You’ve spotted moisture on your walls. Now you need to know what type of damp you’re dealing with, because the wrong fix wastes money and doesn’t solve the problem.
This guide shows you exactly how to identify rising damp versus penetrating damp, what causes each type, and what actually works to fix them.
What’s the Quick Answer?
Rising damp climbs up from the ground through your walls, creating a tide mark usually no higher than 1.5 metres. It often leaves white salt deposits and appears along the base of internal and external walls.
Penetrating damp comes through walls, roofs, or windows from outside. It appears as patches or streaks, gets worse during rain, and can show up anywhere on your walls, not just at floor level.
The main difference: rising damp moves upward from ground contact. Penetrating damp moves inward from outside defects.
How Rising Damp Actually Works
Rising damp happens when groundwater travels up through porous building materials. Think of it like a sponge sitting in a puddle. The water keeps climbing until gravity finally stops it.
What Causes Rising Damp
Your property needs two things for rising damp to occur:
Ground moisture contact. Water in the soil touches your walls or floor. This could be from high water tables, poor drainage, or raised ground levels.
Failed or missing damp proof course (DPC). Older properties built before 1875 often have no DPC at all. Later homes have slate, bitumen, or plastic DPCs that can deteriorate, crack, or get bridged by raised soil levels or internal cement renders.
Where Rising Damp Appears
You’ll spot rising damp in specific locations:
- Along the bottom of ground floor walls
- Usually reaching between 300mm to 1.5 metres high
- On both internal and external wall faces
- At a fairly consistent height across affected walls
- In basements and cellars below ground level
The height limit exists because water can only defy gravity for so long in building materials. Eventually evaporation equals the rising rate.
Visual Signs of Rising Damp
Look for these telltale indicators:
Tide marks. A clear horizontal line where the dampness stops, often with staining above it.
Salt deposits. White, fluffy crystals (efflorescence) or brown hygroscopic salts that draw moisture from the air.
Decaying skirting boards. Wet rot in timber at floor level, causing wood to soften and crumble.
Peeling paint and wallpaper. Decorations fail from the bottom up as moisture pushes them off the wall.
Musty smell. A persistent damp odour particularly noticeable in closed rooms.
Damaged plaster. Crumbling render at the base of walls, sometimes bubbling outward.
The salt content matters because groundwater carries dissolved minerals. When the water evaporates at the surface, these salts concentrate and crystallize, often causing more damage than the moisture itself.
How Penetrating Damp Actually Works
Penetrating damp enters your property through defects in the building’s exterior. When it rains, water finds any weakness and pushes through to your internal walls.
What Causes Penetrating Damp
Several external problems create penetrating damp:
Damaged roofing. Missing tiles, cracked slates, worn flashing, or blocked gutters let water through. According to the Building Research Establishment, roofing defects account for a significant proportion of damp problems in UK homes.
Faulty guttering and downpipes. Leaking joints, cracks, or blockages cause water to overflow and saturate walls.
Cracked render or pointing. External wall finishes develop cracks that channel water inward.
Defective window and door frames. Gaps in seals, damaged sills, or poor installation create entry points.
Solid wall problems. Properties without cavity walls are especially vulnerable as there’s no gap to stop moisture travelling through.
Bridged cavity walls. Mortar droppings, wall ties, or debris in the cavity create a path for water to cross from outer to inner leaf.
High ground levels. Soil, paving, or render raised above the DPC allows water to bypass it and penetrate the wall.
Where Penetrating Damp Appears
Penetrating damp shows up in different patterns than rising damp:
- Patches or streaks on walls, often diagonal
- Around window and door frames
- On chimney breasts and walls below roof defects
- On upper floors and ceilings (never rising damp)
- On external walls facing prevailing wind and rain
- Getting worse during and after rainfall
The location directly relates to the external defect. Water follows the path of least resistance inward.
Visual Signs of Penetrating Damp
Watch for these indicators:
Damp patches after rain. Areas that appear wet during rainfall and slowly dry out afterwards.
Staining patterns. Discolouration that follows the water’s path, often running diagonally down walls.
Internal mould growth. Black spots developing on damp areas, especially in corners or around windows.
Wet ceilings. Water marks or sagging plasterboard from roof leaks.
Isolated damp spots. Moisture appearing in specific locations rather than across an entire wall base.
Window condensation doesn’t explain it. The dampness occurs regardless of internal humidity levels.
Unlike rising damp, penetrating damp rarely produces significant salt deposits because rainwater contains fewer dissolved minerals than groundwater.
Side-by-Side Comparison: Rising vs Penetrating Damp
| Feature | Rising Damp | Penetrating Damp |
|---|---|---|
| Source | Ground moisture moving upward | External water entering through defects |
| Height on walls | Maximum 1.5 metres from floor | Any height, often higher up |
| Pattern | Horizontal tide mark | Patches, streaks, or isolated spots |
| Salt deposits | Common (white or brown crystals) | Rare |
| Timing | Constant, worse in winter | Worse during and after rain |
| Location | Ground floor only | Any floor including upstairs |
| Affected walls | Usually multiple walls at base | Often specific areas near defects |
| Smell | Musty, earthy odour | Mouldy smell |
How to Test Which Type You Have
Don’t guess. Use these practical methods to confirm what you’re dealing with.
The Timing Test
For rising damp: Monitor the affected area over several weeks. Rising damp stays relatively constant, though it may worsen in winter when evaporation rates drop.
For penetrating damp: Check walls during and after heavy rainfall. Penetrating damp appears or worsens when it rains, then gradually improves in dry weather.
The Location Test
Check the height. If dampness extends above 1.5 metres, it’s not rising damp. If it appears only at the base of ground floor walls in a horizontal band, suspect rising damp.
Look outside. Walk around your property during rain. Watch where water flows. Check gutters, downpipes, roof condition, and wall surfaces for obvious defects.
Inspect upstairs. Any damp on upper floors automatically rules out rising damp.
The Salt Test
Scrape any white deposits from your wall and place them in a clear jar with a lid.
Rising damp salts: Usually appear fluffy and white. They’re hygroscopic, meaning they’ll draw moisture from the air and become damp even in the sealed jar.
Penetrating damp: Rarely produces significant salt deposits. Any white marks are more likely to be simple efflorescence from new plaster.
The Moisture Meter Test
Professional surveyors use electronic moisture meters, but these can give false readings if hygroscopic salts are present (common with rising damp).
Carbide test. This laboratory test measures actual water content by weight, not electrical resistance. It’s more reliable but requires a professional.
Thermal imaging. Infrared cameras show moisture patterns and can reveal the source of damp by displaying temperature differences.
When to Call a Surveyor
Get professional help if:
- You can’t identify the source after checking external defects
- Multiple areas show dampness
- You’re planning to buy a property with visible damp
- Previous treatments have failed
- The building is listed or historically significant
A RICS qualified surveyor can provide an independent assessment and recommend appropriate solutions.
Common Mistakes in Damp Identification
People often misdiagnose damp problems. These errors waste money and don’t fix the issue.
Mistake 1: Assuming All Low-Level Damp Is Rising Damp
Many damp proof course companies diagnose rising damp when the real problem is penetrating damp at ground level. Check for:
- Raised ground levels bridging the DPC
- Splashback from hard paving near walls
- Leaking gutters or downpipes at low level
- Cracked render or pointing
Fix these external issues first before assuming you need a new DPC.
Mistake 2: Ignoring Condensation
Condensation forms on cold surfaces when warm, humid air hits them. It often appears at floor level where walls are coldest.
Condensation signs:
- Appears on multiple walls, including internal walls
- Worse in winter
- Associated with misted windows
- Mould growth in corners and behind furniture
- Improves with ventilation
Condensation needs different solutions than rising or penetrating damp: better heating, insulation, and ventilation.
Mistake 3: Relying Only on Moisture Meters
Electronic meters measure electrical resistance, not actual water content. Hygroscopic salts (from old rising damp) can give high readings years after the moisture problem was fixed.
Always combine meter readings with visual inspection and external checks.
Mistake 4: Treating Symptoms Not Causes
Applying waterproof paint or new plaster over damp walls doesn’t work. Water finds another route or pushes the new coating off.
Fix the source:
- For rising damp: install or repair the DPC, improve drainage
- For penetrating damp: repair external defects first, then redecorate
How to Fix Rising Damp
Once you’ve confirmed rising damp, these solutions actually work.
Solution 1: Chemical Damp Proof Course Injection
Contractors drill holes into your wall and inject a water-repellent cream or liquid. This creates a new barrier that stops water rising.
What it involves:
- Drill holes every 100-120mm along the mortar course
- Inject silicone-based cream under pressure
- Allow several weeks for the cream to diffuse through the wall
- Hack off contaminated plaster to 1 metre above highest damp
- Apply salt-inhibiting render and new plaster
Costs: £500-£1,500 for an average semi-detached house (2026 prices).
Success factors: Only works if the diagnosis is correct and external ground levels don’t bridge the new DPC.
Solution 2: Physical Damp Proof Course Installation
For solid walls or where chemical injection isn’t suitable, installers can insert a physical barrier.
Methods include:
- Cutting a slot and inserting a plastic membrane
- Electro-osmosis systems (effectiveness debated)
Costs: £1,500-£3,000 typically, depending on wall length and access.
Solution 3: Addressing the Cause
Sometimes you can solve rising damp without expensive treatments:
Lower external ground levels. Dig out soil, paving, or render that bridges the existing DPC. Create a 150mm gap between ground level and the DPC.
Improve drainage. Install land drains, repair broken drains, or create soakaways to lower the water table around foundations.
Repair the existing DPC. If your slate or bitumen DPC has a small breach, targeted repairs may work.
Re-Plastering After Treatment
Never skip this step. Contaminated plaster contains hygroscopic salts that continue drawing moisture from the air even after the rising damp stops.
Correct process:
- Remove all plaster to 300mm above the highest damp reading
- Let walls dry (can take months)
- Apply a salt-inhibiting backing coat
- Add a sand-cement render
- Finish with multi-finish plaster
How to Fix Penetrating Damp
Fixing penetrating damp means finding and repairing external defects. No internal treatment works until you stop water getting in.
Roof Repairs
Check and fix:
- Missing or slipped tiles and slates
- Cracked or perished flashing around chimneys and valleys
- Damaged roof felt or sarking boards
- Gaps around roof windows or skylights
Costs: Individual tile replacement £75-£150. Complete roof flashing £300-£800.
Gutter and Downpipe Maintenance
Blocked or damaged guttering causes water to overflow and saturate walls.
Regular tasks:
- Clear leaves and debris twice yearly
- Check for cracks, rust, or separated joints
- Ensure gutters slope towards downpipes
- Verify downpipes connect to drains, not soaking into walls
Costs: Gutter repairs £100-£300. Full replacement £400-£1,200 for a semi-detached house.
External Wall Repairs
Repointing: Rake out crumbling mortar and repoint with a lime-based mix (not hard cement on older properties).
Render repairs: Patch or replace damaged render. Ensure breathable lime render on solid wall properties.
Cavity wall problems: Clear bridged cavities by removing debris through inspection holes.
Costs: Repointing £30-£50 per square metre. Render repairs £40-£80 per square metre.
Window and Door Sealing
Check for:
- Gaps between frames and walls
- Cracked mastic sealant
- Damaged window sills that don’t shed water
- Worn weatherstripping
Fix by: Applying new mastic sealant, replacing sills if necessary, or adjusting frames.
Costs: DIY sealant under £20. Professional window sealing £150-£400.
Allow Walls to Dry
After external repairs, give walls time to dry naturally. This can take several months depending on wall thickness and construction.
Use dehumidifiers carefully. They help but can’t substitute for fixing the source.
Special Cases and Complications
Some situations create confusion or require specific approaches.
Solid Wall Properties
Homes built before 1920 often have solid brick or stone walls with no cavity. They’re naturally more vulnerable to both rising and penetrating damp.
Solutions:
- Maintain external walls meticulously
- Use breathable lime renders and paints
- Never apply impermeable coatings that trap moisture
- Consider external wall insulation with breathable systems
Both Types Present Together
You might have rising damp AND penetrating damp in the same property, or even on the same wall.
How to tell:
- Damp at ground level with a horizontal tide mark (rising damp)
- Plus damp patches higher up near roof defects (penetrating damp)
Fix both: Address external defects first, then treat rising damp if it persists.
Old Rising Damp That’s Since Stopped
Previous rising damp can leave hygroscopic salts in plaster. These salts draw moisture from the air, creating damp patches even though no water is rising anymore.
Test: Check with a carbide moisture test to measure actual water content, not just electrical resistance.
Solution: Remove and replace contaminated plaster with salt-inhibiting render.
Listed Buildings
Historic properties need special care. Modern cement-based treatments can damage traditional materials.
Approach:
- Use breathable lime materials
- Avoid injected chemical DPCs if possible
- Improve drainage and ventilation instead
- Consult conservation specialists
Prevention: Stopping Damp Before It Starts
Prevention costs less than cure. These simple maintenance tasks reduce damp risk.
Annual Checks
Create a yearly routine:
Autumn: Clear gutters before winter rain. Check roof condition.
Winter: Monitor vulnerable areas during heavy rain to spot leaks early.
Spring: Inspect external walls for winter damage. Check pointing and render.
Summer: Ensure ground levels haven’t risen. Trim plants away from walls.
Keep Ground Levels Low
Soil, paving, and render should always sit at least 150mm below your damp proof course. Never pile earth against walls or lay patios too high.
Maintain Ventilation
Good airflow helps walls dry out naturally:
- Keep air bricks clear
- Don’t block vents with furniture
- Open windows regularly
- Use extractor fans in kitchens and bathrooms
Use Appropriate Materials
Modern cement renders and paints trap moisture in old solid walls. Use breathable alternatives:
- Lime render instead of cement
- Mineral or lime paint instead of vinyl
- Breathable masonry paint externally
Summary
Rising damp and penetrating damp need different solutions. Get the diagnosis right or you’ll waste money on treatments that don’t work.
Rising damp:
- Climbs from the ground
- Creates horizontal tide marks below 1.5 metres
- Leaves salt deposits
- Needs DPC installation or repair
Penetrating damp:
- Enters through external defects
- Appears as patches anywhere on walls
- Worsens during rain
- Needs external repairs to roof, gutters, walls, or windows
Check external defects first. Many cases diagnosed as rising damp are actually penetrating damp from raised ground levels, leaking gutters, or damaged render.
When in doubt, get an independent survey from a RICS qualified professional, not a damp proofing company that profits from selling treatments.
Fix the cause, not the symptoms. Remove contaminated plaster. Give walls time to dry. Only then will your repairs last.
Frequently Asked Questions
Can rising damp and penetrating damp occur together?
Yes, absolutely. A property can suffer from both types simultaneously. You might have rising damp at ground level due to a failed DPC, while also experiencing penetrating damp higher up from a leaking gutter or damaged roof. Each problem needs its own specific fix. Always address penetrating damp issues first by repairing external defects, then assess whether rising damp treatment is still necessary once walls have dried.
How long does it take for walls to dry after fixing damp problems?
Solid walls can take 6-12 months to dry completely after you’ve fixed the source of damp. Cavity walls dry faster, usually 3-6 months. Thickness matters too. A 9-inch solid brick wall holds much more water than a modern cavity wall. Don’t rush to replaster. Use a moisture meter to track progress. Walls should read below 20% moisture content before you apply new finishes. Forced drying with heaters can cause cracking and doesn’t address hygroscopic salts.
Will a dehumidifier fix rising or penetrating damp?
No. Dehumidifiers remove moisture from the air but cannot stop water entering through walls or rising from the ground. They might help walls dry faster after you’ve fixed the actual problem, but they’re treating symptoms not causes. You’ll run the dehumidifier forever without addressing the real issue. Save your electricity money and spend it on proper repairs instead. Only after fixing external defects or installing a DPC should you consider using dehumidifiers to speed up drying.
Is rising damp always caused by a missing or failed damp proof course?
Not always. Many cases diagnosed as “rising damp” are actually penetrating damp at low level caused by bridged DPCs. Common culprits include raised paving bridging the DPC, high soil levels against walls, or render applied below the DPC line. These allow water to bypass a perfectly functional damp proof course. Before paying for expensive DPC installation, check whether simply lowering external ground levels or removing bridging materials solves the problem.
Can I use waterproof paint to stop penetrating damp showing through?
Absolutely not. Waterproof paint traps moisture in the wall, making the problem worse. Water will either find another route out, push the paint off, or cause hidden damage inside the wall structure. Always fix the external defect letting water in first. Once repaired and dried, use breathable paints that allow walls to dry naturally. On solid wall properties especially, breathable lime or mineral paints are essential. Sealing in moisture creates long-term structural problems and encourages rot in any timber embedded in the walls.