Can a damp-proof course be done from outside?

Damp is a problem for a large number of UK households, with an estimated 912,000 properties thought to be dealing with the issue. Some instances of damp have different causes than others, and there are several methods for tackling the problem.

Sometimes, a damp proof course can be installed from the outside only, but this wouldn't be suitable for all types of damp or all situations. 

Kathleen Walsh Interiors

What is damp?

Damp is excess moisture in a building, its walls, or its floors. Damp will cause unsightly dark patches on internal walls and an unpleasant odour. More seriously, however, damp can also trigger mould and mildew growth, which can cause health issues for many people. And damp that is left for too long could even cause structural damage to the property. 

There are two main types of damp: penetrating damp and rising damp. These have slightly different causes, and the approach you would need to take would differ for each.

Penetrating damp

Penetrating damp is when moisture penetrates through external walls. This is usually due to damage, such as wall cracks, crumbling mortar, or leaky gutters/pipes. It can also happen due to particularly heavy rain. 

A damp-proof course of any kind won't help with penetrating damp. The damage that is creating the hole or damage through which moisture can travel needs to be fixed. 

If you have penetrating damp, you should get a professional involved to identify the underlying issue and develop a plan to fix it. 

Rising damp

Rising damp is actually very rare, although it is often misdiagnosed. In this case, water at ground level moves up through the wall, from the very bottom. It travels through the capillary action of the pores in the material. 

Almost all properties will have some sort of damp-proof course at the base of the walls. This is waterproofing that is designed to prevent rising damp. You would usually only see rising damp if this damp-proof course is damaged or nonexistent. 

Different types of damp proofing

The goal of all types of damp proofing is to prevent water ingress by creating a waterproof barrier between the outside and the internal and external walls and floors.

A new damp-proof course is really the only way to fix a rising damp issue. A damp-proofing specialist can investigate and determine whether or not you truly have rising damp. If you do, then they can install damp proofing that will eliminate the problem.

Damp proofing can be achieved in a variety of ways. 

Membrane damp proof course (DPC)

A membrane made of high-density polyethene or a hard plastic is placed horizontally within the walls or under the floor boards. This creates a barrier that interrupts the capillary action that is responsible for rising damp. 

Damp proof injection or chemical damp proof course (DPC)

This type of damp-proof course doesn't use a premade barrier but, instead, holes are drilled at various points in the wall. Through these holes, a chemical is injected, sometimes in the form of a cream.

Brickwork and mortar are porous materials. Because of this, the chemical will be absorbed into the materials. Once it sets, it creates a waterproof barrier that will prevent rising damp.

Can damp proofing be installed through the outside walls?

There are some situations where installing a new damp-proof course can be completed without needing to touch the internal walls.

A chemical DPC can be installed through drilling holes in the exterior walls, without needing to remove any internal plaster. You will need to re-render afterwards, however.

A damp-proof membrane, on the other hand, will usually involve some work on the interior as well as exterior walls especially when trying to prevent damp in a basement.

Only a professional can determine how extensively your internal walls will need to be worked on. 

When can't damp proofing be installed externally?

In some cases, it would be inadvisable to install damp proofing through the external walls only. This is especially the case if the internal surface has been heavily affected by moisture. 

If you have extensive damp patches on your internal walls, then there may be a high level of moisture contained within the walls. In this situation, you should have some work completed on the inside as well as outside walls. 

Hygroscopic salts

The water that seeps into your home from the outside won't be pure. It will also include soluble salts such as nitrate or chloride. When the water evaporates, these salts are left behind on the surface and become concentrated. 

This can start to cause issues because the concentrated salts attract moisture. At this point, any humidity in the air will be absorbed into the wall much more easily, making the damp problem worse. 

There are two approaches to this problem: an extensive set of protective measures or a less invasive procedure, but both will involve work on the internal wall, not just the outside wall. 

Extensive treatment

To truly get to the bottom of severe rising damp problems, the plaster should be removed to a height of 1.2 metres. Then the cavity wall would need to be cleaned out before a damp-proof membrane or chemical damp-proofing is installed. 

Following this, there should be rendering installed with a salt, fungus, and moisture-inhibiting mix, before the new plastering is applied. 

Less invasive treatment

In this course of action, the damp proofing can be installed through the external walls only, while the internal damp issue is dealt with separately. 

The damp patches should be treated using a salt-neutralising chemical that converts soluble salts into insoluble salts. This can resolve the hygroscopic salt issue, although it may need to be completed multiple times. 

Final thoughts

A damp proof course (DPC) is only effective for rising damp, not penetrating damp. This is because rising damp is caused by water from the ground level moving up through the wall via capillary action.

A damp course creates a waterproof barrier that prevents the water from rising. In some cases, a chemical damp proofing course can be done from the outside. Although, a lot of the time, there will also need to be internal work done to prevent issues such as hygroscopic salts.

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Jenny Kakoudakis likes to blog about interiors. She launched award-winning Seasons in Colour in 2014 and the luxury interior design blog All The Pretty Homes in 2024.

When she is not chasing criminals out of the financial system (her day job), she gets creative by redecorating her own home.