Although there are several different kinds of walls that we may find in our homes, for interior work the differences are not very important. Nearly all of them end up with a plaster surface to which our decorations are stuck. This plaster may be applied in the traditional manner as a wet plastic mass smoothed into place, or it may be plasterboard, which is a commercially made flat plaster sheet sandwiched between building paper.
A few walls, especially in converted properties, may have an outer surface of hardboard, chipboard or an insulation board. All of these are made from wood fibre and chippings. There is very little difference between them for practical purposes. However, wall construction can affect the interior of the rooms if chemical reactions take place with the pastes and paints that we use, and of course if they permit damp to penetrate inside.
The chemical reactions are exceedingly complicated and take many forms. The results are various: paint may blister; wallpaper may peel away; distempers may flake off.
Troubles of this kind are not easily dealt with by amateurs because of the difficulty in deciding the exact nature of the reaction.
For example, applying a heavy glue size as a background for papering or painting to a distempered wall will probably result in the distemper being pulled away in flakes.
This happens because of shrinkage of the glue size. Paints may be affected by salts being drawn from the plaster or by salt forced through walls by sea breezes.
Modern cellulose wallpaper pastes are far less affected by moulds and chemicals than the older fashioned flour paste and trouble from this source is much less likely than it used to be. Damp however, does remain a problem and is a very wide subject. Apart from the application of a protective metal film whose only purpose is to prevent decorations being discoloured, you can do very little on the inside of a room to correct dampness in walls. Indeed, it is not usually wise to try.
One vital, general rule about treating walls is that the plaster should be completely dry before anything is put over it. This applies of course to new houses or to those where extensive repair and new plastering has been done. To apply a paper or paint over a still-damp plaster wall will result in all manner of troubles and may even lead to the entire collapse of the plaster.
By dry we mean perfectly dry, through and through. A plaster wall may feel perfectly dry to the touch and yet in fact be quite damp at depth. This is because the plaster is losing the water that comes to the surface to the air more rapidly than it is drawing more from its backing. In warm dry weather a new plaster wall may feel perfectly dry but a few days of humid cold weather may bring out obvious dampness. In both these cases the completion of drying will not take place for several weeks at the minimum.
It is most unwise to start covering new plaster walls until at least three months of good drying weather has elapsed.
The only decorative covering that can be applied in these circumstances is a porous water paint. This will permit the plaster water to continue evaporating yet will give a decorative surface. Provided a good emulsion or washable distemper is used, you will later be able to paper over it.
Continual dampness in a wall near the ground level probably means that you have a defective damp-proof course (DPC).
The object of a DPC is to prevent water rising from the soil into the wall itself.
It consists usually of bitumen or asphalt sheeting embedded in the wall. Sometimes a double row of blue engineering bricks is used instead. This damp-proof course ‘should by law be 6 ins, or more above the surrounding soil level. This is so that when rain splashes on to the ground outside it does not splash the walls above the damp-proof course, so making them damp.
However, one often sees houses with beds of soil raised against the wall to such a height that the damp-proof course is not left 6 ins, above ground level. Indeed it may be buried completely! Water will certainly enter the walls if this is done and will rise to produce the familiar damp patches on the lower parts of the walls.
The remedy here is obvious. Make sure that the damp-proof course is exposed, and is more than 6 ins, from the soil level, all round the house. If the trouble persists even though the damp-proof course appears to be well clear of the soil, then only a builder is likely to be able to give you further
guidance. As examples of what may be wrong we can list: accumulation of mortar droppings between the two halves of a cavity wall; tearing or cracking of the damp-proof course; failure to install a damp-proof course in a solid ground floor so that water can pass from the floor into the brickwork.
Water may also be seeping between the woodwork of door or window frames and the adjacent bricks. Many older houses have no damp-proof course at all and these require very special treatment.
Water rises up through walls and floors that are in contact with the soil for exactly the same reason that blotting paper absorbs ink: capillary attraction. This is a complicated electrical process by which the water is drawn upwards into fine pores of any porous substance. All building materials are porous to some extent, including concrete and bricks and these will suck water to a considerable height. This rise will be stopped if the water meets an impervious damp-proof membrane such as bitumen, blue bricks, or (in modern times) plastic sheet.
We might mention here one extremely effective modern process for correcting damp in walls that have no damp course.
This is known as the electro-osmotic process. Because the rise of water is an electrical matter it is possible to stop it entirely by drilling through the walls and pushing into the holes bars of copper. These are later attached to the earth just as electrical mains are earthed. This copper framework causes the electric charge to be returned to the earth instead of pulling the water higher and higher. Rentokil are one large firm specialising in fitting this device and in severe cases of rising damp on thick walls it is most effective and relatively cheap to install. In fact it has been used successfully on walls several feet thick, as found in old cathedrals.
Inside most homes, though, we are not really likely to meet any of these big problems. Most walls are reasonably sound. The plaster will be well dried out and will probably have been decorated many times already.
We are often asked whether the old decorations must be removed and why cannot new ones be applied directly over them?
The answer is that of course sometimes it is possible to apply new decorations over old ones, particularly paintwork. If a painted surface is in good condition, solid, not flaking or crumbling, then a good washing down with a suitable cleaner, and perhaps a little roughening with sandpaper if the surface is very glossy, will give a surface which can be repainted quite freely. However, if the original paint film is very thick already (as it may be after several applications) the new paint you apply may, in drying, shrink severely. This can pull the whole paint film away from the plaster surface behind. This flaking is sometimes an aggravating problem which occurs after decoration.
When you are wallpapering, old paper should certainly be removed. Otherwise, as you apply the new over it, the wet paste may soak through and loosen the old paste of the first layer. This will then pull away from the plaster, bringing the whole lot down. At the very least it will probably cause unsightly bubbles and bulges.
It is not difficult to remove paper, and although there are proprietary stripping solutions to help this operation, plain, warm water is often as good as any. The key to the easy stripping of a room is to make the old paper very wet and to keep it wet while stripping is done. This both softens the paper and loosens the adhesive behind it. Then when all the old paper is off you can rub down the surface with an abrasive so as to give a clean fresh plaster surface upon which to apply your new wallpaper.
Another common query is why we should bother to size walls? This sizing is, in effect, painting the wall with a very thin glue and allowing it to dry before applying wallpaper. This first application of glue makes the wallpaper slide easily and stick well It is especially helpful if the new paper is fairly heavy. Mention of heavy paper brings us to the final problem of lining paper.
A lining paper is a thin, perfectly plain paper which is stuck to the wall underneath the main paper. Its object is to give a good firm surface to which a heavy paper can be applied easily. It should be a rule always followed that no heavy papers should be hung, except upon a lining of thin white paper.
Linings are often applied horizontally instead of vertically. Good lining paper will then also reduce the tendency of the top paper to shrink apart and expose small gaps between the strips.
It is now possible to buy self-adhesive papers, which save the trouble and mess of pasting.
Another new development in the past few years has been papers which strip easily from a prepared backing. Both these types cost more, but they are certainly very convenient to use.
The use of oil and enamel paints has become much less frequent in recent years. Emulsion paints, which have a water base, are increasing in popularity. They are easy and somewhat cheaper to apply. They also give a very hard and washable surface. Since painting is a very wide subject and tends to be rather technical in some of its aspects when the paint is applied over plaster, we can give only the very broadest outline. On the whole we recommend the use of emulsion paint rather than oil paints for amateur work.