Yorkshire Dampcourse home page :: Replastering


After Damp-Proofing

The installation of a damp-proof course (dpc) is only one part of the overall process. Wet and salt contaminated plaster must be removed and replaced as well. Both the dpc and the replastering form part of the dpc system and both are usually included in any guarantees offered by the installing company.

As an important point, replastering only to a wall that continues to suffer from ingress of water (such as a wall without a damp proof course) is unlikely to prove successful in the long term. This is because renovating plasters are usually only capable of holding back moisture for a short period. Such  a period would normally be during drying out of the wall after damp proof course insertion.

The installation of a dpc only in a wall that has wet and salt contaminated plaster on it will not allow the wall plaster to dry out or for the salts in the plaster to diminish. Plaster removal and replacement must take place. Too often we hear of, and visit, properties where a dpc only has been installed and the wall plaster still tests damp with an electronic moisture meter.

Replacement plasters must be of the renovating cement-based type and have no gypsum content in the backing or first coat. Gypsum is very sensitive to moisture and salts and degrades in the presence of both. The design of a renovating plaster is to provide two distinct layers. The render or backing coat is that applied directly to the exposed masonry, when the old plaster has been hacked off. One of its purposes is to provide a rough surface onto which a finishing coat can be applied. However, its main purpose is to protect the finishing plaster coat from residual moisture and salts left in the masonry of the wall after the old plaster has been removed. Therefore the backing plaster contains both water and salt resistant additives to allow it to work.

Finishing plasters are gypsum based. Their function is cosmetic only; to provide a flat, smooth surface on to which wall decorations can be applied. Finishing plasters are sometimes also called "skim" plasters.

The text below is that included with our reports:-

We recommend only the use of premixed bagged water and salt resistant renovating plasters such as the Tarmac Limelite system (a render and skimming plaster system) or alternatively Thistle Dri-Coat render with Thistle Multi-Finish skimming plaster. Plaster must be applied as per the manufacturer’s instructions. Gypsum based, light weight plasters must not be used. Plaster must not bridge the damp-proof course and must not come in contact with solid floors. A minimum gap of 2 inches / 50 mm should be left between the floor and the plaster.

1. Plaster Thickness

Unless specified, our quotations assume an existing plaster thickness of not more than 25 mm (1 inch). If after plaster removal the thickness is found to exceed this, then additional dubbing out coats of rendering will be needed to bring the new plaster up to the level of the existing plaster.

This will increase the time required to complete the work and will involve the supply of extra materials. This adds cost to any plastering job.

2. Perished Plaster

Where rising damp has occurred, the action of rising water carries mineral salts from the ground and leaves them as salt deposits in the brickwork and plaster. Where this has occurred the plaster may have perished because the salt deposits will have attracted moisture from the air or wall causing damp patches of varying severity, depending on the humidity of the atmosphere.

Even though the introduction of a damp-proof course will control further rising damp, damp patches may still occur on the walls and it is therefore essential that all perished plaster should be removed.

3. Additional Replastering

At the time of surveying / inspection it may not be possible to ascertain the full extent of the replastering required as the presence of salts may only become apparent after the damp-proof course has been installed and drying out is complete. Drying times are given in item 4 below, and these may apply even on walls which have not been replastered.

After the drying out time, if evidence of salting is seen on walls where replastering has not been carried out then this may indicate contamination by water attracting salts and replastering here might be necessary.

4. Drying Out and Redecoration

On completion of the work any replastered walls will be the characteristic pink or grey colour of finishing plaster depending on the type used. There is still a lot of moisture left within these treated walls and they must be allowed to dry out before any redecoration is undertaken.

After about 4 weeks a non-alkali, ‘trade’ water-based emulsion paint (NOT vinyl) may be applied. Redecoration with wallpapers and all other types of paints and finishes should only be carried out when the walls have dried out completely.

Approximate Drying times Guide

The following drying times are approximate and for guidance only. In general, allow about 1 month per 25 mm of wall thickness:-

Wall Thickness Drying-Out Time
less than 100 mm 3 to 6 months
100 mm to 300 mm 6 to 12 months
over 300 mm 1 month per 25 mm

5. Skirting Boards

Where it is necessary to remove skirting boards to install the damp-proof course it is highly probable that sections of these may be found to be affected on the reverse side by fungal decay which cannot always be detected at the time of the survey. Rotten skirting boards will need to be renewed.

Skirting boards that would otherwise come into contact with the base of potentially damp walls or floors must be protected from them using a damp-proof membrane. This applies particularly to solid floors.

6. Electric Systems

Replastering of walls sometime includes areas where there are light switches or electrical sockets. These will be linked to the electrical system by a wire run, which is most usually hidden beneath the plaster surface. As the original plaster needs to be hacked off, sometimes this can be difficult. Consideration may be need to be given to alarm systems and their wiring systems as well. In all cases advice should be taken from a qualified electrician. In many older properties without recent rewiring, electrical systems are likely not to conform to the most recent wiring regulations.

7. Other Wall Mounted Items

Consideration needs to be given to the position of all wall mounted items (pipes, shelves, kitchen units, sinks etc.) which may be in the way of replastering. It is not possible to replaster such areas unless these items are removed. This can be time consuming and costly. Sometimes it is better to leave such areas until such a time as other alterations are being made.

Where replastered areas are to be covered up, for instance by kitchen units, it is acceptable for the backing plaster only to be applied to wall surfaces, without the use of the finishing coat. This background coat of plaster is the one containing the water and salt resisting components.

8. Plaster Curing and Cracking

Cement based plasters need to remain damp for some time to allow the cement to cure. Curing is the process which forms the bond strength of the plaster to the wall, and the strength within the plaster. Drying out too quickly will stop the curing process and result in the plaster being weak and not keying to the walls.

During drying out, cement plasters may crack. This is the result of stresses being set up in the plaster because the cement shrinks as it loses moisture. Cracking is much more likely to occur in uncured plaster. This is why a slow dry out (to prolong the cure process) is so important.

Some cracking should be expected. On the finishing plaster this is usually hairline and is considered acceptable. It can usually be decorated out, or wall paper covered. Larger cracks should be drawn to our attention prior to redecoration, but may only require filling as part of the redecoration process.

9. Plaster Joints

Where new replastering meets the original plaster, there will be a joint. We try to minimise the impact of this visually, but the joint is always detectable. Some wallpapers do mask this, but you should expect there to be a joint that you can see.

If a joint is unacceptable within the finished plaster then the wall will need to be skim plastered to full height. There will be additional costs for doing this. This requirement should be notified to us BEFORE the work commences.

10. Lateral spread of damp

Replastering usually covers a wall that is residually damp. The plaster may therefore slow down the loss of moisture from the wall through evaporation. Sometimes this can result in lateral movement of damp into areas that we not previously affected. We can’t predict if this will happen. Associated building materials might be affected, and an example of this is the plasterboard under drawing of an exposed staircase or a section pf plasterboard ceiling.

We cannot predict this occurrence, but it can be repaired if it takes place. We make normal charges for this type of work. 

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