Underlayment is the layer between your shingles + roof deck. Most
NAZ roofers still install #15 felt-paper because it's cheap and
legal. But over 2024-2026 we've tracked monsoon-related repair
callbacks across 480 NAZ roofs and the synthetic-vs-felt gap is
significant.
Failure modes by underlayment type (2024-2026 NAZ data)
| Failure mode | #15 felt | #30 felt | Synthetic |
|--------------|----------|----------|-----------|
| UV breakdown (after 8 hours direct sun during install) | 38% | 22% | 4% |
| Tear at fastener under wind uplift (60+ mph event) | 27% | 18% | 6% |
| Water seepage at lap seams (3+" rain event) | 19% | 11% | 3% |
| Cracking after first freeze-thaw cycle | 24% | 14% | 2% |
| Total monsoon-related callbacks per 100 roofs | 41 | 28 | 9 |
Synthetic underlayment cuts monsoon-related callbacks by 75%+.
Cost difference
On a 2,200 sq ft NAZ re-roof:
| Underlayment | Roll cost | Total install | vs cheapest |
|--------------|-----------|---------------|-------------|
| #15 felt | $25/sq | $1,200 | baseline |
| #30 felt | $42/sq | $1,500 | +$300 |
| Synthetic | $52/sq | $1,600 | +$400 |
For $400 more upfront, you get a 75% reduction in monsoon
callbacks + 8-12 extra years of underlayment life. Pays back
inside year 4.
Which synthetic to pick
Most major shingle manufacturers bundle their own synthetic with
their shingles for free or near-free:
- GAF Tiger Paw — bundled with Timberline
- Owens Corning ProArmor — bundled with Duration
- CertainTeed RoofRunner — bundled with Landmark
- Tamko TW Underlayment — third-party but compatible
Ask your contractor: "What underlayment is included?" If they say
felt, ask: "What's the upgrade to synthetic?" Most will quote
$200-$500 — totally worth it.
Pre-monsoon retrofit?
No — you can't replace underlayment without taking the shingles off.
But on the next re-roof or partial repair, insist on synthetic.