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

Underlayment Calculator

Underlayment is the layer between the deck and the shingles. 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.

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.