13.1 Roofing Systems
Key Takeaways
- Low-slope roofing includes built-up, modified bitumen, and single-ply TPO, PVC, and EPDM membranes, attached mechanically, fully adhered, or ballasted
- Asphalt shingles generally require a minimum 2:12 slope, with double underlayment required between 2:12 and 4:12
- Positive drainage is required on low-slope roofs; standing water beyond 48 hours after rainfall indicates a slope or drain defect
- Tile roofs in Arizona are watertight because of the underlayment, not the tile — the tile is a wear and shading surface
- Manufacturer installation instructions become the enforceable standard, and a departure usually voids both the warranty and code compliance
Roofing Is Where Warranty Claims Live
Quick Answer: Thermal and Moisture Protection carries 10 of 100 items on the AZ ROC commercial outline and 12 of 100 on the residential outline. Roofing questions concentrate on minimum slope, underlayment, attachment, and flashing, and the governing document in the field is usually the manufacturer's installation instructions.
Roofs generate more complaints than any other single building element, and AZ ROC's workmanship standards treat leaks, exposed fasteners, and missing flashing at penetrations as defects. In Arizona the failure modes are not the ones a contractor from a wet climate expects: the enemies are ultraviolet degradation, thermal cycling, and monsoon wind uplift and wind-driven rain, not months of standing moisture.
Low-Slope Systems
Low-slope roofs (roughly ¼:12 to 2:12) rely on a continuous membrane rather than shedding water by gravity alone.
| System | Description | Notes |
|---|---|---|
| BUR — built-up roof | Multiple plies of felt in bitumen, usually with a gravel or coating surfacing | Durable, heavy, labor-intensive |
| Modified bitumen | Factory-reinforced sheets, torch-, mop-, cold-adhesive-, or self-adhered | Common on smaller commercial roofs |
| TPO — thermoplastic polyolefin | Single-ply, heat-welded seams, typically white | Very common; reflective, good for Arizona cooling loads |
| PVC | Single-ply, heat-welded, chemical resistant | Restaurants, industrial with grease exhaust |
| EPDM | Single-ply rubber, seamed with tape or adhesive | Long service history; typically black |
| SPF — sprayed polyurethane foam | Foam sprayed in place plus a protective coating | Popular in Arizona; seamless, self-flashing, requires periodic recoating |
| Attachment | How it works |
|---|---|
| Mechanically attached | Fasteners and plates through the membrane into the deck; fastest, but every fastener is a potential leak path if misdriven |
| Fully adhered | Membrane bonded to the substrate; best wind performance and smoothest appearance |
| Ballasted | Loose-laid membrane held by stone or pavers; least common on new work, and wind-restricted |
Two requirements govern every low-slope roof:
- Positive drainage. The IBC requires roofs to be designed and built so water drains. Water still standing more than about 48 hours after rainfall points to inadequate slope, a blocked or high drain, or deck deflection — and it is a defect, not a nuisance.
- Secondary (overflow) drainage. Where a parapet can trap water, overflow scuppers or secondary drains are required so a plugged primary drain does not load the roof with water. In monsoon country this is not theoretical.
Steep-Slope Systems
| Covering | Minimum slope (typical) | Notes |
|---|---|---|
| Asphalt shingles | 2:12 | Double underlayment required from 2:12 to less than 4:12; single layer at 4:12 and above |
| Concrete or clay tile | 2½:12 with a code-compliant underlayment assembly | Ubiquitous in Arizona; heavy — verify structural capacity on re-roofs |
| Metal panel, standing seam | Varies by profile; commonly ¼:12 to 3:12 | Follow the manufacturer's listing |
| Wood shakes and shingles | 3:12 to 4:12 | Rare in Arizona; wildfire restrictions apply |
The tile roof point every Arizona contractor should know: a tile roof is not waterproofed by the tile. The underlayment is the water barrier; the tile is a wear surface that sheds most of the water, shades the underlayment from ultraviolet light, and provides the appearance. That is why tile roofs "fail" at 20 to 30 years even though the tile is intact — the underlayment beneath has embrittled. A re-roof that lifts and re-lays tile over new underlayment is standard Arizona work, and quoting it as a repair when the underlayment is spent is a complaint waiting to happen.
Underlayment and ice barriers. Arizona's low deserts have no ice-dam requirement, but the high country does — Flagstaff, the White Mountains, and other elevations adopt the ice-barrier provisions. Never assume a Phoenix detail transfers to a Show Low project.
Flashing is where roofs actually leak: valleys, headwalls and sidewalls, chimneys and crickets, vent penetrations, curbs, and the drip edge at eaves and rakes. AZ ROC's workmanship standards call out missing flashing at penetrations as a defect in its own right.
Arizona-Specific Conditions
| Condition | Effect | Response |
|---|---|---|
| Extreme ultraviolet exposure | Degrades exposed membranes, sealants, and underlayment faster than in most climates | Reflective surfacing; UV-stable products; recoat SPF on schedule |
| Large daily thermal swings | Cyclic expansion and contraction fatigues seams and fasteners | Expansion joints; correct fastener patterns; avoid restraint |
| Monsoon wind uplift | Peels membranes at corners and perimeters | Enhanced perimeter and corner fastening per the wind design |
| Wind-driven rain | Pushes water uphill under coverings | Correct headlaps and side laps; sealed underlayment at vulnerable areas |
| Dust and debris | Clogs drains and scuppers before a storm | Pre-monsoon drain cleaning as a maintenance item |
Roof wind design is zoned: corners take the highest uplift, then perimeters, then the field of the roof. Fastening patterns tighten accordingly, and a crew that uses the field pattern everywhere has built a roof that will unzip from a corner in the first serious haboob.
Exam tip: when a question gives a slope and asks what covering or underlayment is permitted, work from the slope. 2:12 is the floor for asphalt shingles, and 2:12 to under 4:12 requires doubled underlayment. Below 2:12, you are in low-slope membrane territory.
An asphalt shingle roof is being installed at a 3:12 slope. What does the code require?
A 15-year-old Arizona tile roof is leaking although the tiles are unbroken. What is the most likely cause?
Water is still standing on a low-slope membrane roof four days after a storm. What does this indicate?
Why do roof fastening patterns tighten at corners and perimeters?