealing with water ingress before replacing flooring

Older properties can benefit particularly from checking that later alterations have not blocked ventilation or bridged existing damp-proofing measures.

Professional Damp Floor Surveys

Shield Waterproofing & Preservation provides specialist damp surveys and remedial solutions for properties affected by moisture in floors, walls and other parts of the building.

Our experienced surveyors can investigate the likely source of the problem and recommend appropriate next steps.

Depending on the findings, these may include:

• Damp-proof membranes
• Damp-proofing treatments
• Floor repairs
• Improvements to underfloor ventilation
• Timber investigations
• Wet or dry rot treatment
• Plumbing or drainage investigation
• Basement waterproofing
• Further structural assessment where required

The correct solution depends on the construction and the source of moisture rather than applying the same damp-proofing treatment to every floor.

Concerned About Damp in Your Floor?

If flooring is lifting, timber is deteriorating or damp keeps appearing around the edges of a room, it is worth finding out what is causing the problem before replacing the finishes.

A professional damp survey can help identify whether moisture is coming through a solid floor, affecting a suspended timber floor or entering from another part of the building.

Contact Shield Waterproofing & Preservation to arrange an inspection and receive clear recommendations for the most appropriate next steps.