Well there is a good reason why this may occur.
The roof pitch (anything under approximately 17 degrees) is too low to cope with water that travels down the roof. The slightest up force of wind will stop the water flowing correctly. Slowly but steadily water will push back up the roof tile and in to your roof space. This is especially true for flat profile tiles and tiles with minimum overlaps. A Terracotta tile or similar will cope with this problem much better.
All perfectly flat roof tiles must have roof sarking installed prior to installation. This is determined by the Australian Standards. Newly made flat tiles often have raised water course edges to accommodate a lower pitch so the standards are more relaxed for these tiles.
Like all roofing materials, the lower the roof pitch, the more suspect the roof is of leaking when it rains. Roof Sarking or silver foil is installed under the roof tile battens with a 100mm overlap into the gutter prior to the tile installation. The foil paper ensures that if water does enter the roof space, it is carried away into the spouting.
Older roofs with a low pitch may not have been originally installed with sarking. Alternatively, the foil they did use may have rotted over time and be very brittle. Holes would be appearing in places where water is leaking through. In both cases, new roof sarking will need to be installed.
This is a fairly simple, yet time consuming task. The process involves the roof tiles and in most cases the battens to be lifted, the sarking to be installed, and then tiles to be re installed. Often, the roof tilers will need to work the roof tiles back into the roof like a jig saw puzzle which is why this job is always better left to the professionals. Ridge capping that may have been disturbed will also need to be repaired.
The cost for such a repair is usually worked out to about $50 per square meter depending on extent of repairs to be made.
Give us a call to inspect your roof for that annoying leak that wont go away.