Roof underlayment calculator

What underlayment does, and why a roll is not a square

Underlayment is the sheet between the deck and the shingles. It keeps the house dry from tear-off to dry-in and catches the water that gets past the covering for the next twenty years. The guide to underlayment types covers felt against synthetic against self-adhering membrane; this page is about how many rolls to put on the truck.

The number printed on the wrapper is not one number. A roll of TAMKO No. 15 felt is 36 in by 144 ft, which is 432 sq ft of paper, but TAMKO rates its coverage at 408 sq ft because each course laps the one below by 2 in. Synthetic Guard holds 1,001 sq ft and covers 905.67 at its printed 4 in lap. Divide the roof by the coverage, never by the contents.

How to use the calculator

1. Enter the footprint and pick the pitch, or switch to roof squares if you already have a measured surface from the roof square calculator.
2. Pick the field product. The pitch picker marks every slope under 4:12 — those go down in two layers and the coverage per roll halves.
3. Pick a cut-waste tier. These are the same three tiers the shingle takeoff uses: hips, valleys and dormers cut underlayment the same way they cut shingles.
4. Enter the eave length, and tick the ice barrier if your jurisdiction requires one. Give the overhang as a level measurement from the wall face to the fascia.
5. Add the total length of closed valleys. They take a sheet of membrane whether or not the eaves need an ice barrier.

The code lines behind the numbers

Asphalt shingles are allowed down to 2:12. From there up to 4:12 the 2021 IBC requires two layers: a 19 in strip along the eave, then 36 in sheets each lapping the one below by 19 in. That leaves 17 in of each sheet showing, so a roll covers half its usual area. TAMKO prints a 17 in laying line on its felt for exactly this job. For Synthetic Guard on a low slope, TAMKO calls for a 22 in starter and full 42 in sheets lapped 22 in.

At 4:12 and steeper it is one layer, laid shingle fashion from the eave with a 2 in side lap, with end laps of 4 in staggered 6 ft apart. Where the design wind speed is 140 mph or more, every lap goes to at least 4 in, and the sheet has to be held with cap nails or cap staples with caps of at least 1 in: a 12 in grid between the laps and 6 in spacing along them.

Where ice has a history of forming along the eaves and backing water up, the code requires an ice barrier. That means two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet in place of the normal underlayment. It runs from the lowest edge of the roof to a point at least 24 in inside the exterior wall line. Detached structures with no conditioned floor area are exempt. The ice dam guide explains why that line sits inside the heated wall.

For closed valleys (shingles laid across the valley), the code wants a lining of smooth roll roofing at least 36 in wide, or ASTM D1970 self-adhering membrane in its place. An open metal valley needs a metal lining at least 24 in wide. The calculator counts one 36 in sheet of membrane centred on each closed valley. The valley types guide covers how open and closed valleys differ.

A worked takeoff

Take the 48 × 34 ft ranch at 6/12: 1,632 sq ft of footprint × 1.118 is 1,825 sq ft of roof. It has two 50 ft eaves (100 ft in all), 16 in overhangs, an ice-barrier requirement, and two 8 ft closed valleys where an entry gable meets the main roof. The eave strip needs two courses, which is 200 lin ft. Add the 16 ft of valley and that is 216 ft of membrane. Each 64 ft 6 in roll gives 64 ft after a 6 in end lap, so that is four rolls of Moisture Guard.

The membrane covers 571 sq ft at the eaves, which leaves 1,254 sq ft of field. With 15% for the hips and valleys, that is 1,442 sq ft to cover. In Synthetic Guard that is 1.6 rolls, so two. In #15 felt it is 3.5 rolls, so four. In #30 it is 7.1, so eight. Starter, cap and field shingles come off the shingle bundle calculator using the same squares.

Mistakes that short the order

• Dividing by what the roll contains rather than what it covers. On 21 squares of #30, that is 10 rolls ordered against 11 needed.
• Pricing a 3/12 porch or shed roof as if it were the main roof. Take it off as its own section so it gets the double layer.
• Leaving the sheet exposed. TAMKO limits Synthetic Guard to 45 days in the sun and Moisture Guard to 90, so schedule the shingle delivery around dry-in.
• Laying membrane on a cold, dusty deck. Moisture Guard wants the air, deck and roll at 40°F or warmer, and a swept, dry surface. TAMKO limits it to closed valleys and does not recommend it under metal roofing.
• Getting the order wrong at the edges. The drip edge goes under the underlayment at the eave and over it at the rake. The drip edge guide and the shingle installation sequence walk through both.