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Roof Underlayment Calculator

R905.1.1, the doubling bands, and R905.1.2

Two coverings, two double-underlayment bands, and they share a top but not a bottom. At exactly 2:12 asphalt needs two layers and tile is not permitted at all.

Underlayment takeoff

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The doubling bands are quoted from R905.2.2 and R905.3.2, and the ice barrier from R905.1.2. Roll dimensions are yours, off the product.

Specification summary

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Every minimum in R905, on one axis

Prices for roof coverings and tear-off

The published figures for roof coverings and tear-off are on the roof replacement cost calculator, with the byline, the publication date and the caveats that go with them. They are not repeated here, because the same table on two pages is worth less than one table you can find.

This site publishes no price of its own. Why, and what that means for these figures.

Worked example

A 40 ft by 28 ft roof at 3:12 in asphalt shingles, 24 in overhang, ice barrier required, 36 in rolls.

  1. 3:12 is inside the double band, so this roof needs two layers rather than one. R905.2.2 doubles the underlayment from 2:12 up to 4:12, and that is where most people order half of what they need.
  2. A 36 in roll covers 34, not 36. The head lap takes 2 in off every course, so a 300 sq ft roll covers 283. That is 5.6% of every roll you buy and never see.
  3. Thirteen rolls, which is eleven for two layers over 1,452 sq ft plus 10% waste.
  4. The ice barrier is 49.5 in wide, not 24. R905.1.2 measures to a point 24 in inside the exterior wall line, so the 24 in overhang is added first and then the slope factor applies. A 36 in roll does not reach, so it is two courses.
  5. And the two doubling bands do not line up. Asphalt runs 2:12 to 4:12, tile runs 2 1/2:12 to 4:12. Same top, different bottom, because tile is not permitted below 2 1/2 at all.

Underlayment is the cheapest thing on a roof and the layer that is still working when the covering has failed. The two places it goes wrong are the doubling band and the ice barrier width, and both are arithmetic.

The formula

Two things get multiplied that people leave at one:

rolls = area × layers / [(roll width − head lap) × roll length]
layers
2 inside the doubling band for your covering, 1 above it
roll width − head lap
what a roll actually covers, which is not what it measures
area
the sloped roof area including the overhang

On a 3:12 asphalt roof both terms bite at once: two layers, and 5.6% off every roll. Together that is 2.12 times the rolls a naive area-over-roll-area calculation gives.

Two bands, one top, two bottoms

R905.2.2 doubles asphalt underlayment from 2:12 to 4:12. Below 2:12 asphalt shingles are not permitted at all, so the band starts where the covering starts.

R905.3.2 doubles tile underlayment from 2 1/2:12 to 4:12, and again the bottom of the band is the bottom of the covering's permitted range. Tile is not allowed below 2 1/2.

So the two bands share a ceiling and not a floor, and at exactly 2:12 they give completely different answers: asphalt needs two layers, tile needs a different roof. A page that reuses one figure for the other is wrong at the bottom of the range, which is precisely where the doubling matters.

R905.1.1 is the material clause, and its useful sentence is about labelling: underlayment complying with ASTM D226, D1970, D4869 or D6757 must bear a label indicating which. Those standards are not free to read, so the label is the only free way to check what you were sold.

And R905.1.2 is the ice barrier, which is on this page because it is underlayment and because its width is the most commonly miscalculated dimension in Chapter 9.

The ice barrier is measured from inside the house

R905.1.2 says the barrier extends from the lowest edge of the roof to a point not less than 24 inches inside the exterior wall line.

Inside the wall line, measured horizontally. Not 24 inches up the roof, which is how it gets read and how one roll gets ordered.

So the overhang is added before the slope factor. A 24 in overhang plus 24 in inside the wall is 48 in of horizontal run, which at 3:12 becomes 49.5 in measured up the slope. A 36 in roll covers 36 in. It does not reach, and the second course has to lap the first.

The overhang is what drives it. A house with no overhang at all needs 24 in times the slope factor, and one roll does it. Every inch of eave adds an inch to the requirement before the roof pitch multiplies it.

Whether you need one is not a national question. R905.1.2 applies where Table R301.2(1) designates a history of ice damming, which is filled in jurisdiction by jurisdiction. This page asks rather than assuming.

What underlayment is actually for

It is described as a secondary barrier, which undersells it on a low slope roof.

Shingles are not waterproof, they are water shedding. They rely on gravity moving water off faster than capillary action and wind can move it sideways under a lap. The flatter the roof, the longer water spends on it and the more chances it has.

Which is exactly why the code doubles the underlayment at low slopes. The doubling is not belt and braces, it is an acknowledgement that below 4:12 the covering alone is not reliably keeping water out and something underneath has to.

It is also the layer that survives the covering. A shingle roof at the end of its life leaks in a few places; the underlayment is what stops that becoming a rotten deck while the money is found. On a re-roof the underlayment is coming off and being replaced, and it is the cheapest line on the job.

And it is the layer that protects the deck during the build. A deck dried in with underlayment can sit through weather; one waiting for shingles cannot. That is a scheduling benefit rather than a permanent one, and it is the reason it goes on the same day the sheathing does.

Frequently asked questions

When does roof underlayment need to be doubled?
Between 2:12 and 4:12 for asphalt shingles, per R905.2.2, and between 2 1/2:12 and 4:12 for clay or concrete tile, per R905.3.2. The two bands share a top and not a bottom, because each starts where its covering starts being permitted.
How many rolls of underlayment do I need?
Roof area times the number of layers, divided by what a roll actually covers. A 36 in roll with a 2 in head lap covers 34 in, so a 300 sq ft roll covers 283. On a 1,452 sq ft roof needing two layers that is 11 rolls, or 13 with waste.
How wide does the ice barrier have to be?
From the lowest edge of the roof to a point not less than 24 inches inside the exterior wall line, measured horizontally. That means the overhang is added first and then the slope factor applies. A 24 in overhang at 3:12 needs 49.5 in up the slope, so a 36 in roll does not reach.
Do I need an ice barrier?
Only where Table R301.2(1) designates a history of ice forming along the eaves, which is filled in jurisdiction by jurisdiction. It is a question for your building department, not a national rule, and this page asks rather than assuming.
Why does low slope need more underlayment?
Because shingles shed water rather than being waterproof. They rely on gravity moving water off faster than wind and capillary action move it under a lap, and the flatter the roof the longer water has to find a way. The doubling is the code acknowledging that below 4:12 the covering alone is not reliably keeping water out.
What standard does underlayment have to meet?
R905.1.1 points to ASTM D226, D1970, D4869 and D6757 depending on the material, and requires the product to bear a label showing which it complies with. None of those standards is free to read, so the label is the only free way to verify what you have been sold.
Does the head lap really matter?
On the order, yes. A 2 in head lap on a 36 in roll is 5.6% of every roll, and combined with a doubled layer on a low slope roof it takes the requirement to 2.12 times what a naive area divided by roll area would give.

Check these numbers yourself

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-09. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Whether an ice barrier is required is local. R905.1.2 applies where Table R301.2(1) designates a history of ice damming, which is filled in jurisdiction by jurisdiction, so this page asks rather than assuming. Roll dimensions and head laps are the manufacturer's. The ASTM standards R905.1.1 points to are not free to read, so the product label is the only free way to check compliance.