What is bridging damp? How it happens and how to fix it

Bridging damp happens when moisture finds a path around the building’s damp-proof course (DPC) so it reaches your internal walls and skirting. If you see damp patches rising above the skirting or plaster that touches a concrete floor, the DPC may be bridged and you’ll need to remove the bridge and fix ground levels or linings to stop further moisture.

You’ll learn how bridging forms — from raised external ground or paths, to plaster or mortar left against the DPC, or missing floor membranes — and how to spot the difference between bridging and true rising damp. The article then shows practical fixes you can expect from a survey, simple repairs you might do yourself, and when to call a damp specialist.

Understanding Bridging Damp

Bridging damp happens when moisture finds a path over or around the damp-proof course so it reaches internal walls and finishes. You will see specific physical causes, typical signs on surfaces, and particular areas of the building that are more vulnerable.

Definition of Bridging Damp

Bridging damp occurs when the damp-proof course (DPC) is bypassed by a material or change in level, allowing ground or rainwater to move into the wall above the DPC. The DPC itself is usually intact; the problem lies in something creating a continuous wet route—such as mortar, render, paving or internal plaster—between the external ground or another wet surface and the wall above the DPC. You should treat bridging as a moisture-path issue rather than automatic DPC failure. Diagnosis focuses on locating the material or change in level that creates the bridge

Signs and Symptoms

You will commonly notice tide marks, blistering or flaking plaster on walls a few centimetres above floor level where moisture has risen past the DPC. Paint blistering, salt deposits (efflorescence) on mortar or plaster, musty smells and decayed skirting boards are typical. Moisture metre readings often show damp higher than expected for true rising damp and can be patchy rather than uniform. External clues include soil or paving touching the wall, or render and decking running down over the DPC line.

Common Causes

High external ground levels and added paving, decking or steps that sit over the DPC are frequent causes. Render, cement fillets, or extended mortar beds that bridge from the ground up over the DPC create a continuous wet path. Internal plaster or floor finishes that extend below the DPC can also wick moisture past the barrier. Blocked or poorly routed guttering and splashback from hard surfaces can worsen the problem by keeping the wall wetter for longer. DIY attachments like sheds, trellis or storage units fixed against the wall may unintentionally bridge the DPC.

Areas Most at Risk

External ground-to-wall junctions receive most attention: exposed rear gardens with raised paving, patios, or infill soil near the external DPC are high-risk spots. Conservatory abutments and extensions where finishes overlap original wall DPCs commonly bridge. Door thresholds, pathway edges and areas around downpipes and drains are vulnerable because splashback and pooling raise moisture levels. Inside, low skirting areas, plinths and walls where plaster runs down to floor level often show the first signs, especially in older solid-walled properties.

How to Fix Bridging Damp

You will assess where moisture is bypassing the damp-proof course, decide the right repair method for that exact cause, and then apply changes to stop water reaching internal plaster or floor levels.

Initial Assessment and Diagnosis

Start with a visual inspection of external ground levels, paths, and render relative to the damp-proof course (DPC). Check internal plaster: does it extend below the DPC or touch the floor?
Use a hygrometer and moisture meter to map moisture readings at several heights and positions rather than relying on one spot. Record readings and photograph evidence.

Look for secondary causes: blocked or leaking gutters, downpipes, breached cavity trays, or failed external render. Remove skirting and inspect the wall base and floor junction if safe to do so.
If readings or signs are unclear, commission a bridging damp survey from a qualified damp specialist who will produce a moisture pathway diagram and recommend targeted remedial actions.

Effective Remediation Methods

Remove any material that physically bridges the DPC: trim plaster back to the correct height, lower raised external paths, and remove render that contacts the ground. Work carefully to avoid damaging the DPC itself.
Repair or replace defective gutters, downpipes, and external drainage to eliminate rainwater splash and groundwater pooling against the wall.

Where internal plaster has been removed, replaster with a salt-resistant render or a breathable lime-based plaster to allow walls to dry. Use a damp-proof membrane on floors only when appropriate and compatible with the wall DPC.
For complex cases, use chemical or physical DPC upgrades only after confirming bridging is eliminated; inappropriate injection when bridging remains will fail. Keep records and photos of all remedial steps.

Preventative Measures

Lower external ground levels or paths so they sit below the DPC by at least 150 mm where possible. Install or repair drip detail on pathways and add a small gravel strip against the wall to prevent splash-back.
Maintain gutters and downpipes with regular cleaning and ensure soil or turf slopes away from the building at a minimum 1:40 fall for the first metre.

Use breathable external finishes and avoid cement render that can trap moisture against the DPC. Fit proper cavity trays and ensure any penetrations (soil stacks, vents) are sealed and flashed correctly to prevent new bridging paths.

Frequently Asked Questions

You will find clear causes, signs, fixes, prevention steps and guidance on DIY versus professional work. Each answer names specific checks, repairs and the situations where each approach applies.

What causes damp to bridge across a damp-proof course?

Damp bridges when something bypasses the damp-proof course (DPC) so moisture can reach the inner wall. Common causes include raised external ground or paving, internal plaster or screed taken below the DPC level, and penetrating defects such as faulty gutters or downpipes.

Building materials and construction detail also matter. Render or external finishes that sit against the ground, and clogged or leaking drainage that soaks the wall, commonly create a path for moisture.

How can you identify bridging damp in your property?

Look for damp patches and salt deposits on walls at or above skirting level rather than only at floor level. Compare internal plaster height to external ground or paving; if plaster runs below the known DPC line, bridging is likely.

Check outside for paths, patios or soil levels that sit higher than the DPC and inspect gutters, downpipes and flashing for leaks. A structured survey with moisture readings and damp maps confirms the diagnosis.

What are the most effective solutions for treating bridging damp?

Lower or relandscape external ground and paving so they sit below the DPC where possible. Remove internal plaster or screed that bridges the DPC and replace it with a membrane or apply a suitable breathable repair plaster.

Repair leaking gutters, downpipes and defective flashing to stop water ingress. In many cases you will not need chemical DPC injection if the physical bridging is removed and drainage corrected.

Are there preventative measures to protect against the risk of bridging damp?

Set external paving, paths and turf at least 150 mm below the DPC where practical. Fit and maintain gutters, downpipes and ground drains to keep roof and surface runoff away from walls.

Use breathable wall finishes and avoid applying solid renders or internal plasters down to the floor level. Regularly inspect and clear external gullies and drainage routes.

What is the difference between bridging damp and rising damp?

Bridging damp occurs when an external or internal detail bypasses an existing DPC, allowing moisture to reach the wall fabric. Rising damp traditionally describes moisture drawn up through wall materials by capillary action where no effective DPC exists or it has failed.

Diagnosis differs: bridging is fixed by removing the bypass and correcting levels or defects; rising damp may require a combination of DPC remediation, drying and salt removal depending on construction and moisture sources.

Can DIY treatments for bridging damp be effective or should professional help be sought?

You can successfully address simple cases yourself by lowering external paving, repairing gutters and removing internal plaster to the correct level. These tasks require basic landscaping, plastering and an awareness of the DPC position.

Seek professional help when you cannot confirm the DPC location, when works affect structural elements, or when damp persists after initial repairs. A damp surveyor or RICS/PCA-accredited contractor can provide moisture mapping, correct specification and warranty-backed repairs. Written by: Damps https://damps.co.uk