Fashion can often deal multiple blows to common sense – and even engineering science…
This seems to be the case with roof tiles. Home owners seem to like the ‘slate’ look at a cheaper price. The flat tile look looks low profile and modern. And when promoted as a cheaper alternative to slate, many people are sold.
There is a reason that traditionally, roof tiles are contoured with crests and valleys. The edge of the roof tile finishes at the crest – so all the laps are on the high side of the roof tile. There are no lap joints at the troughs where most of the water will travel. Makes total scientific sense.
Any sensible engineer would design a travel path for the water that is kept away form the vulnerable lap joint. This way, a minimum amount of water runs between the tiles.
The traditional contoured terracotta tiles are designed to be ‘cross bonded’ (staggered laying pattern between adjacent rows). And the troughs on the tiles divert water away from the sensitive laps. This is the reason why these tiles have withstood the test of time.
With the contoured concrete roof tiles, they could be laid either straight (no staggering between adjacent rows) or cross bond. The laps were always on the high (crest) edge of the tiles however they were laid. and therefore protected against heavy streams of water.
When flat roof tiles began production and the tile manufacturers would nominate a ‘cross bond’ laying pattern only. No straight laying.
On a flat roof tile, rainwater is spread evenly over the surface of the tile. So, the lap gets as much water as the rest of the roof tile. The design of the water (drainage) course on the lap becomes critical, now that there is much more water to be carried…
But if all the roof tiles are laid straight, the water courses will line up. This forces the downstream tile to pick up the water flows from the adjacent upstream tile at the drainage course. And as we go down the roof, the water load gets worse with each tile and it will overflow the water (drainage) course… causing leaks.
Most (if not all) roofers will cross bond flat concrete (and terracotta) roof tiles – because the manufacturer tells them to. So, the laying pattern is not usually an issue. It is what the laying pattern does to the roof tile that is causing a lot of roof leaks in Sydney.
Firstly, the design of the nail hole location (roof tiles are secured to the battens using nails).
On old production runs of some flat profile roof tiles, the nail hole was located at the centre of the roof tile. This means that the tile that is laid on top on the next row has the drainage course resting directly on the nail. Roofers never bang the nails flush to the top of the roof tile because of the danger of splitting the tile. So, the fragile water course sits on the head of the nail.
Any load on the tile and the drainage course snaps.See the picture above.
Sometimes, the fragile water course cracks even without the aid of a nail.
Now, you have a heavy water load on the drainage course due to the tile design – plus the broken water course (that is hidden and hard to see). The two add up to a steady trickle of water into the roof space when ever there is heavy rain.
Check out the video below to see how this happens and what the new roof tile design looks like (to overcome the nail issue):
There are other problems with flat roof tiles.…And the main one is what happens when the roof terminates at walls.
Water seeps between the underside of the lead apron flashing and the top surface of the roof tile along a side wall. This is not a problem with a contoured roof tile because the seepage water usually drips into the trough.
But with a flat roof tile, the seepage continues to travel all the way across the roof tile … and since there are no valleys (troughs) to break the flow of water and carry it down the roof, the seepage dribbles into the roof cavity.