Damp in Old Houses

A large part of the repair work that I do is concerned with damage caused by moisture. The damp problems are in many cases the result of the conflict between old houses and modern materials.

Quite often a new damp proof course is suggested as the solution to damp in internal walls. However the retrospective installation of a damp proof course can not only cause significant damage to the fabric of a building, but it is very expensive and may not actually tackle the cause of the damp.

This page is intended to give some helpful advice about damp in old houses, including identifying the cause, and what the remedial actions might be. The statements below are based on my practical experience of dealing with damp issues in a wide variety and age of old houses.

Independent & Proper Advice

If you have a problem with damp in your house, before committing to any remedial works I would urge you to seek some professional independent advice. This can be in the form of a surveyor who specialises in historic buildings, or a builder who has significant experience of old properties.

Certainly if you have just bought, or are buying an old house which has possible problems such as damp, woodworm, rot etc you are well advised to seek specialist advice before embarking on any work.

I appreciate that gaining this specialist advice can cost up to a few hundred pounds, but that is very little compared to the money that could be spent on unnecessary and expensive treatments.

Damp Proof Courses

DPCs started life following the 1875 Public Health Act and the subsequent 1877 Building Laws. At that time the lack of proper public sewer systems meant that waste ended up in the street and the liquid element seeped into the soft masonry of houses.

From 1877 DPCs began to be built into new houses. They are still built into new houses today although one wonders why considering that today’s new houses are generally built of hard impervious masonry, plasters, and paints which effectively prevent any moisture from entering the house. (And of course we now have functioning public sewer systems).

If your house was built after 1877 it will more than likely have a DPC. Short of someone hacking them out, they do not break. If there is damp in your post 1877 house it might not be rising damp, but another type of damp, and therefore another DPC may not solve the problem.

If your house was built before 1877 it does not need one. It would have been originally built with the types of masonry, mortar, paint etc that allowed it to manage moisture. A pre-1877 house that is experiencing damp problems has most likely had that moisture management system upset, and installing a retrospective DPC is not going to restore that system.

Moisture Management

Walls, floors, roofs and their finishes in old houses are meant to be “breathable”. What this basically means is that they should be able to effectively manage moisture.

For example a stone or brick external wall whether left exposed or covered with a lime render will absorb and hold rain until it is evaporated by wind and sun. Similarly an internal brick, stone, or lath wall covered with lime plaster will hold moisture until it is evaporated by ventilation and heating. A stone or tile floor laid on earth or a lime mortar will allow any residual ground moisture to escape and evaporate.

One of the biggest causes of damp issues in old houses is when the fabric is “blocked” from breathing by the use of modern impervious materials.

Types of Damp

1.Penetrating Damp

This type of damp goes horizontally through a wall and will in most cases be seen as a dark patch on the internal wall which can develop mildew if untreated.

2.Rising Damp

There is currently some debate about whether or not “rising damp” actually exists as a number of academics have failed to produce it under laboratory conditions. However I have encountered several cases of damp coming from a floor and rising up the wall. So until someone tells me what it should be called this is what I mean by rising damp.

3.Condensation

This is basically warm human and/or animal breath, or evaporation of water from cooking or washing, which hits a cold surface and turns back to moisture. It is most noticeable on windows; however the same amount of moisture will be on the walls or there may be some on the ceiling and these may be less obvious. Excessive amounts can form damp patches. Cold moisture does the opposite of warm air and drops down the wall. The condensation will therefore be more severe at the bottom of the wall. Because of this condensation can sometimes be mis-diagnosed as penetrating or rising damp.

What Can Cause Damp?

Below are my top ten which I have come across over the years:

1.Plumbing leaks

Not all plumbing leaks comprise of water dripping (or even spurting) from a pipe in full view of the building’s inhabitants. Quite a few leaks are not at all obvious, and can be very small. However they don’t stop and after time they can cause areas of damp as well as rot and other damage to wood and masonry. Examples of these are:

Leaking gutters or downpipes which can cause penetrating damp.

Cast iron hoppers which can rust away at the back where they join the wall and leak unseen leading to penetrating damp.

Broken drains causing rising damp.

Poorly sealed shower trays or wastes.

Lost roof tiles or flashing allowing water into the loft which can cause damp in a ceiling or wall.

These types of leaks are more common than might be thought and my advice is to check for plumbing leaks if damp is found before looking for other reasons.

2.Cement-based external wall render

Many old houses were rendered with lime; this could be because the stone or bricks were not of high quality and needed to be covered, it could have been done for decorative reasons, or as added protection against the elements. Lime render can cope with minor movement of the house, and it will give way before the masonry as it is softer than the bricks or stone, and it will allow the wall to manage moisture.

Quite often lime render has been replaced by cement-based render, or a previously unrendered house has been rendered with cement.

Cement renders are not a good idea on old houses. They effectively block the pores of the old masonry and prevent any management of moisture by the external wall. They cannot cope with movement and are liable to crack. These cracks let in water which gets trapped behind the render and works it way through the wall causing penetrating damp. And finally because they are harder than the masonry, and because of the way they glue themselves to soft stones and bricks, when they do crack and pieces fall off, inevitably they pull the facing of the masonry off as well.

3.“Lost” pointing

This is a very common cause of damp patches on internal walls. Basically some pointing between bricks or stones has dropped out and therefore water is getting into the wall. It occurs quite often below a windowsill which means that rainwater falls over and then under the sill and concentrates itself into the gap thereby causing penetrating damp.

4. Concrete floors

Floors in old houses were meant to breathe thereby allowing any moisture from the ground below to evaporate, and to deal with condensation. Breathable floors might be stone or tiles laid directly onto the earth or onto a lime mortar, or floorboards suspended above the ground and ventilated via air bricks.

A concrete floor has no element of breathability and allows no ventilation. It is therefore perhaps not surprising that any moisture in the ground below tries to escape up the wall which then gives the symptoms of rising damp.

5. Stone cladding

Sometimes the external walls of old houses have been faced with artificial stone cladding instead of render. This reacts with an old house in a similar way to cement renders. It prevents moisture management and when it cracks it traps water which goes into the wall.

6.Gypsum plaster

Gypsum plaster is also an impervious finish which blocks the pores of internal walls and ceilings. It cannot cope with moisture which develops inside the wall and will flake and crumble when presented with damp or consistent condensation.

7.uPVC windows & doors

Plastic windows, doors and their respective frames have been added to many older houses, in particular those from the Victorian and Edwardian periods. There is no innate reason, provided that they are properly installed, why they should be a cause of damp problems. Unfortunately not all of them are properly installed. The gaps between the frames and the walls are sometimes poorly filled with fillets of cement based mortar. These can crack and let in water causing penetrating damp.

8.Lack of ventilation/condensation

Old houses are sometimes difficult to heat, or the cost of doing so can be prohibitively expensive. There can therefore be a temptation to seal the house in order to conserve warmth. However the reduction in ventilation will allow the humidity to increase due to the vapour from cooking, washing, bathing etc, which can cause a build up of condensation.

9.Floors and their coverings

Covering previously breathable floors with non-breathable coverings such as lino, vinyl, and rubber underlay will result in the moisture being trapped below them. This usually results in rising damp as the moisture tries to find an escape route up the wall, and the floor underneath the covering will become very wet.

10.External pathways & borders

Concrete slabs next to a house will trap rainwater which will then work its way through the wall of the house. Similarly vegetation growing next to a house can encourage water into the wall. Both can cause rising or penetrating damp.

There can of course sometimes be a mixture of these causes, and there are indeed other causes of damp problems, and that is why it is highly recommended to get independent advice from someone who knows what they are talking about.

Possible Remedial Actions

1.Plumbing Leaks

Obviously the leak should be fixed. It is important to then to allow any masonry to dry out before redecoration.

2.Cement Render

Either all of the cement render is removed, or just the damaged sections are taken off.

Care needs to be taken if all of the cement render is to be removed as the underlying stone or brick work can be damaged in the process. It can also be costly to have a whole building or wall re-rendered. There is a lot to be said for doing only patch repairs and allowing the cement render to crack and fall off in its own time. The disadvantage of this approach is that an amount of repair work might be needed every year or so.

Any replacement render should be lime-based and a breathable paint applied.

3.Lost Pointing

This is usually relatively simple and inexpensive to remedy. Any loose mortar is raked out of the joints and replaced with lime mortar.

4.Concrete Floors

Removing a concrete floor is not a job for the faint-hearted, and it can be very disruptive. It is better to manage damp by other means if possible.

What I have done in some cases is to remove a c12” width of the cement from around the edges of the room right down to the earth. This trench has then been filled with limecrete which is breathable and helps any rising damp to evaporate before it goes up the wall.

5.Stone Cladding

Once the cladding cracks there is little point in trying to repair it – my recommendation would be to remove the lot. This can usually be done without any significant damage to the masonry. The brick or stone work can then either be left exposed if good enough or rendered with lime.

6.Gypsum Plaster

Removing gypsum plaster, like cement render, can damage the masonry underneath it. If there are no damp issues, or if any damp in the house is allowed to evaporate, then I would not recommend removing the gypsum plaster just for the sake of it.

Quite often though gypsum plaster can be ruined by a long-standing or serious damp problem and therefore will need repair. What it is repaired with depends on whether or not the potential for damp, particularly rising damp still exists. If for instance the floor of the room is concrete then there might always be a risk of damp. In that case it is certainly advisable to replace all, or at least the bottom metre, of gypsum plaster with lime plaster.

Consideration then needs to be given to subsequent decoration – see “Some Words about Paints” below.

7.UPVC frames

If they have not already fallen out any cement fillets should be replaced with lime mortar.

8.Lack of Ventilation/condensation

The simplest and cheapest solution is to open some windows and leave them on the latch whenever the weather allows.

During cold months it can be better to keep central heating on all day at the lowest setting thereby keeping the walls and windows at a constant temperature.

9.Floor Coverings

Any impermeable covering should be removed and replaced with nothing or else a breathable covering.

10.External Pathways & Borders

Concrete slabs against the walls of the house should be removed, as should any soil and vegetation borders that are likely to hold water.

What I usually do then is to dig a 12” wide by 12” deep (foundations allowing) trench against an outside wall and fill this with 10mm limestone chippings. The bottom of the trench should be sloped away from the house.

Some Words about Paints

If you have gone to the trouble of applying breathable external render or internal plaster to your walls it does not make sense to then cover them with non-breathable paints. There is also the issue about what paint to use if for instance you have replaced the lower part of a gypsum-plastered wall with lime plaster as the same paint and colour would normally be desired.

There are an increasing number of “Eco-Paints” on the market, and there are a number of well-known branded paints that are either marketed at period house owners, or may have words such as natural, period, heritage etc in their title. This does not guarantee that they are breathable. It is best to ask for a technical specification if unsure. If the specification does not say either that the paint is breathable, or that it has high vapour permeability then it is not what you are looking for.

Options known to be breathable are as follows:

1.External Walls

Lime Wash

Silicate Paint

2.Internal Walls & Ceilings

Lime Wash

Clay Paint

Casein Paint

Natural Emulsions

3.Mixed Walls

Natural Emulsions

Issue 1 – October 2013

Disclaimer

The information in this paper is for guidance only and is given in good faith but without warranty, since site conditions and applications skills are beyond the control of Old House Repairs.