If you have been told that your property needs a new damp proof course, the first question to ask is whether the existing one has actually failed. In many cases it has not. Damp at the base of a wall is frequently caused by something bridging the damp proof course, by rainwater or leaks, or by condensation, and none of these is likely to be resolved by injecting a new one. This guide explains what a damp proof course (DPC) does, how it can be bypassed, why an independent diagnosis should come before injection treatment, and what to fix first.
From a surveying perspective, the cause should always be understood before a cure is recommended.
What is a damp proof course?
A damp proof course is a horizontal barrier built into a wall, above external ground level, to prevent moisture in the ground from rising through the masonry by capillary action. Government guidance for new work, including extensions, expects the DPC in an external wall to be at least 150mm above ground level and to consist of materials such as impervious sheet material, engineering brick or slates in a suitable mortar.
Older buildings may have a DPC of slate or engineering brick, while many older solid-walled buildings were built without a DPC at all. The absence of a DPC does not, in itself, mean that a building has a damp problem.
How a damp proof course can fail or be bridged
A DPC which has been bridged has not necessarily failed. BRE guidance on rising damp states that where a building already has a physical DPC, it is unlikely to have failed, as most DPC materials have a long life, and that defects such as mortar droppings in a cavity wall can make a DPC ineffective by bridging it. Common routes around a DPC include the following.
Raised external ground levels
Paths, patios, driveways and flower beds are often built up against external walls over time. Where the ground or paving rises close to, or above, the DPC, moisture can pass into the wall above it. Lowering the ground level is often the first and most effective remedy.
External render
Render carried down over the DPC and into the ground can bridge it. Render can also conceal the DPC and any cracking, which makes diagnosis more difficult.
Internal plaster and floor finishes
Inside the property, plaster carried down to meet a solid floor, or a floor raised above the level of the DPC, can create a route for moisture around the barrier.
Debris in the cavity
In cavity walls, mortar droppings and building debris can collect at the base of the cavity. Where debris builds up above the DPC, moisture can cross from the outer leaf to the inner leaf. The remedy is to clear the cavity, not to install a new DPC.
Damaged or missing DPCs
A DPC may occasionally have been damaged by alterations or building movement, or be missing in parts of the building. These situations do occur, which is why any recommendation should be based on evidence, not assumption.
Why genuine rising damp is less common than often diagnosed
Rising damp does exist, particularly in older buildings without an effective DPC. However, it is diagnosed more often than it is present. Damp at low level can equally be caused by:
- Penetrating damp from defective gutters, downpipes, pointing or render
- Rainwater splashing back from hard surfaces at the base of the wall
- Plumbing and drainage leaks
- Condensation on cold wall surfaces, particularly behind furniture and in corners
- Hygroscopic salts left in plaster by earlier damp
Traditional buildings also behave differently from modern ones. A joint position statement published in 2022, co-authored by RICS and Historic England, explains that traditional construction absorbs moisture but allows it to evaporate when conditions become drier, whereas modern construction relies on impermeable barriers. It also recognises that moisture in such buildings may not be due to a construction failure. Our guide to the types of damp explains the differences.
The limits of moisture meters where salts are present
Most hand-held moisture meters measure electrical resistance or conductance, not the actual water content of masonry. They are useful for mapping patterns, but salts, carbon in some plasters, foil-backed wallpaper and metal fixings can all produce high readings on a dry wall.
BRE guidance makes the same point: some accumulation of salts can occur in the walls of old buildings even when they do not have a damp problem, so high readings from a moisture meter alone are not conclusive. Drilled samples taken at different heights and depths can measure the moisture actually present, as explained in our guide to the calcium carbide test. A recommendation for a new damp proof course that relies on meter readings alone should therefore be treated with caution.
Why an independent diagnosis should come before injection treatment
Chemical injection is a recognised method of forming a remedial DPC and, in the right circumstances, it may be appropriate. However, it is invasive, it is commonly accompanied by replastering, and it will not resolve damp that has another cause, such as a bridged DPC, a leaking gutter or condensation.
Many damp-proofing contractors provide careful and competent advice, and the joint position statement expects contractors who provide quotations to report objectively and suggest only proportionate works. Nevertheless, where the person diagnosing a problem is also quoting for the remedy, it is sensible to have the diagnosis confirmed by someone with no interest in the outcome. An independent damp survey from an RICS Chartered Surveyor separates the diagnosis from the sale of any treatment.
Before agreeing to injection treatment, it is reasonable to ask:
- What evidence shows that the moisture is rising from the ground and not coming from another source?
- Has the existing DPC been located and, if so, why is it considered to have failed?
- Have ground levels, render, internal finishes and drainage been checked for bridging?
- Is the diagnosis based on meter readings alone, or on measured moisture content and salt analysis?
- Is the proposed plaster compatible with the wall, and what does any guarantee cover?
What to fix first
The joint position statement recognises that dealing with damp is often a staged process: the simpler causes are addressed first, a period is allowed for monitoring and natural drying, and the position is reassessed before other treatments are considered. Measures commonly considered first include:
- Ground levels – lowering external ground or paving so that it sits well below the DPC, and removing soil or planters heaped against walls
- Drainage – repairing gutters, downpipes, gullies and leaking drains so that water is directed away from the base of the wall
- Ventilation – clearing blocked air bricks to suspended timber floors, and improving extraction and background ventilation to reduce condensation
- Bridging – cutting back render above ground level, separating plaster from solid floors and clearing debris from the base of cavities where it is found
- Breathable materials – in older solid-walled buildings, replacing impermeable renders, plasters or coatings with vapour-permeable materials, such as lime, where appropriate, so that the wall can dry out
Walls can take a considerable time to dry once the source of moisture has been removed, so the outcome should not be judged too early.
When a new damp proof course may be appropriate
A remedial DPC may be a reasonable option where the investigation shows that:
- There is no DPC, or the existing DPC is damaged or ineffective
- Measured moisture content follows a pattern consistent with rising damp
- Bridging, drainage defects, leaks and condensation have been ruled out or addressed
- The wall has not dried as expected after the simpler measures
BRE guidance describes two ways of providing a new moisture barrier where one is genuinely needed: inserting a physical DPC, or chemical injection. The method and any replastering materials should suit the construction of the building.
How an RICS Chartered Surveyor can help
An RICS Chartered Surveyor can provide an independent assessment of low-level damp and establish whether a new damp proof course is actually required. Our damp and mould survey includes an external inspection of ground levels, drainage and the position of any DPC, moisture meter mapping, thermal imaging, environmental readings and, where necessary, calcium carbide testing and salt analysis. The written report explains the cause, includes photographs and a risk rating, and sets out prioritised recommendations. Lansdown Associates does not sell damp-proofing products or carry out remedial works.
Frequently asked questions
My house was built without a damp proof course. Does it need one?
Not necessarily. Many older buildings were built without a DPC. Whether a remedial DPC is needed should depend on evidence of a moisture problem that simpler measures have not resolved.
The wall was treated before and is damp again. What should I do?
If the work is covered by a guarantee, contact the company that carried out the work in the first instance. An independent survey can also help to establish whether the damp relates to the original treatment or has a different cause.
Do I need Building Regulations approval for a new damp proof course?
In most instances, installing or replacing a DPC does not require Building Regulations approval, although approval may be needed where the structure or thermal performance of the building is affected. Listed building consent is likely to be required before introducing new modern materials to a listed building. If you are unsure, contact your local authority building control department.
Get an independent view before you commit
An independent diagnosis can confirm whether a new damp proof course is necessary or whether a simpler remedy will resolve the problem. Speak to one of our RICS Chartered Surveyors on 0151 374 0905 or email office@lansdownassociates.co.uk (Monday to Friday, 09:00 to 17:00).
