Although less in use these days following the introduction of modular home construction, parapet walls were popular in older houses where they are frequently the cause of damp.
Parapet Wall – a double-skin wall that extends through the roof
Signs of Damp – bubbly, flaking paint, powder on raw brickwork
When builders build a wall comprising of an inner and outer layer of bricks they frequently leave gaps in the cement filler. Sometimes they do this deliberately in order to run services in the cavity. Where water enters this gap, it flows down to where it reaches an obstruction and then collects there, causing damp. This is why so many damp spots are at the bottom of a parapet wall. Fixing the damp by drilling breathing holes, or applying a proprietary coating on the inside, is expensive and usually useless too. Do not get tradesmen quotes for this. Rather fix the problem at the source – permanently.
There are two ways that builders finish off parapet walls – they either cap them with a tile, or with mortar. In the latter case, the mortar should be sloped to one side to lead rainwater away. These cappings fail for two reasons: either the tiles work loose, or the cement capping was badly done. If you are scared of heights, then find a tradesman to do the work for you. If not, the job is relatively easy.
Before you start the job, attach yourself with a safety rope to a secure point, or have a mate hold it for you in case you start to overbalance. Chip away at the bad mortar and re-do it, or reattach the loose tile. Tradesmen in London often prefer to cover the finished job with membrane along the entire length of the parapet top, because some London buildings are old and crumbling.
When you have done the job, you can attend to the damaged paint or plaster, but only after the wall has dried completely. Mr-Skill provides this information without guarantees.