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P Property Improve Roof calculator Methodology About Reviewed by a Texas-licensed GC

Property Improve · Roofing

Underlayment Calculator

Get the right number of rolls — synthetic, 15# felt, or 30# felt — accounting for overlap and current pricing.

Inputs

Use the roof sqft, not home sqft. Estimate it here.
Underlayment laps each course over the next. 15% is typical for synthetic; 18–20% for felt (more laps, narrower rolls).
Tear-resistant, walkable, UV-stable for 90+ days. Adds 20–35 yr to the underlayment system.
Effective area (with overlap)2,760 sqft
Rolls needed3
Material cost range$270–$480
Why synthetic is worth the upcharge. On a typical 2,400 sqft roof, synthetic costs $250–$450 vs $80–$200 for 15# felt. The synthetic upcharge is ~$200 — and synthetic adds an extra 15–20 years of failure-resistance, plus tearing protection during a partial tear-off and re-cover. Cost-of-failure math heavily favors synthetic.
→ Ice & Water Shield Calculator
For eaves, valleys, and low-slope sections — the membrane that goes UNDER the underlayment.
→ Roof Replacement Cost Calculator
Underlayment is bundled into the material per-square cost on the main calc.

More about this estimate

The synthetic vs felt fight, settled

Synthetic underlayment is now standard on virtually every residential roof in the South. The case is straightforward:

  • Tear resistance — felt rips when a contractor walks on it. Synthetic doesn't.
  • UV stability — synthetic stays usable for 90+ days exposed; felt degrades in days.
  • Lighter — one synthetic roll covers ~1,000 sqft vs 432 sqft for 15# felt. Faster install.
  • Lifespan — synthetic is rated for the life of the shingle; felt is not.

On a typical 2,400 sqft roof, the upcharge from felt to synthetic is $200–$400. Cost-of-failure math heavily favors synthetic; if your bid quotes 15# felt, ask why.

When 30# felt makes sense

30# felt is heavier and slightly more durable than 15#. It's required by code under tile roofs in some jurisdictions, and sometimes specified for low-slope sections. Outside of those use cases, synthetic outperforms it on every dimension.

The order of layers, top to bottom
  1. Shingles or panels
  2. Underlayment (this calculator) — covers the entire roof
  3. Ice & Water Shield at eaves, valleys, low-slope sections, and around penetrations (calculator)
  4. Drip edge at eaves goes under I&W; drip edge at rakes goes over underlayment (calculator)
  5. Roof decking (OSB or plywood)
  6. Rafters / trusses
About this calculator

Reviewed by Eurocraft, a Texas-licensed general contractor. Pricing reflects mid-2026 South-region distributor cost.