Chimneys are more exposed than any other part of a building so it is not surprising that they are particularly vulnerable to rain penetration issues.
The BRE recommend that,
“ideally, chimneys and parapets should be worked on at the same time as the roof, to minimise damage to roof coverings, and cut down any scaffolding costs.” 
Dampness issues related to chimneys are noticed when chimney breasts become damp. The underlying cause is usually rain penetration through the chimney structure or condensation of the flue gasses.
Typical repairs carried out to deal with rain penetration through the chimney structure include the fitting of caps or cowls, repair of flashings, repair of flaunching around chimney pots, inserting a DPC at roof level, repointing of brickwork, and applying a water-repellent coating (e.g. Stormdry Masonry Protection Cream) to the chimney stack.
Condensation problems are usually dealt with by increasing ventilation through the chimney (e.g. by opening up fireplaces) or by introducing a flue liner.
Damp Chimney Breasts (Secondary Chimney Damp)
Even when the underlying source of dampness has been dealt with, damp patches on chimney breasts can be very persistent.
This is due to the fact that most chimney breasts are heavily contaminated with moisture-attracting hygroscopic salts that have accumulated in the chimney structure due to the burning of coal and wood over many decades. Trying to solve the problem by simply repainting or replastering using a standard gypsum plaster will not usually be effective in these situations as the hygroscopic salts will be able to migrate to the surface and the damp patches will reappear.
To overcome this problem, the traditional method of dealing with damp chimney breasts has been to hack off any salt-contaminated plaster and apply a cementitious waterproofing slurry such as Vandex BB75 to the affected chimney breast before a renovation plaster such as Dryzone Damp-Resistant Plaster is applied. Alternatively, a studded “plaster membrane” such as Oldroyd Xp can be applied to the chimney breast before replastering takes place.
Both of these methods can be very effective, but they can be time-consuming to apply. For this reason the Dryzone Express Replastering System is now our most popular system from replastering chimney breasts that have been affected by dampness. The system is approved by the British Board of Agrement for use on damp and salt contaminated chimney breasts and benefits from the following advantages:
- Applied more quickly than traditional methods
- Can be redecorated over more quickly than traditional methods
The method of application for treating chimney damp using the Dryzone Express Replastering System is as follows:
- Deal with underlying cause of chimney damp – e.g. condensation in flue, rain penetration though chimney stack, defective flashings, etc…
- Remove salt-contaminated plaster from chimney breast
- Apply Dryshield cream to exposed brickwork – this acts as a barrier to moisture and disrupts salt movement
- Fix standard plasterboard to the chimney breast using Drygrip Adhesive (this has been specially developed to resist the passage of salts and moisture)
- Either apply a plaster skim to the plasterboard or (for faster results) use a dry jointing method
- Apply a breathable paint such as Dryzone Mould-Resistant Emulsion Paint
For full application instructions for the Dryzone Express Replastering System can be downloaded from this website.
 BRE Good Repair Guide 15 “Repairing chimneys and parapets”.