Why does my tile roof leak when all the tiles look fine?
Because the tiles are not the waterproof layer. A tile roof is two roofs: the tiles shed the bulk of the water, and an underlayment beneath them is what actually keeps it out of the house. Tiles commonly last 50 years or more and the felt underneath them is usually rated 20 to 30, so the normal failure is a roof whose armour is perfect and whose waterproofing has expired. Telling that apart from broken tiles is the difference between a repair and a tear-off, and it is why so many sound tile roofs in Dallas end up as shingle.
What actually fails on a tile roof
Underlayment, first and most expensive. When it goes, the tiles come off, get stacked, new felt goes down and the tiles go back on. It is a big job, but it is a job that renews a roof for another few decades rather than replacing it with a material that will need doing again in fifteen years.
Ridge and hip pointing, second and far cheaper. The tiles along the ridge are bedded and pointed in mortar, and that mortar cracks and washes out. Water then enters at the very top of the roof, which is the worst possible place, and re-pointing is one of the highest-value small repairs on any tile roof.
Individual broken tiles, third. Usually from hail, a fallen branch, or somebody walking on them. Concrete tile has a load path along the headlap and the batten; step in the middle of one and it cracks under your weight, which is how an inspection generates its own repair list.
Debris, fourth and cheapest to prevent. Leaves and grit accumulate in the tile channels, water backs up over the headlap and reaches the underlayment. Clearing them is unglamorous and it is what stops the expensive version of this job.
Slate fails around the slate, not because of it
Good slate lasts between 75 and 200 years. The nails holding it and the flashing woven through it do not, so an old slate roof is normally a flashing and fastener problem rather than a material one.
The material does eventually end, and the test for it is sound. Tap a slate: a sound one rings, a failing one thuds, because it is delaminating along its layers. Flaking at the exposed edges is the visual version. If a large share of a roof thuds, patching has stopped making sense.
How a slate is replaced matters enormously. It is held by two nails under the slate above, so the correct method is a slate ripper to cut the old nails and a copper bib to retain the new piece. The shortcut is face-nailing the replacement through its visible surface and sealing the nail heads with caulk. That is visible from the ground for the next fifty years and it starts leaking within five, and it is the most common way a sound slate roof is quietly ruined.
Why the tear-off gets recommended anyway
Not usually dishonesty. Tile and slate need specific tools, specific technique and crews who have done it, and a company set up to replace composition shingle efficiently has none of those. Faced with a tile roof, the honest version of that company's answer really is that they should not touch it.
The cost of that is borne by the homeowner, because what gets recommended instead is a tear-off, and a tile or slate roof that has been shingled over cannot be put back. It is worth asking any contractor directly what they would repair on your roof, and asking a tile question of anyone quoting on tile: what is under the tiles, and how do they know.
Related questions
How long does underlayment last under tile?
Typically 20 to 30 years for traditional felt in North Texas heat. Modern synthetic and self-adhering underlayments last longer, which matters because relaying tile is expensive labor you would rather do once.
Can broken tiles be matched?
Often, and the best match usually comes from the roof itself: tiles taken from a hidden back elevation and replaced there with the nearest current profile, so the mismatch ends up where nobody sees it.
Is a slate roof worth keeping in Dallas?
Almost always, if the slate itself is sound. It is a material that outlives everyone involved in installing it, and the alternative is a shingle roof that will need replacing three or four times over the same span.
How can I tell if a roofer knows tile or slate?
Ask what they will do about the underlayment on a tile roof, and ask whether they own a slate ripper. Both questions are answered instantly by someone who works the material and awkwardly by someone who does not.
