Shingles & materials
Underlayment: felt vs synthetic, coverage and overlaps
The layer under the shingles is the roof's second line of defence. Choosing it is easy; counting it right means counting the laps.
What underlayment does
Shingles shed water; they are not waterproof. Wind drives rain up under the laps, ice dams back meltwater up the slope, and a shingle blown off in a storm leaves a hole. The underlayment, a continuous sheet over the roof deck, catches what gets through and carries it down to the eave. It also keeps the deck dry while the roof is being built, separates the shingles from resin in the wood, and is required by the International Residential Code (R905.1.1) under asphalt shingles.
The two families
Asphalt-saturated felt is paper or fibreglass mat soaked in asphalt, sold as #15 and #30 (the numbers once meant pounds per square; today they are ASTM D226 Type I and Type II). A #15 roll is 36 inches by 144 feet, four squares nominal; a #30 roll is 36 inches by 72 feet, two squares. Felt is cheap, grips well under boots and breathes a little, but it wrinkles when it gets wet before the shingles go on, tears in wind, and should be covered the same day.
Synthetic underlayment is woven or spun polypropylene or polyethylene with a coating, typically 48 inches wide in 250-foot (ten-square) or 100-foot (four-square) rolls. It weighs a fraction of felt per square, does not wrinkle, resists tearing and is rated for weeks or months of exposure before shingling, which is why reroofers dry a house in with it. It can be slippery when wet or dusty, and it needs plastic-cap fasteners rather than plain nails.
| #15 felt | #30 felt | Synthetic | |
|---|---|---|---|
| Roll | 36 in × 144 ft | 36 in × 72 ft | 48 in × 250 ft (also 100 ft) |
| Nominal coverage | 4 squares | 2 squares | 10 squares |
| Net coverage, single (2 in or 4 in lap, 6 in end lap) | about 4.1 squares | about 2 squares | about 9.1 squares |
| Exposure before covering | same day | same day | 60 to 180 days by product |
| Fasteners | cap nails | cap nails | plastic-cap nails or staples |
| Strengths | cheap, grippy | heavier, more durable felt | light, strong, UV-rated |
Laps eat coverage
A roll never covers its nominal area. Each course overlaps the one below by the side lap (2 inches for felt, 3 to 4 for synthetic, along a printed line), and where one roll ends and the next begins there is an end lap of about 6 inches. The net coverage per roll is the net width times the net length.
A 20-square roof in synthetic
2,000 square feet, 48 in × 250 ft rolls, 4-inch side lap, 6-inch end lap, 10% waste.
- Net width: 48 − 4 = 44 in
- Net per roll: 44 in × 249.5 ft = 915 sq ft
- Rolls: 2,200 ÷ 915 = 2.4, so 3 rolls and about 1,000 cap fasteners
The same roof in #15 felt with a 2-inch lap needs 6 rolls; in #30, 11. The underlayment calculator runs all three.
Double coverage on low slopes
Between 2/12 and 4/12 the code and the shingle makers require two layers of underlayment everywhere. In practice that is one roll laid with each course overlapping the one below by half its width plus an inch: 19 inches on a 36-inch roll, leaving 17 inches exposed. Every point on the roof then has two thicknesses. A roll covers a little under half its nominal area: a 2-square #30 roll covers about one square.
A porch roof at 3/12
1,200 square feet in #30 felt, double coverage, 15% waste for its hips.
- Net width: 36 − 19 = 17 in; net per roll: 17 in × 71.5 ft = 101 sq ft
- Rolls: 1,380 ÷ 101 = 13.6, so 14 rolls
Below 2/12 shingles are not allowed at all; the roof needs a membrane. The pitch calculator tells you which band a roof is in.
Where underlayment is replaced, not added
At the eaves in cold climates and in every valley, a self-adhering ice and water membrane goes on the bare deck in place of the underlayment, and the underlayment laps over its top edge. On a modest roof the overlap is inside the waste allowance; on a long eave with two courses of membrane and several valleys, subtract the membrane area first. Ice and water shield, where the code wants it covers the rules, and the ice and water shield calculator counts it.
Fastening
Felt is fastened with cap nails (a roofing nail through a plastic or metal disc) along the laps and in rows across the field; it can be held with fewer if shingled the same day. Synthetic needs plastic-cap nails or cap staples in the pattern printed on it, typically every 12 inches on the laps and in two rows at 24 inches across the field: about 50 per square. Plain nails pull through synthetic in wind.
Order of layers at the edges
At the eaves, drip edge goes on the bare deck and the underlayment laps over it, so water running down the underlayment lands on metal. At the rakes, the underlayment goes on first and the drip edge over it. Reversing either sends water behind the fascia. Drip edge, flashing and valleys shows the full sequence.
Which to choose
Synthetic for most reroofs, especially where the roof may stand open for a day or more or in wind. Felt where budget is tight and the roof will be shingled the same day, or where a product’s warranty asks for it. Both work when installed with the right laps; the coverage arithmetic, not the material, is where orders go wrong. The squares come from the shingle calculator or your own measurement.