why roofs in Sydney leak only in heavy rain 

Some roofs in Sydney leak in heavy rain (and not in light rain) and there is usually only one reason why this happens.
• Water flow exceeds the capacity of the roof elements.
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In my experience, roof leaks can be separated into 3 categories:
1. The roof leaks everytime it rains;
2. The roof only leaks when it rains heavy;
3. Only prolonged rain makes the roof leak.
On this post, I will examine the second category.
Leaks on tiled roofs
The most common reasons why tiled roofs leak during heavy rain are:
1. No sarking under the roof tiles. If you go into your ceiling cavity and you can see the underside of the roof tiles - then you have no sarking. Stay there during heavy rain and you will feel a fine mist coming through the gaps between the tiles. Usually, this is not a big problem because it is not enough moisture to wet the ceiling. And there are no ceiling stains.
2. Low pitched roof tiles. Roof tiles start to do poorly when the roof pitch gets less than 20 degrees. Manufacturers of roof tiles have limits on roof pitches because of this. Yet, many roofers ignore these for the sake of ‘looks’.
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