Roof underlayment is the layer that keeps water out
Roof underlayment is the water-resistant layer fitted between the roof deck and the covering you can see. On a tile roof it is the part that actually keeps water out of the house. The tiles shed most of the rain and shield the layer beneath, but the underlayment is the barrier.
That one fact explains most of the confusion homeowners have about tile roofs here. A roof can look sound from the street, with every tile in place and no obvious damage, while the layer doing the real work has already reached the end of its life.
It also explains why the honest answer to “how is my roof doing?” cannot be given from the driveway. The component that decides the timing is the one you cannot see without lifting tiles.
Heat ages the underlayment faster than the tile
Heat and ultraviolet exposure age the underlayment faster than weather wears the tile. At the Scottsdale Municipal Airport weather station, NOAA’s 1991 to 2020 normals average about 93 days a year at or above 100 degrees Fahrenheit and about 161 days at or above 90.
An attic under a tile roof in that climate runs hotter still, for most of the year (NOAA NCEI). A City of Scottsdale homeowner guide describes attic heat expanding and contracting roofing components and working nails loose over time, which is the same daily cycle that embrittles the membrane.
Concrete and clay tiles, by contrast, are close to inert. They crack when something strikes them or someone walks on them, but heat does not consume them. The result is a roof where the visible material outlives the hidden one by a wide margin, which is the opposite of how most people assume a roof ages.
The Phoenix-area record gives a sense of the exposure involved. National Weather Service figures show an average of 21 days a year at or above 110 degrees over the 1991 to 2020 period, and 2024 alone produced 70 such days (NWS Phoenix).
Felt, synthetic and the main underlayment types
There are three families in common use. Asphalt-saturated felt is the traditional product, sold in weights such as 30-pound. Synthetic underlayment is a woven polymer sheet. Self-adhered membrane, sometimes called peel and stick, bonds directly to the deck and is usually reserved for the areas that take the most water.
Felt is the material most older local roofs were built with, and it is the one that becomes brittle with age. As it dries out it loses flexibility, then cracks along fastener lines and at laps, which is where leaks start.
Synthetic underlayment is lighter, tears less readily during installation and generally tolerates heat and ultraviolet exposure better than felt. It is now the common choice on replacement work in this climate, which matters because the point of the exercise is to buy another two or three decades.
Self-adhered membrane is the premium option and is often specified in valleys, at eaves and around penetrations rather than across the whole roof. Those are the places where water concentrates, so the extra cost is spent where failure is most likely.
One thing worth knowing before comparing products: no underlayment is designed to be a finished roof surface. All of these materials are rated for a limited period of sun exposure during construction, after which the covering is meant to be on. A membrane left exposed, or left exposed by tiles that have slipped, ages far faster than the same product properly covered.
Which specification a quote actually includes is worth checking line by line, because it is one of the easiest places for two quotes to differ without either being wrong. Replacing tile underlayment: what it costs and when it is due covers how to read that part of a quote.
Tile roof underlayment is the layer that sets your timetable
On a tile roof the tiles are a rain screen and a sunshade, and the tile roof underlayment is the waterproofing. That division of labour is why the standard repair is not a new roof: the tiles are lifted, the old membrane is stripped, new underlayment goes down and the same tiles are re-laid.
Scottsdale’s housing stock makes this a common conversation locally. Census American Community Survey 2023 five-year estimates put 102,546 of the city’s 138,422 housing units, about 74 percent, as built before 2000, and the city’s own homeowner guide records that clay and concrete tile became the material of choice on Scottsdale homes in the 1990s.
Put those two together and a large number of local tile roofs are carrying original underlayment that has now spent two or three decades under this sun (US Census Bureau). Those are survey estimates with margins of error, so treat them as a picture of the housing stock rather than a count.
Service life in this climate
There is no honest single number, and anyone offering one without looking at your roof is guessing. Service life depends on the material used, how hot the attic runs, how much ventilation the roof has, how well the original installation was detailed, and how much direct sun each slope takes.
What can be said is the direction of travel: this climate is harder on underlayment than most, and a felt membrane installed in the 1990s is well into the part of its life where an inspection is worth more than an assumption.
We have deliberately not published a lifespan figure for local conditions, because no measured source for one exists that we have been able to verify. The way to turn the question into an answer is to have the membrane looked at directly, in several places, and get the finding in writing.
Signs the underlayment is failing
The symptoms of a failing underlayment show up indoors before they show up on the roof, which is why they are so often misread. A stain that appears only in heavy, wind-driven rain is the classic one, because light rain never reaches a defect sitting under a tile.
Leaks in more than one place, or a leak that moves, point at the membrane rather than at a single broken component. One failed pipe boot produces one stain in one spot. An underlayment at the end of its life produces several, often in different parts of the house.
Debris in the gutters that looks like dark grit or fibre can indicate felt breaking down, although it is not conclusive on its own. Slipped or displaced tiles are worth attention too, because tiles that move expose the membrane to direct sun and accelerate exactly the process described above. Cracked, broken and slipped roof tiles covers what causes that movement.
The one sign that is not reliable is the look of the roof from the ground. A tile roof in good visual order tells you about the tiles and nothing at all about what is underneath them.
A replacement is a strip and re-lay, not a demolition
The job is a strip and re-lay rather than a demolition. Tiles are lifted and stacked, the old underlayment is removed, the deck is inspected and repaired where needed, new underlayment and flashings go on, and the tiles are re-laid with replacements for whatever broke in handling.
Breakage during handling is normal and should be allowed for in the quote rather than discovered as an extra. Ask how replacement tiles will be matched, particularly on an older roof where the original profile or colour may no longer be made. Concrete and clay tile in Arizona covers the differences between the two and what that means for matching.
Flashings are the part worth asking about explicitly. Replacing a membrane while re-using tired flashing at the valleys, walls and penetrations leaves the most leak-prone details untouched, and those are the places water concentrates.
The deck underneath is the other variable. Most decks are sound and need nothing, but where water has been getting in for a while there may be damaged sheathing to replace, and that is work nobody can quote for accurately until the tiles are off. A quote that says nothing about how deck repairs will be priced is a quote with an open end in it, so ask for that rate in writing before the job starts rather than after something is found.
Permits, inspections and who does the work
Permit requirements depend on the work and on the city, so confirm your specific job locally before assuming either way. Scottsdale adopted the 2021 International Residential Code, along with the 2021 IBC and IECC, effective in January 2023 (City of Scottsdale).
The city’s older homeowner guide ties the permit trigger to work on the roof sheathing or structural framing, lists flashing and felt as one of the inspection stages, and says the property owner is ultimately responsible for making sure a permit is issued. That guide predates the current code, so treat it as the city’s published guidance rather than a current rule and confirm the requirement for your own job.
Keep the paperwork afterwards. Arizona law allows a complaint to be filed with the Registrar of Contractors within two years after a project is completed (Arizona Revised Statutes 32-1162), which is a good reason to document the work as it happens.
One safety point applies throughout. Tile is brittle and breaks underfoot, and the city’s own guide says roof work can be dangerous for the inexperienced. Assessing underlayment means lifting tiles on a roof, which is not a homeowner task.
Where to start
If your tile roof dates from the 1990s or earlier and the underlayment has never been replaced, the useful next step is an inspection that lifts tiles in several places and reports what the membrane is actually doing. That converts an unknown into a date you can plan around.
If you already have a stain or a leak, start there instead, and treat the underlayment question as the thing the diagnosis will settle. Our guide to roof problems in Scottsdale sets out how the different failures present and which is which.
If you would rather have someone look at it, Residential Roofing Scottsdale works across Scottsdale and the surrounding Valley. Call (480) 919-1173, or read our roof replacement in Scottsdale page for what the work involves.
Frequently asked questions
These are the questions homeowners in this area ask most often about roof underlayment, the layer most of them had never thought about until a stain appeared. Each answer is kept short deliberately, with a link to the detailed article where one exists, so you can find the right starting point.
What does roof underlayment do?
It is the water-resistant layer between the roof deck and the covering. On a tile roof it is the barrier that actually keeps water out of the house, while the tiles shed most of the rain and shade the layer beneath them from the sun.
Can you replace underlayment without replacing the tiles?
Yes, and on most tile roofs that is the standard job. The tiles are lifted and stacked, the old membrane is stripped and renewed, and the same tiles are re-laid, with replacements for any that break during handling.
Is synthetic underlayment better than felt in Arizona?
Synthetic products are lighter, tear less readily during installation and generally tolerate heat and ultraviolet exposure better than asphalt-saturated felt. Which specification a quote includes is worth checking line by line, because it is an easy place for two quotes to differ.
How do I know if my underlayment has failed?
The signs usually appear indoors first: a stain that only shows in heavy wind-driven rain, or leaks in more than one place. The look of the tiles from the ground tells you nothing about the membrane underneath them.
Does replacing underlayment need a permit in Scottsdale?
It depends on the scope of the work, and the city’s published guidance on this point is dated. Confirm the requirement for your specific job with Scottsdale’s permit services before any work starts rather than relying on a general answer. See do you need a permit to re-roof in Scottsdale? for what is known.
Can I check the underlayment myself?
No. Checking it means lifting tiles while on the roof, tile is brittle and breaks underfoot, and the city’s own homeowner guide calls roof work dangerous for the inexperienced. This is a job for a professional with the right access equipment.