Chimneys are a potential source of water ingress and it is quite common to find damp areas around the chimney breast. So what’s going wrong?
Well there are a few key factors to bear in mind.
1. Failed pointing. This is quite common as the chimney gets a lot of wind driven rain and this can easily get behind the mortar if cement has been used and where the old lime mortar has been eroded away. When repointing remember to use a breathable mortar (a lime based one). Whilst this does allow moisture into the mortar it also allows it to dry out again. So ensure that the mortar joints are scraped out (to a depth of twice the height of the mortar bed) and then get it moist and repoint using the lime mortar. This should be finished in a flush manner to the bricks. Make sure you replace any broken bricks as well.
2. Cracked flaunches. Flat surfaces are not good for waterproofing as rain can settle onto the flat surface and soak in. Maintaining an angle / slope on the flaunch is really important if it made from a porous material. Cracking here is very common (again when cement is used), so either use a lime based mortar (and paint it with a mineral paint for a long lasting breathable finish) or look at a replacement option with a impervious capping material. This will help to reduce any water ingress into the stack.
3. Chimney pot not covered. Cowls are really important as we need to stop rain water getting into the main structure. Depending on whether the chimney is being used or no, there are a range of options to help reduce the amount of rain that are able to enter the pot.
4. Flashing. It is quite common for flashing to be fixed with cement. This again cracks easily and hence should not be used. Flashing needs to be set into the brickwork properly, not just abutted to the chimney. The flashing should be a lead one rather than the modern ‘fix’ that is glued on. Where mortar has been used, again it should be a lime mortar should be used rather than cement.
5. Vegetation growing out of the stack. Roots from plants like buddlia allow moisture into the structure and so these must be removed and the resultant holes filled with lime mortar.
I would warn against using cement renders, water proofing paints etc as they are much more likely to trap moisture behind them in the long term due to the effects of wind driven rain. So using materials that breathe and also have some movement potential in them makes them more suitable for these high exposure elements of our homes.
If the chimney has too many problems to mention (and this can happen when cement renders pull off the finish on bricks) then you may have to render / re-render them. Oddly enough I would recommend the use of a lime render. Make sure that the render has a good cap though as even lime renders require protection from above, so the flaunch /capping is really important to get right.