What changes between roof underlayment types?
Felt, synthetic and peel-and-stick describe product families, not interchangeable specifications. The decision turns on the exact roof cover, deck, slope, attachment, laps, exposure period and approval route.
This comparison uses three documented examples: G.A.P. 15 ASTM D4869 Type I felt, G.A.P. GuardShield synthetic and Polyglass Polystick TU Plus self-adhered underlayment. Their numbers are not category-wide rules. Start with the finish system in the roofing materials comparison, calculate the plane with the roof pitch and slope guide, then check the exact underlayment documents and applicable approval together.
Felt: concrete instructions, one unresolved fastener phrase
G.A.P. 15 is an asphalt-saturated organic felt listed as ASTM D4869 Type I and used over the deck before asphalt shingles. The product sheet identifies the material, standard and use. That makes it a useful felt example, not a stand-in for every product sold as felt.
The felt installation instructions require a deck clear of debris and moisture and direct the installer to run smooth, wrinkle-free horizontal courses from the eaves. On slopes from 2:12 through 4:12, the instructions describe a double-layer method with a 19-inch starter strip and 19-inch successive overlaps; at 4:12 or greater, they describe one layer with 2-inch course laps. The document therefore includes 4:12 in both ranges. Do not resolve that overlap by guessing: get the manufacturer or authority having jurisdiction to identify the applicable method for an exact 4:12 project. End laps are 4 inches.
The fastener sentence says fasteners must be applied no less than eight inches at the top edge, middle and eave. That wording does not identify a fastener type or provide an unambiguous maximum-spacing schedule, so it should not be rewritten as 8 inches on center. Record the project fastener type and spacing only after the manufacturer, specification and authority having jurisdiction settle that point.
These felt instructions recommend installing the primary roof covering immediately after felt installation. They do not give a 30-, 60- or 90-day exposure allowance. They also say they are not for the Florida Building Code 2007 High Velocity Hurricane Zone, so this document is not current HVHZ approval evidence.
Synthetic: GuardShield under asphalt shingles
GuardShield is a coated woven polypropylene sheet with a nonwoven upper surface, mechanically attached to roof sheathing as secondary protection under asphalt shingles. The technical data sheet describes that construction and use. Keep every number below tied to GuardShield rather than extending it to all synthetics.
The instructions require a clean, smooth, dry, continuously supported deck with no existing covering or underlayment, voids, battens or uneven surface. Read the complete GuardShield instructions. GuardShield is identified as a vapor barrier and must be installed over properly ventilated space. The instructions require installation above 40°F.
GuardShield has a stated minimum slope of 2:12; the instructions describe slopes from 2:12 to 4:12 as double-felted with 50 percent overlaps. Because the normal lap instructions and this range meet at 4:12, do not invent a different boundary rule for the exact 4:12 case. Normal horizontal laps are 4 inches and end laps are 6 inches.
In normal wind zones, GuardShield uses ring-shank roofing nails with approved 1-inch plastic caps at 6 inches on center on overlapping edges and 12 inches on center in the field, and staples are prohibited. High-wind requirements remain jurisdiction-specific, and the instructions say the product is not intended for HVHZ.
The final roof covering must be installed within 90 days of GuardShield installation. Where a separate severe-climate barrier is required, the instructions call for an ASTM D1970 or AC48 self-adhered membrane beneath GuardShield, a 4-inch overlap, and the locally required extent. GuardShield itself should not be relabeled as that separate barrier.
Peel-and-stick: TU Plus requires pressure and a defined approval route
Polystick TU Plus is a self-adhered, dual-reinforced waterproofing underlayment designed for adhesive-foam or mechanically fastened roof tile applications; its product data sheet also discusses slate, metal and multi-ply uses. The July 2025 PDS defines those manufacturer-listed uses. Those possibilities still need an applicable approved assembly.
The PDS requires a dry substrate and air, deck and membrane temperatures of 40°F and rising, and it prohibits direct application over existing shingles or other roof coverings. TU Plus uses split release film and firm, even pressure during placement, with minimum 3-inch side laps and 6-inch end laps. After adhesion, apply uniform pressure over the whole surface with a 35- or 75-pound weighted roller or a water-filled lawn roller; brooming is allowed on steep roofs where rolling creates a safety concern. Where steep-slope back nailing is required, the next sheet must cover every nail. Use PolyPlus 50 or PG 500 at non-factory fabric or granule laps and the listed details and accessories, rather than treating primer as a universal step.
Polystick TU Plus must be covered within 360 days unless the authority having jurisdiction imposes a shorter limit. This is a product-specific maximum, not permission to leave any peel-and-stick exposed for that period.
The March 31, 2026 substrate bulletin requires priming for its listed concrete and gypsum products. It also requires priming for listed metal surfaces. It says primer is recommended but not required for new APA-rated exterior-grade OSB. Read the substrate bulletin. It also strongly recommends an anchor sheet where plywood or OSB has an underside radiant barrier and calls for bond-breaker tape or an anchor sheet at unsupported wood-sheathing joints in the stated low-slope condition. These are manufacturer substrate guidelines, not proof that every listed option falls under one evaluation report.
For the route evaluated in ICC-ES ESR-1697, TU Plus installation is limited to plywood, slopes of at least 2:12, ventilated attics and roof coverings mechanically fastened through the membrane to the sheathing or rafters. Read ESR-1697. That route also requires installation above 40°F and excludes roof coverings involving hot asphalt or coal-tar pitch. ESR-1697 says it governs if it conflicts with the manufacturer instructions. OSB, foam-set tile or another manufacturer-listed substrate or cover therefore needs a separate applicable approved assembly rather than being claimed under this evaluation route.
Decision table: where each documented example fits
| Example | Deck and preparation | Supported slope and cover in cited route | Fastening or adhesion | Laps | Exposure | Best fit in this comparison |
|---|---|---|---|---|---|---|
| G.A.P. 15 D4869 Type I felt | Deck clear of debris and moisture; smooth horizontal courses from eaves | Asphalt shingles; double layer from 2:12 through 4:12 and one layer at 4:12 or greater, leaving exact 4:12 to project clarification | Instructions say no less than 8 inches at top, middle and eave, but do not state type or an unambiguous maximum spacing | 19-inch successive overlap in double-layer method; 2-inch course laps in one-layer method; 4-inch end laps | Cover immediately; no numeric window stated | Shingle job that accepts this felt and has a resolved fastener specification (instructions) |
| G.A.P. GuardShield synthetic | Clean, smooth, dry, continuously supported bare deck; ventilated space; above 40°F | Asphalt shingles only in these instructions; minimum 2:12; 2:12 to 4:12 uses 50 percent overlap | Normal wind: ring-shank roofing nails with approved 1-inch plastic caps, 6 inches on center at overlapping edges and 12 inches in field; no staples | Normal courses: 4-inch horizontal and 6-inch end laps | Final cover within 90 days | Mechanically fastened asphalt-shingle field underlayment where this exact schedule and ventilation condition fit (instructions) |
| Polyglass Polystick TU Plus self-adhered | Dry bare approved substrate; air, deck and membrane 40°F and rising; substrate-specific primer or anchor-sheet rules | ESR-1697 route: plywood, at least 2:12, ventilated attic and mechanically fastened cover; other PDS uses need their own applicable assembly | Self-adhered with split release film and full pressure; roll with 35- or 75-pound weighted roller or water-filled lawn roller; cover required back nails with next course | Minimum 3-inch side and 6-inch end laps; specified mastic at non-factory fabric or granule laps and details | Cover within 360 days unless the authority having jurisdiction is stricter | Tile-oriented field or detail membrane where the selected cover, substrate and approval route all match (PDS; ESR-1697) |
The table supports a real first cut: felt needs its fastener ambiguity closed; GuardShield supplies a complete normal-wind schedule for an asphalt-shingle assembly; TU Plus replaces mechanical field attachment with substrate preparation, adhesion pressure and route-specific approval.
Approval checklist before ordering
Use the table to narrow the product, then close the submittal with this checklist:
- Roof cover: Name the exact shingle, tile, metal, slate or other cover and the document approving the pairing.
- Deck: Record plywood, OSB, concrete, metal, an anchor sheet or another substrate. Do not move a broad manufacturer list into a narrower evaluation route.
- Slope: Measure every plane. Resolve the overlapping 4:12 language in the G.A.P. documents rather than choosing the easier instruction.
- Attachment: Put fastener type, cap, spacing and wind zone on the felt or synthetic submittal. For self-adhered material, record temperature, primer or anchor sheet, pressure method, back nails and repair method.
- Laps and details: Record side, end, valley, eave, ridge and penetration treatment, including where mastic is required.
- Exposure: Schedule from the exact instruction: immediate cover for this felt, within 90 days for GuardShield and within 360 days for TU Plus unless the authority having jurisdiction is stricter.
- Moisture path: Coordinate vapor-barrier underlayments with the attic ventilation guide. Treat the eave condition as part of the roof-and-attic system described in the ice-dam prevention guide.
- Approval: Match the current product label, evaluation report, local amendment, roof-cover instructions and project specification. A dated PDF or marketing badge alone does not establish current local acceptance.
Frequently asked questions
Is synthetic underlayment always better than felt?
No. GuardShield provides a clear normal-wind fastening schedule and a 90-day cover limit, while the cited felt gives concrete slope and lap methods but ambiguous fastener wording. That comparison belongs to these products and this shingle use, not every synthetic and felt.
What happens at exactly 4:12 in the G.A.P. instructions?
Both documents use ranges that include 4:12 in overlapping methods. Do not invent a boundary. Ask the manufacturer or authority having jurisdiction which method applies to the exact product and project, then put that answer in the submittal.
Can roof underlayment remain exposed during a delay?
Only within the exact product instruction and any shorter project or jurisdiction limit. This felt says to cover immediately, GuardShield says within 90 days, and TU Plus says within 360 days unless the authority having jurisdiction is stricter.
Does ESR-1697 approve TU Plus over OSB or under foam-set tile?
Not through the route described here. ESR-1697 limits its recognition to plywood and mechanically fastened roof coverings through the membrane. A broader PDS or substrate bulletin option needs another applicable approved assembly.
Does GuardShield replace an ice barrier?
Not in its own instructions. Where a severe-climate barrier is required, the document calls for a separate ASTM D1970 or AC48 self-adhered membrane beneath GuardShield, with a 4-inch overlap and locally required extent.