Damp Proofing: How To Treat Rising Damp

What Is Rising Damp?

Damp is caused when moisture leaks into the structure of a building. This can be caused by weakened exterior walls that allow rain and condensation to penetrate the brickwork, or perhaps by work that has compromised the integrity of the internal walls. This can lead to mould on the walls, which looks unsightly, gives off a musty smell, and can damage the decor of your property.

How We Approach Rising Damp Treatment

The first step is to remove existing skirting boards and set aside to re-use where sound. Skirtings may be unavoidably damaged in the process of removal and where this occurs we do not accept responsibility. It is highly probable that sections of skirting may be found to be affected on the reverse side by fungal decay, which can’t always be detected at the time of the survey. Any replacement found necessary in this case may be charged as an additional cost. Existing plaster must be removed to 900mm above internal floor level.

Next, the appropriate brick course is drilled and injected with a silicone cream or injection mortar. The corresponding external course is also injected where the wall is of two bonded leaves. If the wall is of cavity construction, the outer leaf will be injected 150mm above external ground level. Vertical barriers are also installed to prevent moisture ingress from untreated walls such as adjoining properties or inaccessible areas.

We recommend only the use of pre-mixed, bagged water- and salt-resistant renovating plaster, such as the Tilcon Limelite system. There are a couple of alternatives, however. You can use a render sand, cement, and skimming plaster system, or a thistle Dri-Coat render using Thistle Multi-Finish skimming plaster. Plaster must be applied as per the manufacturer’s instructions. Gypsum-based lightweight plasters must not be used unless a membrane system is in place. Plasters must not bridge the DPC, and must not come in contact with solid floors. A minimum gap of 2”/50mm should be left between the floor and the plaster.

The walls are finished with a multi-finish to a smooth, even surface, and are not over-polished. Drill holes are plugged or filled with sand and cement to the outsides. In some instances, damp proof injection can be from one side only, especially in listed buildings where external drill holes are undesirable.

The initial quotation assumes an existing plaster thickness of no more than 1”/25mm. If after plaster removal the thickness is found to exceed this, then additional dubbing out coats of rendering would be needed to bring the new plaster up to the level of the existing plasterwork. Watertight Homes reserves the right to make an additional charge for the extra thickness required.

Chimney Breasts

The guarantee for rising dampness will not apply to chimney breasts, because of the following factors:

  • Interstitial condensation can arise from burning of fuels or previously burnt fuel deposits, which can cause tar deposits to leach out through plaster finishes.
  • The inside of the chimney breast may be built upon or contain a hearth which lacks an effective damp proof membrane, thus lateral dampness may also occur.
  • Rain may penetrate from open chimney pots or stacks.

A Guide To Plaster Drying Times

On completion of any re-plastering work, the drying time will vary depending on whether it’s pink, cream, or grey in colour. There is still a lot of moisture left within the walls, and they must be allowed to dry out before any redecorating is carried out. You should wait a minimum of four weeks, after which time a non-alkali, non-vinyl, water-based emulsion paint may be applied.

Decorating with wallpaper and all other types of paints and finishes should only be carried out when the walls are dried out completely.

(N.B. The following drying times are approximate, and should be used for guidance purposes only.)

In general terms, you should allow one month per 1”/25mm of wall thickness. This is worked out to the following approximations:

  • <100 mm to 300 mm — 3 to 6 months
  • 100 mm to 300 mm — 6 to 12 months
  • >300 mm — one month per 1”/25mm

Additional Work

Once drying is complete, if evidence of salt deposits is seen on walls where re-plastering has not been carried out, this may indicate contamination by water-attacking salts. In these circumstances, the client should contact Watertight Homes to arrange a further inspection. This will determine whether any additional work is necessary.

If additional work is required, it will involve the supply of extra materials, and will increase the overall completion time. Costs for this are calculated pro-rata, based on the original quotation.

(N.B. Walls subjected to additional work during the initial drying out period may experience the sudden or gradual reappearance of damp patches. Water staining such as this is usual, and not a sign the work has been ineffective. These will dry out completely over time.)

No inspection under guarantee will be made during the anticipated drying out period.

Important Information

The Party Wall Act 1996 requires neighbouring homeowners to be notified of works affecting the party wall. Clients should ensure this is done prior to commencement of works.

A supply of water and electricity is required on site.

Where ground levels permit sufficient clearance in non-cavity walls, the damp proof course may be installed below timber floors. Where these are not independently supported on sub floor walls, clients are advised that all floor coverings and furniture must be removed to at least six feet away from the areas to be treated.

Every care is taken to minimise inconvenience, but it should be realised that the work involves a great deal of disturbance and creation of dust — some of which may linger for a while after work is completed. Health and safety advice will be given verbally or in writing prior to execution of the works.

It may not be possible to ascertain the full extent of the re-plastering required upon initial inspection/survey, as the presence of salts may only become apparent after the damp proof course has been installed and drying out is complete. It is required that the client complies with the drying times on walls which have been re-plastered.

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