Tile Roof Underlayment Replacement: How a Lift-and-Relay Works

A tile roof is two systems stacked together. The tile is the armor: it takes sun, hail and foot traffic and sheds most of the rain. The TRI Alliance installation manual describes a properly installed tile roof as "a highly effective water shedding assembly."¹ Shedding is not sealing. Tiles overlap with a head lap and interlock at the sides, but those joints are not watertight. Wind-driven rain, water backing up at a valley, and runoff at a wall all put some water under the tile.

That water lands on the underlayment, the membrane laid across the roof deck, which carries it down to the eave. This is why the manual calls the underlayment "critical to the success of the roof system."¹ The tile protects the underlayment from sun and impact. The underlayment protects the house.

The two layers age at very different rates. Atrium's Central Texas planning ranges are 40–50 years for concrete tile and 50+ years for clay tile, but 20–30 years for the underlayment under either one. Attic heat below and trapped heat above cook asphalt-based underlayments over time. The membrane dries out, gets brittle, cracks at the batten lines and fastener holes, and stops carrying water to the eave. The tile above it can look perfect the whole time.

The failure is usually concentrated where water is concentrated: valleys, the lower edges of walls and chimneys, around vent pipes, and the eave courses that carry the whole slope's runoff. That is why tile leaks often look random from inside the house. Water enters at one spot, travels on top of the failing membrane, and comes through somewhere else.

You can't see underlayment from the ground or from on top of the tile. The useful evidence is indirect:

• Age. If the original underlayment is near or past the 20–30 years range, treat it as suspect even without a leak.
• Attic evidence. Water stains on the underside of the deck, damp insulation after rain, or daylight at the deck seams.
• Leaks in more than one place, or a leak that comes back after a cracked tile was replaced.
• Brittle material at the edges. At the eave or at a lifted tile, old felt that cracks or crumbles when bent is past its working life.

The direct test is to lift tile in sample areas, especially valleys, the bottom of walls and the eave, and look at the membrane itself. That is part of a proper tile inspection. Atrium's free inspection includes a written report you keep, so you can compare it against any other quote.

1. Stage and protect. The work area is protected, and the tile plan is set: which slopes come off first, and where tile will be stacked so it doesn't overload any one spot.
2. Lift and stack the tile. Tile is removed by hand, sorted, and stacked. Some breakage is unavoidable on any relay, which is why the scope should say how broken tile will be replaced.
3. Strip to the deck. Old battens, underlayment, fasteners and flashings come off, so the deck can be seen.
4. Inspect and repair the deck. The TRI manual tells installers to verify the deck has no significant delamination or warping and to repair it as needed to meet local code.¹ Rotted or delaminated sheathing gets replaced here, while it is exposed.
5. Install new underlayment in the number of layers the slope calls for, lapped shingle-fashion so water always runs over a lap, never into one.
6. Install new flashings and battens. Valley metal, wall pans, headwall flashing and penetration flashings go in, then battens with drainage gaps.
7. Relay the tile. Sound tile goes back on, fastened as the manual requires. The manual is explicit that cracked or broken tile must be removed from the roof, so damaged pieces are replaced with salvaged or new tile.¹
8. Close out. Hips, ridges and rakes are finished, the site is cleaned, and you get the documentation.

The point of the process is that everything you can't see afterward is new. That is also why it is the moment to fix ventilation and any problem in the deck. For how this compares with spot repair and full re-tiling, see tile roofing.

The TRI Alliance manual lists ASTM D226 Type II felt (No. 30 felt²), ASTM D2626 or ASTM D6380 Class M mineral-surfaced roll roofing, and self-adhered membranes meeting ASTM D1970 as underlayments. For synthetic underlayments it directs installers to the underlayment manufacturer's instructions.¹

Slope sets the layer count. For roof slopes from 2.5:12 up to 4:12, the manual calls for a minimum of two layers; at 4:12 or steeper, a minimum of one. Tile on slopes below 3:12 is considered decorative and goes on counter battens.¹ Low-slope tile sections, like porches tied into a main roof, are where a one-layer shortcut causes trouble first.

Heat is the Central Texas factor in the choice. Under tile, the underlayment sits in a hot, dark space for decades, so the product should be rated by its manufacturer for use under tile and for high temperatures. The trade-offs between felt and synthetics in general are covered in synthetic underlayment vs felt. Whatever is chosen, the proposal should name the product and the number of layers.

Battens are wood strips fastened across the slope that the tile hooks onto. On slopes of 4:12 to 24:12, the manual has them fastened to the deck at no more than 24 inches on center, with drainage provided by a ½-inch gap at the batten ends every 4 feet or by shimming.¹ Those gaps matter. A continuous batten is a dam: water and debris that get under the tile pile up behind it, and the underlayment at that line fails first. In its cool and humid climate category, the manual requires raised, scalloped, counter or other batten systems that provide positive drainage.¹

Flashing handles the places where tile alone can't. The manual's minimums include valley flashing that extends 12 inches each way from center with a 1-inch splash diverter rib, wall pan flashing with a minimum 6-inch trough, and headwall flashing with at least a 3-inch lap over the tile. Metal flashing must be at least 0.019-inch galvanized (G90) steel or one of the other metals the manual lists.¹

Old flashing is usually as old as the old underlayment, and it is only accessible while the tile is off. Reusing it saves little and leaves the most leak-prone parts of the roof at their original age. How each flashing type fails is covered in flashing failure modes.

• Whether the job reuses your existing tile or supplies new tile.
• The underlayment product by name, and the number of layers on each slope.
• New battens, their fastening, and how drainage is provided.
• Which flashings are replaced: valleys, wall pans, headwalls, penetrations.
• How deck repairs are handled and priced if rot is found.
• How broken tile is replaced, and the plan if your profile is discontinued.
• Whether attic ventilation is being addressed while the roof is open.

Two tile quotes on the same house can differ a lot simply because one is a relay and the other is all-new tile, or because one replaces flashings and the other doesn't. Ask for the scope in writing so the comparison is real. Our tile roofing page lists ballpark ranges, and Atrium quotes the final price per roof after inspection. General cost drivers are on the roof cost guide.

Central Texas heat is the main reason tile underlayment wears out on the schedule it does here. Long summers keep the space under the tile hot for months, and asphalt-based membranes age faster with heat. Hail is the second factor: it cracks tile, and every cracked tile puts more water on an underlayment that may already be tired. After a hail event on a tile roof, document cracked pieces slope by slope; see hail damage.

If your tile home in San Antonio or the Hill Country still has its original underlayment and is approaching that range, plan the relay before it becomes an emergency. If you are planning solar on a tile roof, sequence the relay first, because removing an array later to reach the underlayment is avoidable cost; see solar panel detach and reset.

If the underlayment still has life and the problem is isolated, such as a few cracked tiles, one valley, or one wall flashing, repair is the right call. Atrium's rule of thumb: repair wins when it costs under 20 percent of the full lift-and-relay. Once leaks show up in several places, a repaired spot leaks again nearby, or the original underlayment is past its expected life, spot repairs are chasing a membrane that is failing everywhere, and a relay is the better use of money. All-new tile is only needed when the tile itself is failing or can't be matched. Small jobs are scoped on roof repairs.