Ideally whilst the best defense against a leaky building is to undertake waterproofing in the construction phase, rectification of faulty waterproofing membranes and poor waterproofing can be done later and is often needed after a number of years to address membrane which is starting to leak due to ongoing element exposure. The key is figuring out where the water is coming from, because more often than not, the leak is never directly below the area of weakness.