Weathertightness of buildings is complex .We not only need to keep water out. We also need to let water out.A ventilated space presents a much smaller risk to the building than an unventilated space if water is present.

In this traditional brick cavity system moisture is picked up and removed by circulating air. The bricks and mortar also allow moisture to pass through them.

The red arrows shows the pathway for air to enter and exit the cavity.

It is very difficult to prevent any moisture ever entering a building.If water can not escape from a building, even small amounts can cause big problems.This is why we try to provide ways for moisture to leave the building.

The full lintel, beam , joist or stud often require replacing to maintain the structural integrity of the building.

Common points of water entry are .

Penetrations.

The top ends of window and door flashings

Decks over living areas

This wall has a high risk of leak damage especially if it has a monoclad system with out a cavity.

The parapet flashing should be deeper and potentially will allow water to blow into the building

Using the correct flashing detail is important here

When you flash any part of the building you take responsibility for its weathertightness.If you have doubts you can try to create multiple protections so that if one fails another will divert water away. A flexible flashing such as butynol beneath a metal flashing can be a very effective second line of defence.