Calculator
Roof Underlayment Calculator
A roll of underlayment stamped 400 square feet does not put 400 square feet of new coverage on a roof, because every course after the first gives up part of its width to the course below it. The overlap is what keeps water shedding downhill across the seam instead of finding it, and it is also material the roll paid for that never becomes new roof surface — a cost the number on the wrapper does not mention.
This calculator works out what a roll actually covers once its head lap and end lap are subtracted, rather than dividing roof area by the stated roll size. The gap between stated and effective coverage is small on a normal-pitch roof with a standard 2 inch lap and much larger on a low-slope roof laid to a double-coverage detail, where the lap itself can claim more than half the roll's width.
Calculate your quantity
Sloped area, not the footprint.
2 in for felt on a normal pitch; 19 in for double coverage on a low slope.
Cuts at hips, valleys and penetrations.
Rolls needed
7 rolls
- Roof area
- 2,200 sq ft
- Roofing squares
- 22 squares
- Exposure per course
- 34 in
- Stated coverage
- 400 sq ft
- Effective coverage
- 376.4 sq ft
- Cap nails
- 1,100 nails
- Laps cost you 23.6 sq ft of every 400 sq ft roll — about 5.9%.
Shopping summary
- • 7 rolls of underlayment
- • 1 box(es) of cap nails (3,000 per box)
This is an estimate — confirm structural work with a professional.
How this calculation works
A roll's usable width in any one course is its full width minus the head lap — the strip that disappears under the course above it. A 36 inch roll laid with a 2 inch head lap only exposes 34 inches per course; laid with a 19 inch head lap for a double-coverage low-slope detail, it exposes just 17. That ratio, exposure over stated width, is applied directly against the roll's stated coverage to find what it actually delivers, before a smaller correction for the end lap where one roll's run meets the next along its length.
Because the loss scales with the fraction of the roll's width the lap consumes, a wide head lap does not cost a fixed number of square feet — it costs a fixed proportion, which gets worse as the lap grows relative to the roll. Going from a 2 inch lap to a 19 inch lap on the same 36 inch roll is not a modest adjustment; it turns a roll that loses roughly 6% of its stated coverage into one that loses over half.
- Head lap is set by the roofing material and the pitch, not chosen freely — a low-slope application commonly needs the wide double-coverage lap precisely because a shallow pitch gives water more time sitting near a seam.
- End lap loss is proportionally small compared to head lap loss on most roofs, since it only affects where one roll's run ends and the next begins, not every course along the roll's width.
- Cap nails are estimated separately from rolls and scale with total roof area rather than with roll count, since nailing pattern doesn't change with how many rolls the job happens to use.
- This page assumes felt or synthetic underlayment applied in overlapping courses. Self-adhered membrane, which bonds directly rather than relying on mechanical lap and fasteners, is a different product with different coverage math.
The formula
exposure = rollWidth − headLap; effectiveCoverage = rollCoverage × (exposure ÷ rollWidth) × (1 − endLapLoss); rolls = ceil((roofArea × (1 + waste)) ÷ effectiveCoverage)
- rollWidth − headLap
- The exposed width of each course — what actually becomes new coverage once the portion tucked under the next course up is subtracted.
- rollCoverage × (exposure ÷ rollWidth)
- The roll's stated coverage scaled down by the fraction of its width the head lap consumes — this is where most of the gap between the wrapper's number and the real number comes from.
- 1 − endLapLoss
- A smaller second correction for the overlap where one roll ends and the next begins along the length of a course, applied on top of the head lap correction.
- waste
- Extra material for cuts at hips, valleys, ridges and roof penetrations — ten per cent is the working default.
- roofArea
- The true sloped roof surface, not the flat footprint below it — underlayment is ordered against the same inflated area shingles are, not against a ground-level measurement.
Where these numbers come from
- 2 in head lap for a normal-pitch roof; 19 in head lap for double coverage on a low slope
- Trade convention — the standard lap for felt underlayment above roughly a 4-in-12 pitch against the wider double-coverage detail commonly used on shallower pitches, where slow-moving water sits near a seam longer.
- 200 sq ft (30 lb felt), 400 sq ft (15 lb felt), 1,000 sq ft (synthetic) roll sizes
- Manufacturer-published stated coverage, printed on the roll's own packaging — the starting figure this page corrects down to an effective coverage once head lap and end lap are subtracted.
- Effective coverage as a proportional loss, not a fixed square-footage loss
- Derived on this page from the ratio of exposure to full roll width — the loss grows as a share of the roll the wider the lap gets, which is why a double-coverage detail loses proportionally far more than a standard 2 inch lap does, not just a bit more.
- One cap nail per roughly 2 sq ft of roof area
- Trade convention for a synthetic underlayment nailing pattern. Felt underlayment is more commonly stapled or nailed at a wider spacing, so this figure is most accurate for synthetic products specifically.
Worked examples
A 2,200 sq ft roof in 15 lb felt at a normal pitch
| Roof surface area | 2200 sq ft |
|---|---|
| Roll type | 15 lb felt (400 sq ft) |
| Roll width | 36 in |
| Head lap | 2 in |
| End lap | 6 in |
| Waste factor | 10 % |
| Rolls needed | 7 rolls |
|---|---|
| Roof area | 2200 sq ft |
| Roofing squares | 22 squares |
| Exposure per course | 34 in |
| Stated coverage | 400 sq ft |
| Effective coverage | 376.4 sq ft |
| Cap nails | 1100 nails |
A 2 inch head lap on a 36 inch roll leaves 34 inches of real exposure per course, which brings a 400 sq ft roll's effective coverage down to about 376 sq ft — a loss of roughly 6%, most of it from the head lap rather than the smaller end-lap correction. Against 2,200 sq ft of roof with 10% waste, that comes to seven rolls.
That 6% loss is close to the smallest this calculation ever produces, because a 2 inch lap is close to the minimum a normal-pitch roof uses. The next example shows what the same roll looks like once the lap requirement changes for the pitch, not the roof size.
A 600 sq ft low-slope porch roof, double-coverage 30 lb felt
| Roof surface area | 600 sq ft |
|---|---|
| Roll type | 30 lb felt (200 sq ft) |
| Roll width | 36 in |
| Head lap | 19 in |
| End lap | 6 in |
| Waste factor | 10 % |
| Rolls needed | 8 rolls |
|---|---|
| Roof area | 600 sq ft |
| Roofing squares | 6 squares |
| Exposure per course | 17 in |
| Stated coverage | 200 sq ft |
| Effective coverage | 93.7 sq ft |
| Cap nails | 300 nails |
This roof is less than a third the size of the first example and yet needs one more roll. A 19 inch head lap on the same 36 inch roll leaves only 17 inches of real exposure per course — barely half the roll's width — which drags a 200 sq ft roll's effective coverage down to roughly 94 sq ft, a loss of over 53%. The smaller roof simply cannot outrun a lap that severe.
This is the case worth flagging before ordering by feel: a low-slope porch or dormer roof, even a small one, can consume more underlayment per square foot than a much larger roof at a normal pitch, purely because the code-required lap detail for a shallow slope claims so much more of every course.
Common mistakes
- Ordering rolls against the stated coverage printed on the wrapper instead of the effective coverage after lap loss. On a normal-pitch roof this under-orders by a small margin; on a double-coverage low-slope roof it can under-order by over half.
- Measuring roof area from the building's flat footprint rather than the sloped surface. Underlayment is ordered against the same inflated sloped area as the shingles above it, and a footprint measurement misses the pitch multiplier entirely.
- Assuming a bigger roof always needs proportionally more rolls than a smaller one. A small roof at a steep lap requirement can need more rolls per square foot than a large roof at a shallow one, which is easy to miss without running the actual lap math.
- Using felt underlayment's cap-nail spacing assumptions for synthetic underlayment or the reverse. The two products are commonly nailed at different patterns, and this page's nail estimate is tuned to a synthetic-style pattern.
- Treating end lap as trivial and skipping it in the estimate entirely. It is a smaller loss than head lap on most roofs, but it is not zero, and leaving it out compounds with an already-tight order on a large roof.
Shopping summary
The rolls figure above already has waste built in — buy that many, plus cap nails at the count in the breakdown. A roof whose true sloped area is still a guess is the bigger risk here: how to measure a roof turns a ground footprint and a pitch reading into the actual figure this page and the roofing calculator both need as an input.
In cold-climate installations, ice barrier membrane replaces standard underlayment across the eave zone rather than sitting underneath it — the ice and water shield calculator sizes that zone against a horizontal code dimension projected up the slope, a genuinely different calculation from the lap-loss math this page runs.
Edge and penetration materials installed over this underlayment — drip edge and step flashing among them — are their own orders with their own geometry, not a percentage of the underlayment total.
FAQ
Why does my underlayment order come out lower than the roof area divided by the roll's stated coverage?
Because the roll's stated coverage assumes no overlap, and every real course overlaps the one below it. This page corrects for that overlap directly, so the effective coverage per roll is always somewhat below the number printed on the wrapper — sometimes only a little, sometimes drastically, depending on how wide the required head lap is.
Why does a low-slope roof need such a wide head lap compared to a steeper one?
Water moves more slowly across a shallow pitch and has more time sitting near a seam before it drains past it, so the double-coverage detail widens the overlap enough that two full layers of felt cover the entire roof rather than a single layer with a narrow seam. It is a real difference in how the water behaves, not an arbitrary code number.
Does the end lap correction matter as much as the head lap correction?
No — on most roofs it is a distinctly smaller loss, since it only affects where one roll's length ends and the next begins along a course, not every course's width the way head lap does. It is still worth including rather than ignoring, particularly on a roof needing many rolls end to end.
Can I use a wider roll to reduce how many rolls I need overall?
Sometimes, but check the lap requirement against the wider roll specifically — a wider roll with the same fixed lap in inches loses a smaller proportion of its width, so it can genuinely improve effective coverage, but not every underlayment product is available in more than one width.
Is synthetic underlayment's coverage math the same as felt's?
The lap-loss geometry on this page applies to any rolled underlayment regardless of material, but synthetic rolls are typically sold in much larger stated coverage per roll and are commonly nailed at a different cap-nail spacing than felt, both of which change the final numbers even though the underlying formula is unchanged.
Do valleys and hips need extra underlayment beyond what this page calculates?
Yes — this page estimates the field coverage for the general roof area, and the 10% waste default is meant to absorb ordinary cutting at hips, valleys and penetrations, but a roof with an unusually high number of valleys relative to its size may need a higher waste allowance than the default assumes.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.