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Roofing Underlayment Calculator

Estimate how many rolls of synthetic roofing underlayment you need beneath your shingles.

  • Answers as you type
  • Every formula cited
  • Calculated in your browser
SettingsSettings for this calculationUS
Market
Imperial · sales tax
The total sloped roof surface area, not the flat footprint.

Use the same roof surface area (accounting for pitch) as the Roofing Shingle Calculator.

Membrane spent on side and end laps, and on the double layer run in valleys.

A steep, simple gable laid at the product's minimum laps sits near ten. A low-slope roof laid with deeper end laps, a wide-lap product, or a roof of many valleys and hips run double needs more.

Underlayment rolls needed

3 rolls

High confidence
Area to cover (with overlap allowance)
2,365 sq ft
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • A standard synthetic roofing underlayment roll covers approximately 1,000 sq ft (10 roofing squares)

Inputs used

Roof Surface Area
2150 sq ft
Overlap Allowance (%)
10

Intermediate steps

Area to cover (with overlap allowance)
2,365 sq ft
Final result3 rolls

What this calculation does not cover

  • Overlap is one percentage of your area rather than a lap width, so at the same setting a wide-lap product or a shallow roof laid with deeper end laps gets exactly the same allowance as a steep one; raise it for those.
  • Coverage is divided by a single 1,000 sq ft (93 m²) roll figure and then rounded up to a whole roll, so the offcut left on the last roll is never reported and two roofs a hundred square feet apart can return the same number to order.
  • One area figure is all the arithmetic ever sees — valleys and hips where the membrane is run double, and skylights or chimneys you would normally deduct, are all left to that same overlap allowance.
  • Your figure is used exactly as typed with no slope multiplier anywhere in the calculation, so entering the plan-view footprint instead of the sloped surface comes out short at every pitch above flat.
  • Eave and valley areas that will be covered by self-adhered membrane instead are not subtracted before the division, which means the doubled-up zones are ordered as full underlayment as well.
  • Cap nails, staples, seam tape and the drip edge that runs beneath it fall outside the count, which returns rolls of membrane and nothing else.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Part of bigger jobs

This trade is one line of several job takeoffs. Run the whole job and every other trade comes back with it, off the same measurements.

Or plan the space itself: measure it once, doors and windows included, and this figure comes back worked out on that space, with what goes with it.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-17 · v1.1.0

Regulatory standards & verification citations1
  1. A standard synthetic roofing underlayment roll covers approximately 1,000 sq ft (10 roofing squares)
Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Tools and safety for this job

To lay the roof underlay. Generic types, no brands, no prices.

Protection this work requires

  • Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
  • Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.
Show the 3 tools this job needs

Essential

  • Tape measure

  • Utility knife

Recommended

  • Staple gun

Also needed as materials: replacement blades, staples.

what each roofing tool is for, and the spec that decides which to buy where one does.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Called something else where you work? Sarking and roof underlay · Eaves · Roof valley — the term in each market, how close the equivalence really is, and the standard that governs it.

Already have some? How much roof will my rolls of underlayment cover?

Worked example: Shingle roof replacement — step 6 of 11

The data behind it: Production rates by operation

How to calculate roofing underlayment in 3 steps

  1. Roof Surface AreaThe total sloped roof surface area, not the flat footprint.
  2. Overlap Allowance (%)Membrane spent on side and end laps, and on the double layer run in valleys.
  3. Underlayment rolls neededThe tool computes the underlayment rolls needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Underlayment rolls needed by roof surface area

Page defaults, not your figures above.

Roof Surface AreaUnderlayment rolls needed (rolls)
1,000 sq ft2
1,500 sq ft2
2,000 sq ft3
2,500 sq ft3
3,000 sq ft4
3,500 sq ft4
4,000 sq ft5

Frequently asked questions

Is underlayment the same as ice & water shield?
No — underlayment is a lighter-duty water-resistive layer covering the full roof deck, while ice & water shield is a heavier, fully self-adhered membrane typically used only at eaves, valleys, and other vulnerable areas. Most roofs use both.
Felt or synthetic underlayment — does coverage differ?
Yes — traditional asphalt felt is commonly sold in smaller rolls (covering 2-4 squares), while modern synthetic underlayment is typically sold in larger rolls (around 10 squares) — this calculator assumes the more common synthetic roll size.
Why is underlayment needed if shingles already shed water?
Underlayment acts as a secondary water barrier in case wind-driven rain, ice damming, or a failed/damaged shingle lets water reach the deck — it's a required part of most modern roofing systems and building codes, not optional.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.