The layer that decides when a tile roof is finished
A tile roof is two roofs. The tile is the one you can see, and it sheds most of the water and all of the sun. The underlayment underneath is the one that keeps the house dry, and it is on a far shorter clock than the tile above it.
- 1 The ridge. The highest, hottest course, and the one the sun works on all day.
- 2 The field. Tile or shingle above, and the underlayment that actually keeps water out below.
- 3 Valleys. One channel carrying what two slopes shed, at the pace this coast rains.
- 4 Flashing. Where the lanai and the enclosure meet the house, and where neither trade owns the seam.
- 5 Penetrations. Every vent and curb is a hole the underlayment has to seal, and the first place it stops.
- 6 Eaves and metal. Drip edge and fasteners, which salt air reaches years before it touches the covering.
Why the visible layer misleads
Concrete tile is close to inert. It can sit on a roof for decades looking much as it did on day one, which is exactly the problem: nothing about its appearance reports on the material beneath it. Owners routinely conclude a roof is young because the tile is unmarked.
The underlayment lives in a harsher place than the tile does. It sits in a ventilated cavity that runs hot for most of the year, and heat is what ages it. When it goes brittle it stops sealing around the fasteners that hold everything above it, and every one of those fasteners is a hole.
Where it opens first
Penetrations lead. Plumbing vents, mechanical curbs and anything cut into the plane are where the underlayment is asked to turn a corner and stay sealed, and they are where the first leaks appear on almost every tile roof on this coast.
Valleys come next, because a valley concentrates the water of two slopes into one narrow run and asks the underlayment to carry it. Then the perimeter, where the covering has the least depth of material working for it.
What that means for the decision
It puts a real number on a vague question. If the underlayment is the limiting layer, the useful age of a tile roof is the age of that layer, not the age of the tile. On much of the housing here that means the twenty-year mark is the one to plan around, whatever the tile looks like.
It also changes what a good quote contains. Removing and resetting tile over new underlayment is a different job from tearing off and replacing, with different labour and a different result. A quote that does not say which one it is has left out its largest variable.