Roof Types, Underlayment & Thermal/Moisture Protection

Roof Types, Underlayment & Thermal/Moisture Protection

Quick Answer: Know the major roof shapes (gable, hip, mansard, shed, gambrel, flat/low-slope), treat underlayment as the secondary weather barrier over sheathing, add ice barriers where cold-climate rules apply, install drip edge in the correct eave-then-rake relationship to underlayment, and keep thermal/moisture control continuous so heat, vapor, and water do not destroy the roof assembly.

Modules 27202 (Roofing Applications) and 27203 (Thermal and Moisture Protection) feed the Commercial Carpenter roofing domain. Assessment items expect you to name roof forms, sequence weather barriers, and explain why underlayment, ice protection, and ventilation work together—not just how to nail a shingle.

Why Roof Shape Matters on the Exam and the Job

Roof geometry controls how water leaves the building, how many valleys and hips you flash, how complex the framing is, and which coverings are appropriate. Slope (pitch) further separates steep-slope systems from low-slope systems. Do not confuse shape names with framing names: a gable roof can be framed with common rafters or trusses; the shape is what the finished outline does.

Exam scenario: A building has four sloping sides and no vertical gable wall at the ends. That is a hip roof, not a gable. If the roof is a single plane leaning one way over a lean-to addition, call it a shed roof.

Steep-slope vs low-slope: Manufacturer literature and codes draw the line near about 2:12 to 4:12 depending on product. Asphalt strip shingles typically need adequate slope and often special underlayment below about 4:12. Low-slope commercial decks use membranes designed for standing water and ponding resistance. Choosing shingles on a dead-flat commercial roof is a materials error, not a style preference.

Roof Deck, Sheathing, and What “Dry-In” Means

Before finish roofing, carpenters close the structure with roof sheathing (commonly OSB or plywood rated for roof use) fastened to rafters or trusses per spacing and nailing schedules. Hips, valleys, and ridges must land on solid framing or approved supports. Gaps, damaged panels, and missing clips at unsupported edges create soft spots that telegraph through shingles and puncture membranes.

Dry-in means the building is weather-resistant enough that interior work can proceed: sheathing on, underlayment (and often ice barrier) complete, openings flashed or temporarily protected. Dry-in is not final roofing—it is the intermediate weather shell.

Underlayment: The Secondary Weather Barrier

Underlayment sits on the roof deck under the primary roof covering. It sheds water that gets under shingles or metal during wind-driven rain, ice dams, or damaged finish materials. Types you should recognize:

• Asphalt-saturated felt (historically 15-lb / 30-lb designations; modern labels use ASTM product specs)—economical, traditional, more sensitive to wrinkling and UV if left exposed too long.
• Synthetic underlayment—lighter, often more tear-resistant, longer exposure ratings; still requires manufacturer-approved fastening and overlap.
• Self-adhered (peel-and-stick) membranes—used as full underlayment on some roofs and as ice barrier at eaves and valleys in cold climates.

Overlap and fastening: Courses generally run horizontal, starting at the eave and working up so upper courses lap over lower courses (shingle fashion). Side and end laps follow product instructions. Fasteners are typically capped nails or staples where allowed—never sparse random nailing that allows wind uplift of the underlayment before finish roofing.

Cold-climate ice barrier awareness: In cold regions, model codes (IRC style) typically require an ice barrier from the eave upward a specified distance inside the exterior wall line (often described as at least 24 inches inside the warm wall, which usually means several feet of membrane measured up the roof slope from the eave edge). Valleys often get ice barrier as well. The barrier seals around fasteners and resists water that backs up from ice dams. Carpenters must know that ice barriers exist and where they go—not invent dimensions from memory when the drawing or code table is available.

Drip Edge Sequence (Eaves and Rakes)

Drip edge is metal flashing at roof edges that directs water into the gutter or off the fascia and protects the deck edge. A common teaching sequence (confirm always with manufacturer and AHJ):

1. At eaves: install drip edge first, then run underlayment over the eave drip edge so water that gets under the covering still drains onto the metal and into the gutter.
2. Apply underlayment (and ice barrier where required) up the roof.
3. At rakes (sloped gable edges): install drip edge over the underlayment so wind-driven rain on the rake cannot get under the underlayment at the edge.

That eave-under / rake-over relationship is a frequent training and exam concept. If instructions reverse a product-specific detail, the product instructions and approved drawings win—but “eave drip edge under underlayment; rake drip edge over underlayment” is the default literacy answer when the item describes standard asphalt-shingle preparation.

Thermal and Moisture Protection at the Roof (Module 27203 Lens)

Roofs fail from water and from heat/moisture mismanagement:

• Heat shortens asphalt shingle life and drives attic temperatures that stress HVAC and sheathing.
• Moisture from interior vapor, wet construction materials, or leak paths can condense on cold sheathing, grow mold, and rot framing.
• Ice dams form when warm attic air melts snow that refreezes at cold eaves—hence ice barriers, air sealing, insulation at the exterior wall line, and balanced ventilation.

Insulation and ventilation work together. Ceiling insulation should reach the full required R-value without crushing ventilation baffles at the eaves. Soffit intake vents need clear air paths to ridge or high exhaust vents. Blocking intake with over-stuffed insulation is a classic defect. Continuous air barriers at the ceiling plane reduce moist indoor air from loading the attic.

Vapor retarders / retarders: Placement depends on climate and assembly design. The carpenter’s job is to install the specified membrane orientation and seal penetrations—not to redesign vapor control on the fly. At roof penetrations (vents, chimneys, skylights), thermal/moisture continuity means boots, flashings, and underlayment are integrated so water cannot enter around the hole.

Field Sequence Snapshot: Weather Barrier Stage

1. Verify roof deck is clean, dry, fastened, and within flatness tolerances.
2. Install eave drip edge (as specified).
3. Install ice barrier in cold-climate eave/valley zones if required.
4. Install underlayment up the slope with correct laps; integrate valleys per product method.
5. Install rake drip edge over underlayment.
6. Flash walls, chimneys, and curbs as work proceeds (detail next chapter sections).
7. Install primary roof covering (shingles, metal, or commercial membrane).

Commercial vs Residential Awareness

Residential steep-slope work centers on felt/synthetic underlayment, ice barriers, and shingles or metal. Commercial low-slope work may use cover boards, base sheets, vapor retarders over decks, insulation layers in multi-ply assemblies, and fully adhered or mechanically attached single-ply membranes. The principle is the same: continuous water-shedding layers, protected edges, and controlled moisture. The products change with slope and specification.

Exam Traps to Avoid

• Calling every peaked roof a gable (hips have no tall end gables).
• Treating underlayment as optional cosmetic wrap.
• Putting rake drip edge under underlayment the same way as eaves without thinking about wind-driven rain.
• Blocking soffit vents with insulation “to get more R-value.”
• Using steep-slope shingles on a low-slope commercial deck without manufacturer approval.

Mastering roof-type names, underlayment purpose, ice-barrier zones, drip-edge sequence, and the thermal/moisture link between insulation and ventilation covers the prep half of 27202/27203 before shingles and commercial membranes go on.