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Hello Phil

If you look at the house itself, its surroundings and how the occupants live in the house you may identify a number of contributing factors.
In my opinion it is better to start by eliminating as many potential causes as possible (provided they do not involve major work or cost) rather than to try to determine exactly which one is contributing the most to the problem. Maybe an example will help. I went to see a house with mould problems even on some internal walls!!! 20+ possible causes or contributing factors became evident immediately:

– the worst problems were in the bedroom (1 – people spend many hours in bedrooms releasing moisture) located in in the South (2 – so no solar heat gains) corner of the house (3 a corner, therefore bigger heat losses) where two people slept (4 – more people, more moisture) and which remained unheated (5) and had no insulation in ceiling (flat roof), under the floor or in the walls (6). The house had been broken into before so windows remained closed most of the time (7). The house was built in the 1940’s and the walls were finished with plaster (8), which tends not to provide goood wall moisture management and possibly not good ventilation of the wall (19) and flat roof (20) which has less ventilation and more chances of leaks than one with trusses. The plaster had a few very thin cracks in which mould was growing outside (9 – a sign that moisture is getting in or accumulating in the cracks). The grass was very green around the base of the downpipe in that corner, so water from the downpipe may have just been seeping into the ground, which is not uncommon in older houses (10). The ground along one of those walls fell towards the house rather than away from it (11). There was no polythene laid on the ground under the house and ventilation to the subfloor was rather limited as it often is in houses on a concrete ring foundation (12). Palmerston North is built on swamps and lagoons and the water table is high so many areas in town have damp ground (13). This house was not very far from the river (14). Extract fans were ineffective or missing (15). Weeds could be seen growing in a couple of places inside the gutters (16). The joint between windows and plaster could have also allowed water in places, so that would have been something else to check (17). The house was a flatting situation, in which in my experience people tend to leave their bedroom doors closed so ventilation is poor (18).

Instead of trying to figure out if the downpipe is contributing more to the problem than the gutter or the cracks in the walls or the lack of ventilation, I think that it is better to address as many of those possible causes as possible, given that if they turn out not to be causing problems now (maybe moisture is not getting all the way through the cracks in the plaster yet or the gutters are not backflowing into the roof and wall yet) they may well become a problem in the near future.

Some of the solutions may be cheap or zero cost but others will require that the owner of the house considers his/her options very well. A good example in this case would be the roof. Maybe spending a bit more and replacing the flat roof with a pitched isulated roof is the best option, as it would address more than one problem and it would help to prevent leaks.

Sorry for the length of the reply. I hope it helps.