Asphalt Shingles and Wood Roofing Materials

Asphalt Shingles and Wood Roofing Materials

The roof is the building's primary weather barrier. A home inspector must understand how each roofing system is assembled, recognize the signs of failure, and document findings precisely enough that a client, contractor, or insurer can act on them.

The Asphalt Shingle Roof Assembly

A standard asphalt shingle roof is built in layers, each serving a specific function.

Decking (sheathing) is the structural base, typically plywood or oriented strand board (OSB) fastened to the rafters or trusses. Check for sagging, soft spots, and visible deterioration at the eaves.

Underlayment lies directly on the decking and acts as a secondary water barrier. Traditional felt underlayment (No. 15 or No. 30 felt) has largely been joined by synthetic and self-adhering ice-and-water-shield products. Ice-and-water shield is required at eaves, valleys, and other vulnerable locations in most jurisdictions — always note when it appears to be absent.

Starter course is a specially cut or manufactured strip installed along the eaves before the first full course of shingles. It fills the gaps at shingle cutouts and prevents wind-driven rain from entering under the first course.

Field shingles make up the main roof surface. Three-tab shingles have uniform cutouts and a single flat layer. Architectural (laminated) shingles bond two layers together during manufacture, producing a dimensional appearance and generally longer rated service life. Field shingles must be nailed in the manufacturer's specified nail zone; nailing too high (high-nailing) reduces wind resistance and can cause blow-offs.

Ridge cap covers the peak where two slopes meet. Purpose-made ridge cap shingles or cut field shingles are overlapped to shed water away from the peak.

Flashing — metal or flexible membrane — seals transitions at walls, chimneys, skylights, and valleys. Step flashing, counter flashing, and valley flashing are the most common types. Missing, rusted, or improperly lapped flashing is a leading cause of roof leaks.

Common Asphalt Shingle Defects

During inspection you should look for at least these defects:

• Granule loss: bare or thin spots where the ceramic granules have worn away, exposing the asphalt mat. Granule loss accelerates UV degradation.
• Cupping and curling: edges that turn upward (cupping) or corners that lift (clawing) indicate age, improper ventilation, or moisture imbalance in the shingle.
• Blistering: bubbles in the shingle surface caused by volatile gases trapped during manufacture or poor ventilation.
• Fishmouthing: an open, lifted seam at the bottom edge of a shingle, often caused by high-nailing or an out-of-flat deck.
• Improper nail placement (high-nailing): nails placed above the manufacturer's nailing strip leave the fastener head under only one shingle layer, reducing holding strength.
• Exposed fasteners: nail or staple heads visible on the surface instead of covered by the overlapping course above.
• Missing tabs: sections of three-tab shingles blown away, creating unprotected gaps.
• Improper overhang: eave overhang outside the range typically specified by manufacturers (commonly ¼ to ¾ inch) — too little allows water to wick back under; too much allows wind uplift.
• Hail damage: circular impact marks, often with a soft, bruised feel to the mat; may appear as a random pattern across the slope.
• Algae and moss growth: dark staining (algae) or raised green growth (moss). Moss can lift shingle edges and trap moisture against the deck.

Wood Shake and Shingle Roofing

Wood roofing uses either shingles (sawn on both faces, uniform thickness from butt to tip) or shakes (split on at least one face, creating a rougher texture and greater thickness variation). Both are most commonly western red cedar.

Key assembly differences from asphalt: wood roofing requires an open or spaced deck (skip sheathing) or, on solid decking, an interlayment of felt between each course to allow the wood to breathe and dry. Without proper ventilation and drainage, the wood rots prematurely.

Common wood roofing defects:

• Splitting, cracking, or excessive checking beyond normal weathering
• Cupping or warping of individual shakes/shingles
• Missing or displaced pieces
• Moss or lichen growth holding moisture
• Rot at butt ends or along shake edges
• Improper exposure (too much shingle exposed to weather reduces the effective number of weathering layers)
• Inadequate headlap (the amount each course overlaps the course two layers below it)
• Debris accumulation in valleys holding moisture

Recording Findings with Correct Terminology

Field notes must specify what the defect is, where it is (use clock-position on the roof plane — e.g., "north slope, 3-o'clock position, approximately 6 feet from the ridge"), and a recommended action ("monitor," "repair," or "replace" with a brief reason).

Safety During Roof Inspection

Never climb a roof that is wet, icy, excessively steep (generally above 6:12 for most inspectors), or visibly deteriorated to the point of structural risk. Wear rubber-soled footwear with good grip. Use a properly secured ladder (three points of contact at all times). If conditions prevent safe walking, inspect from the eaves, from a ladder, or with binoculars and a camera — and document why you did not walk the roof. Fall protection (harness and anchor) is required whenever OSHA-regulated work is performed at height; inspectors operating under their own professional guidelines should follow InterNACHI and ASHI safety standards for roof access.