Roofing Paper: Felt Underlayment Types

Roofing paper — tar paper, felt paper, roofing felt; the names all describe the same product — is the asphalt-saturated underlayment that has gone under American shingles for more than a century. It’s the roof’s second line of defense: when wind lifts a shingle or an ice dam backs water up the slope, the paper is what keeps the deck dry. Synthetics have taken over most professional installs, but felt still holds real advantages in specific situations, and knowing 15 lb from 30 lb from synthetic is basic literacy for anyone buying a roof. Here’s the complete picture: types, weights, coverage math, when felt still wins, and how to install it right.

What Roofing Felt Actually Is

Traditional felt starts as a mat of recycled cellulose and rag fiber (organic felt) or fiberglass, saturated with asphalt until it’s water-resistant. The result is a heavy, slightly tacky black paper sold in 36-inch-wide rolls. Its job description under a shingle roof:

• Secondary water barrier against wind-driven rain and minor shingle failures.
• Deck separation: it isolates shingles from resins and uneven sheathing, and lets minor deck moisture escape (felt is vapor-permeable enough to breathe, unlike some films).
• Temporary dry-in: it keeps the house dry between tear-off and shingling.
• Fire assembly: felt is part of many Class A fire-rated shingle assemblies.

15 lb vs 30 lb Felt

The pound ratings are historic — originally the weight of 100 square feet of product. Modern “No. 15” felt actually weighs 7-12 lb per square and “No. 30” about 16-27 lb, which is why spec sheets now say “Type I” and “Type II” (ASTM D226/D4869). The practical differences:

Rule of thumb: if you’re using felt at all in 2026, use 30 lb. The upcharge on a whole roof is $50-$150 and everything about it is more forgiving. Codes commonly allow 15 lb on slopes 4:12 and steeper with single-layer coverage; between 2:12 and 4:12, code requires two layers of felt lapped 19 inches — at which point synthetic or membrane usually makes more sense.

Felt vs Synthetic Underlayment

Synthetic underlayments are woven or spun polypropylene/polyethylene sheets, and they’ve conquered the market for good reasons — but the comparison isn’t a shutout:

• Weight and coverage: a 10-square synthetic roll weighs about 25-30 lb; the felt equivalent is 2.5 rolls at 60 lb each. Fewer laps, fewer trips up the ladder.
• Toughness: synthetics resist tearing at fasteners, don’t wrinkle with moisture, and offer better walking traction on steep slopes.
• UV exposure: synthetics are rated for 60-180 days exposed; felt wants covering within days.
• Price: entry synthetics now cost roughly the same per square as 30 lb felt ($4-$8/square material); premium synthetics run $10-$20/square.
• Where felt pushes back: felt’s modest vapor permeability suits unvented or marginal attics better than vapor-tight synthetics, which can trap deck moisture on poorly ventilated roofs. Felt lies flatter under slate and tile where crews want a traditional slip sheet, it’s cheap everywhere, and some historic-district and manufacturer specs still call for it. It’s also self-healing around nails in hot weather to a degree synthetics aren’t.

Verdict: for most asphalt reroofs, synthetic is the better tool — especially on complex or steep roofs and anywhere dry-in time stretches. Choose 30 lb felt when budget rules, when the attic ventilation story is questionable, or when a tile/slate/cedar spec demands it. Whichever you pick, underlayment is secondary defense — shingle selection and installation carry the roof, and our asphalt roof guide covers that layer.

Coverage and Buying Math

Roof area in squares = footprint × pitch factor ÷ 100. A 1,800 sq ft footprint at 6:12 pitch (factor 1.118) is about 20 squares. Underlayment shopping list for single coverage with standard laps:

• 15 lb felt: 20 ÷ 4 = 5 rolls; buy 6 for laps and waste.
• 30 lb felt: 20 ÷ 2 = 10 rolls; buy 11-12.
• Add self-adhered ice-and-water membrane at eaves (code in snow country: from the edge to 24 inches inside the warm wall), valleys, and penetrations — felt is not a substitute there.

Installing Roofing Felt Right

1. Start on a dry, swept deck. Felt over damp sheathing wrinkles and telegraphs through shingles.
2. Drip edge first at the eaves, felt over it; at the rakes, drip edge goes over the felt. This tucks each edge with gravity.
3. Roll horizontally from the eave up, 2-inch horizontal laps, 4-inch end laps, upper courses always over lower.
4. Fasten with cap nails, not staples — plastic caps hold 10× better in wind and don’t slice the paper. Pattern: every 8 inches on laps, 12-16 inches in the field; tighter if it will sit exposed.
5. Keep it flat. Wrinkles show through thin laminated shingles. If dew wrinkles overnight, let the sun relax the paper before shingling.
6. Lap the peaks: carry felt over ridges and hips 6 inches each way.
7. Walk carefully. Felt on dusty plywood is a toboggan run; wear soft-soled shoes and use anchors on anything steep. Then shingle promptly — our shingling guide picks up from here.

Other Jobs Roofing Paper Does

Tar paper’s usefulness leaks well beyond roofs, which is why a spare roll is worth keeping:

• Under siding and behind stucco — Grade D building paper is its cousin and still standard behind stucco in the West.
• Under hardwood flooring: 15 lb felt is the classic squeak-and-dust slip sheet over wood subfloors.
• Concrete curing and protection: slip sheets, temporary surface protection, bond breaks.
• Shed and outbuilding roofs: two lapped layers of 30 lb can serve alone as a short-term roof on a low-value structure for a season or two.

Storage and Handling

Felt is fussier in the driveway than on the roof. Store rolls standing on end, indoors or tarped — a roll left flat in the sun deforms into an oval that never rolls out straight again, and a wet roll delaminates from the inside. In cold weather, felt gets brittle: below about 40°F, unroll it and let it relax in the sun before fastening, or it cracks along the laps as you bend it over ridges. In heat, the opposite problem — saturant softens and boot traffic scars it, so walk high-melt-point (or synthetic) on 95°F days. Carry rolls up on a shoulder or hoist them; a 60-lb roll of 30 lb felt dropped on a drip edge folds it flat. And label your leftovers: a half roll stored dry is genuinely useful for the flooring, siding, and shed jobs above, so keep it standing in the garage rather than sending it to the dumpster with the tear-off.

FAQ

Can felt roofing paper get rained on?

Once installed and lapped, it sheds a rain fine — that’s its dry-in job. Extended exposure, ponding, or foot traffic on wet felt breaks it down. Cover within a few days; retack any wind-lifted laps immediately.

Is roofing paper waterproof?

Water-resistant. It sheds shingle-lapped water; it will not hold standing water, and every unsealed fastener is a potential path. For waterproof, you want self-adhered membrane.

Do I need underlayment at all on a reroof?

Yes — code (IRC R905) requires underlayment beneath asphalt shingles, and shingle manufacturers require it for warranty. Over an existing layer of shingles in an overlay, requirements vary; tear-offs always get new paper. Never reuse old underlayment.

Which is better under metal roofing?

High-temperature synthetic or ice-and-water rated for metal is standard now; felt under metal can stick to panels and cook. Metal systems also change the fastening story entirely — see our metal roofing screws guide for that side.

Bottom Line

Roofing paper is the $30-a-roll insurance layer under every shingle roof. If felt fits your job, buy 30 lb, cap-nail it flat with honest laps, and cover it quickly. If your roof is steep, complex, or will sit exposed, spend the extra few dollars per square on synthetic. Either way, never let anyone shingle over bare wood — the day one shingle lifts, the underlayment is the roof.