Underlayment by the roll, and the lap that eats it
Underlayment is sold by coverage, in squares, and every roll loses some of that coverage to the lap printed on it. A 4 square roll of #15 felt is nominally 432 square feet of material and covers about 400 once you have lapped it 2 inches course to course, which is why it is sold as 4 squares rather than 4.32. Synthetics are wider, longer and lighter, and a 10 square roll that one person can carry up a ladder is the reason most people stopped buying felt.
| Product | Common roll | Covers | Roughly |
|---|---|---|---|
| #15 felt | 36 in x 144 ft | 4 squares | 400 sq ft |
| #30 felt | 36 in x 72 ft | 2 squares | 200 sq ft |
| Synthetic, small | 48 in x 100 ft | 4 squares | 400 sq ft |
| Synthetic, standard | 48 in x 250 ft | 10 squares | 1,000 sq ft |
| Self-adhered ice barrier | 36 in x 66.7 ft | 2 squares gross | about 183 sq ft net at a 3 in lap |
The 10 percent added on top of the bare square footage in this calculator is for the ends of runs, the piece you cut around a chimney and throw away, and the extra lap you end up running where the deck is not straight. On a plain gable it is generous. On a roof with six dormers it is not.
Drip edge goes two different places
At the eave, drip edge goes on the deck first and the underlayment goes over it, so that water running down the underlayment lands on the metal and gets thrown clear of the fascia. At the rake, it goes on top of the underlayment, so that wind-driven water running sideways across the roof cannot get under the edge. Getting these the wrong way round is the most common detail error on an otherwise competent roof, and on the eave it is the one that rots the fascia quietly over ten years.
Sticks are commonly 10 feet 6 inches, which after a 2 inch end lap gives about 10 feet 4 inches of coverage each. Some suppliers sell 10 foot 0 inch. Measure one before you divide, because the difference over a 150 foot perimeter is a stick and a half. Corners take an extra piece each, mitred or lapped, and this count does not add for them.
Ice barrier, and the measurement that is not the roll width
The distance an ice barrier has to reach is not measured at the eave and it is not measured horizontally. The requirement is stated as a distance past the interior face of the exterior wall, and it is measured along the roof plane. So the calculation runs: overhang, plus wall thickness, plus the reach past the inside face, gives a horizontal distance in from the fascia; multiply that by the slope multiplier and you have the distance up the slope; divide by the course exposure and you have the number of courses.
Worked through: an 18 inch overhang on an 8 inch wall with a 24 inch reach inside is 50 inches horizontally. At 6/12 the multiplier is 1.1180, so that is 55.9 inches up the slope. A 36 inch roll with a 3 inch lap exposes 33 inches, so it takes two courses. Cut the overhang to 6 inches and the horizontal reach is 38 inches, 42.5 up the slope, and it is still two courses — the second one only nine or ten inches wide, which is the worst value roll on the job. At 6/12 with a 24 inch reach inside the wall a 36 inch roll never fits in one course however short the overhang gets, because the wall thickness and the 24 inches alone come to 32 horizontal and 35.8 up the slope. What moves that boundary is a shallower pitch, a wider roll, or a jurisdiction asking for less than 24 inches. Measure the actual overhang anyway, since it decides how much of the second course you are buying.
Valleys are separate. They commonly get a full width strip of the same membrane centred in the valley, run before the eave courses so the eave laps over it, and the length is the valley length rather than anything derived from the pitch.
Read the wrapper, then ask the building department
Two authorities sit above everything on this page and they do not always agree. The first is the building code your local jurisdiction has adopted, which decides whether an ice barrier is required at all in your climate, how far it has to reach, and how many layers of underlayment a given slope needs. Jurisdictions adopt different editions of the code and amend them locally, so the requirement in the next county is not evidence of the requirement in yours. The second is the manufacturer's installation instructions for the specific product on the pallet, which is what the warranty is written against and which is often the stricter of the two. If you install to the code and against the instructions, the roof may pass inspection and still have no warranty. Get both before the material is ordered. The quantities here are arithmetic and they will not tell you which detail you are supposed to be building.
Questions people ask
How many rolls of felt do I need for a 2,000 square foot roof?
Twenty squares of roof, plus about 10 percent for laps and cuts, is 22 squares of coverage. In #15 felt at 4 squares per roll that is 6 rolls. In #30 at 2 squares per roll it is 11. In a 10 square synthetic it is 3. If the 2,000 figure is your footprint rather than the roof surface, apply the pitch first — at 6/12 the roof is 2,236 square feet and the synthetic count goes to 3 rolls still, but the #30 count goes to 13.
How far up the roof does ice and water shield have to go?
The common requirement in cold regions is to a point at least 24 inches inside the interior face of the exterior wall, measured along the slope. That means the roll has to cover the overhang, the wall thickness and then 24 inches more, all multiplied by the slope factor. On a typical house with an 18 inch overhang at 6/12 that comes out near 56 inches up the slope, which is two courses of 36 inch membrane. Whether it is required at all, and whether your jurisdiction asks for more than 24 inches, depends on the code your building department has adopted and on your climate. Confirm it locally.
Does drip edge go over or under the underlayment?
Under at the eave, over at the rake. At the eave the metal has to be beneath the felt so water shedding down the underlayment runs onto the drip edge and off the fascia rather than behind it. At the rake the metal goes on top so wind cannot lift water in under the edge from the side. If ice barrier is being used, it goes over the eave drip edge as well, since it takes the place of the underlayment there.
Do I need two layers of underlayment on a low slope roof?
Most asphalt shingle manufacturers call for a doubled underlayment, or a fully self-adhered deck, on slopes between 2:12 and 4:12, and some jurisdictions require it too. Above 4:12 a single layer is the normal installation. The double layer is run at half the exposure so every point on the deck has two thicknesses over it. Which detail applies to your roof comes from the wrapper on the shingles and from your adopted code, not from a general rule, and it is worth confirming before the material arrives because the quantity roughly doubles.
How many sticks of drip edge for a 150 foot perimeter?
At a 10 foot 6 inch stick with a 2 inch end lap you get about 10 feet 4 inches of coverage each, so 150 divided by 10.33 is 14.5, meaning 15 sticks. Add one per outside corner if you are lapping rather than mitring, and buy a spare, because the first cut on a mitre almost always goes the wrong way. If your supplier stocks 10 foot 0 inch sticks the same perimeter takes 16.