Tile roofs leak with nothing broken
Tile works as a rain screen more than a waterproof skin. It sheds most of the water, but wind-driven rain gets under the laps and runs down the underlayment to the eaves. The underlayment is the actual water barrier, and on a tile roof it spends its life in a hot, dark space under heavy material.
Clay and concrete tile can outlast the house. The felt under most older tile roofs does not. Years of heat cook the oils out of it until it cracks and tears, worst at the eaves, in valleys and around penetrations where water collects. The result is a roof that looks perfect from the street and stains the ceiling inside. When a roofer lifts a few tiles near a leak and the felt crumbles, the rest of the field has had the same years of heat and is rarely in better shape.
Shallow tile roofs fail sooner because water moves slowly and more of it gets under the tile. Tile is allowed down to a slope of 2.5:12, but IRC chapter 9 requires a double layer of underlayment below 4:12. The estimate prices a single layer, so a roof in that band needs more material and time than shown. The roof pitch calculator tells you which side of 4:12 you are on.
How a lift and relay is priced
The calculation follows the job as a tile crew does it. Tiles come off course by course and are stacked on the roof or the ground, which is the largest block of the 45 person-hours. The old felt and battens are stripped and the deck is checked for soft spots. Synthetic underlayment ($202 to $392) goes down with new drip edge at the eaves, and the tiles go back in their original courses. Tile labor is billed here at $92 an hour, above the rate this site uses for shingle replacement.
Some tiles crack in handling, old clay especially, and the estimate carries spares for about 5 percent breakage at $125 to $250. That allowance only helps if replacements exist. Profiles and colors get discontinued, and a roof whose tile is no longer made has to borrow from salvage yards or from a slope nobody sees. Ask where the spares will come from before the first tile is lifted; tile replacement covers the cost of swapping broken ones.
Not priced: new battens (the old ones are stripped, but no batten line is carried, so ask whether replacements are in the quote), rotten decking found under the felt, new flashing at walls and chimneys, and re-bedding ridge and hip tiles where they were set in mortar. Flashing is worth replacing while the tile is off, since the next chance is the next relay.
Felt or synthetic under tile
Felt is a paper or fiberglass mat saturated with asphalt. It is cheap and familiar, and it is what most aging tile roofs have. That is much of the reason they are leaking now: the asphalt slowly bakes out under hot tile.
Synthetic underlayment is woven or spun polypropylene with a coating. It is lighter, much harder to tear when a crew is walking among stacked tile, and the high-temperature grades are made for use under tile and metal. The estimate prices synthetic for those reasons. In hurricane-prone regions, local rules may call for a self-adhered membrane or a specific underlayment system under tile, so find out what the building department expects before comparing quotes.
Whatever goes down is the part of the roof you are paying for this time. A quote that names the product, the number of layers and how the laps are sealed tells you far more than one that says "new underlayment".
On an asphalt roof, this is a replacement
Shingles do not survive being lifted. Each one is fastened through the underlayment by nails that also pin the course beneath it, and glued to its neighbor by a sealant strip, so pulling it tears the nail holes and usually cracks the mat. By the time the underlayment is exposed, the roof has been torn off. If you have a shingle roof and have been told the underlayment is failing, what you are pricing is a roof replacement with tear-off, underlayment and new shingles.
A leak in one spot is different. A roofer can strip a small area, replace the underlayment there and weave in new shingles, which is a repair you can price with the roof repair cost calculator. To count underlayment rolls for a full re-roof, the roof shingle calculator includes them in its material list.