With decayed or failing chimneys, there are two strategies, depending on whether you wish to use a fireplace with a flue to that chimney.
If the chimney is not to be used, some people say that a poor chimney can supported with a concrete plug in the flue hole. However, this is not recommended:
The chimney should also be capped but ensure ventilation at the top and bottom of the chimney; fit a vent if fireplaces have been removed and do not use a plate or other blocking device in the fireplaces which remain. If the draughts are excessive in winter, you may use a special 'balloon' device but remove it in the spring so that the flow of air can again maximise evaporation.
The tips below are also relevant to disused chimneys.
If the chimney is to be used, consider one or more of these solutions.
The flue can be lined with a metal or concrete liner.
Small areas can be protected with a mix of sand, lime and brick dust. Consider rendering the whole stack with lime mortar as an alternative to rebuilding the chimney.
Check the ventilation indoors; if the fireplace has been filled in, a vent is essential to maintain a flow of drying air up the chimney. If the fireplace is still open, check that nothing has been stuffed up the flue to reduce the draught; again this can allow a build-up of moisture. Above the blockage, there is usually a pile of crumbled mortar and soot so get a vacuum cleaner ready to suck it all up. Opening the flue up will increase ventilation and evaporation.
Check the roof tiles higher up the roof from the chimney; damp may travel down inside the loft, reach the stack and then continue down to the ceiling.
Consider removing cement-rich pointing, replacing it with a lime mortar. If the chimney is painted and there are damp problems, removal of the paint may improve evaporation. A cement-based render may cause the same problem but removal of the render may cause damage to the bricks. If this occurs, either have the stack rebuilt or use a lime and sand render.
Restore the lead flashing at the junction of stack and roof tiles.
In the worst cases, the chimney may have to be rebuilt.
In rebuilding a chimney, beware of the risks of damage to the lower parts of the chimney; these tend to be weak because they have borne the weight of the bricks above for many decades. The shocks of demolishing the stack may cause the rest of the chimney to fail. Consider strapping the chimney during the work. Make sure that lime is used rather than cement because future rebuilding will be easier.
Take care also that as the chimney is taken down, any parts of the roof relying on the chimney for structural strength are supported properly.
If the chimney is being rebuilt, it is worth incorporating a lead tray, like a damp-proof course that extends into the flue with an upturned edge. It may be at the level of the flashing so that the outside edge can be continued out and down so that water coming down the chimney is carried out.
A proper pot should be used. Use the same design as other similar houses in the area.