What's your guess?
Is it because houses tend to face away from the prevailing winds?
Is it because roofers start at the front and then rush the last bit in order to get home in time to beat the traffic?
The clue lies in the fact that older houses in Perth (pre-1980's) tended to be smaller than houses built today. As lifestyles and priorities shifted in favour of floor space over yard space, homeowners usually decide they don't need a quarter of an acre out the back for a chicken coop and a veggie garden, and would prefer an extra bedroom or a decent-sized kitchen instead.
The most common solution was simply to add a room on the back. But as there was not a whole lot of height to work with, the roof at the back tended to be a lot lower-pitched than the rest of the house. Such a flat, low-pitched rear roof is commonly referred to as a skillion roof.
The builders then tiled it, invoiced the owners and hoped it didn't rain before they got paid, leaving the owners with a roof that would, sooner or later, begin to leak at the tiniest movement of the rafters.
Fast forward to today and that beautiful old Federation home with the wonky extension out the back is a problem that just won't go away.
Unfortunately, the solution is not simply replacing broken tiles. And it is not sarking. Sarking is a lightweight insulation foil which is primarily a thermal and dust barrier. It is designed to handle very small amounts of moisture and should not be relied upon to keep rainwater out of your building. That is the tile's job.
Short of major structural work the only guaranteed watertight solution is to replace the tiles with metal sheeting. The good news is that for a rear extension this is usually an inconspicuous solution that is not visible from the street.
Here is a project we recently completed in Kensington. As you can see, the pitch of the rear extension is significantly lower than the rest of the roof. The owners had recurrent ceiling stains and despite their previous repair attempts, the stains always returned at the first rains.
Acting on mistaken advice the diligent owners had tried to fix it themselves by removing all of the tiles and installing sarking underneath. Unfortunately this was to no avail. So when we worked out a solution that would match their existing patio sheeting and guarantee it would be watertight, the decision to replace the tiles with Colorbond was a no-brainer.
We were also able to use this opportunity to install steel battens and anticon insulation, thus improving the thermal rating of the house at the same time. We then sheeted it in brand new Colorbond steel to match the patio.
Once finished and flashed in, the new roof section proved a harmonious and inconspicuous addition to the home - all in all, a success!
They can now repaint their ceiling with utter certainty that the drips will not return.