I have concrete roof tiles of the following design:

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Water started dripping through the ceiling and I located the leak to the gap between 2 tiles after poking a hole in the ceiling (it was rotten anyway) and pulling away the felt.

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I went on the roof and it struck me what a poor design the tiles were. Unlike flat clay tiles where the nth row fully overlaps the (n-2)th row, there is a considerable gap between those rows where it is down to the intermediate row to keep the water out. Also, unlike terracotta type tiles where the whole tile forms a deep and wide gulley for water to travel down and has a lip that comes right up and underneath the tile next to it, it simply has 2 pretty pathetic, thin, shallow looking gulleys that interlock with the tile next to it that IMO could easily get blocked up with moss or dirt and overflow; moss was actually bunging up the end of it, which I removed.

I completely cleaned out the interlocking in question by shoving a rod in the two gulleys, and ran a hose on the roof downwards and away to simulate rain, and the water was still dripping through, in fact water was gushing out of both gulleys. It had gone into the gap between the tiles, started gushing out the first gulley, overflowed into the second gulley and started gushing out of that, so clearly it wasn't going to take long to spill over the 2nd gulley.


  • 2
    How old is the roof.
    – JACK
    Jun 25, 2022 at 20:31
  • @JACK 50 years. Jun 26, 2022 at 21:22

1 Answer 1


So you have a roof that has lasted 50 years and you think you've discovered a poor design? (I'm not chuckling). Those little overlapping rows aren't there just meant to keep water out, they're meant to secure the tiles to each other. Driving wind and rain can also force water under the tiles. The underlayment and flashing provide the main waterproofing for a roof and they are usually the first items to go.. The tiles protect the underlayment. Many tile roofs have strips at the bottom that have weep holes in it to allow any water that got under the tiles to drain out. My guess would be that your underlayment has started to deteriorate. The heat over many years can do that and 50 years ago, the underlayments were not as good as they are today. When my concrete tile roof started to leak after 30 years, we have extreme heat in Florida, I told the roofer the tile roof was supposed to last 50 years. He laughed and said "The tile will last 50 years, the underlayment will last 30".

You can remove the tiles in the area where the leak occurred and replace that part of the underlayment. If you determine that the whole underlayment is starting to crack, then it's time to have it replaced. You can reuse you roof tiles if you have them removed carefully.

Using a hose to simulate rainfall never works and exaggerates the outcome.

  • so are you saying that it's okay for some water to drip onto the underlayment as long as it's installed correctly? Admittedly, the water between those two tiles only became an issue and dripped through the ceiling because the underlayment was coming away near the edge of the roof and was funneling the water down near the edge of the ceiling. I ripped it away slightly further to expose the tiles, it was breaking off very easily anyway Jun 26, 2022 at 22:54
  • @LewisKelsey Yes, it's OK. After a hurricane, people will get a new underlayment , felt & roll roofing, and wait for weeks/months to get the tile installed
    – JACK
    Jun 26, 2022 at 23:08

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