6.1 Building Construction Types, Framing, and Roofing Assemblies

ISO Commercial Building Construction Classes

In property insurance underwriting and claims adjustment, building structures are classified according to their combustibility and fire-resistance characteristics. The standard classification system developed by the Insurance Services Office (ISO) establishes six distinct commercial construction classes (Classes 1 through 6). A public adjuster must master these classifications to properly interpret policy coverage, verify underwriting classifications, assess structural fire spread, and calculate replacement costs.

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Residential Wood Framing Systems

Residential property in Illinois is predominantly constructed using light-frame wood construction. Understanding framing systems and their individual structural members is essential when scoping fire, wind, water, and structural impact losses.

Platform Framing vs. Balloon Framing

The two primary wood framing systems encountered by adjusters in Illinois represent distinct architectural eras and present radically different forensic fire dynamics.

          BALLOON FRAMING (Pre-1940)                    PLATFORM FRAMING (Modern)

       Roof Rafters                                  Roof Rafters
       ┌──────────┐                                  ┌──────────┐
       │          │                                  │          │
       │ 2nd Fl.  ├────────┐                         │ 2nd Fl.  ├────────┐
       │ Joists   │ Studs  │                         │ Joists   │ Studs  │
       │ (Ribbon) │ Run    │                         └──────────┴────────┘
       │          │ Con-   │                      ═══ 2nd Floor Subfloor Platform ═══
       │          │ tinuous│                         ┌──────────┬────────┐
       │ 1st Fl.  │ From   │                         │ 1st Fl.  │ Studs  │
       │ Joists   │ Sill   │                         │ Joists   │ Stop   │
       │          │ To     │                         └──────────┴────────┘
       │          │ Roof   │                      ═══ 1st Floor Subfloor Platform ═══
       │          │        │                         ┌───────────────────┐
  ─────┴──────────┴────────┴─────               ─────┴───────────────────┴─────
        Foundation Wall                                Foundation Wall

  * UNRESTRICTED VERTICAL FLUE CHANNELS          * INHERENT HORIZONTAL FIRE-BLOCKING
  * RAPID SPREAD: BASEMENT TO ATTIC              * SLOWER COMPARTMENTALIZED SPREAD

1. Balloon Framing (Historic Construction)

• Era & Distribution: Dominant from the 1880s through the late 1930s, balloon framing is widespread throughout older Chicago neighborhoods, historic residential areas in Peoria, Rockford, Springfield, and across rural Illinois.
• Structural Anatomy: The exterior wall studs are continuous, single pieces of lumber (often actual 2x4 dimensional lumber) extending uninterrupted from the foundation mud sill all the way to the top plate supporting the roof rafters. Second-story floor joists rest upon a 1x4 inch horizontal board called a ribbon (or ledger board) notched into the continuous studs.
• Forensic Fire Hazard: Because the stud cavities are unbroken from basement to attic, they form completely open, vertical convective flues (chimneys). When a fire ignites in the basement or first floor, hot buoyant gases and flames bypass the living quarters and draft upward through the hollow wall cavities directly into the attic within minutes. In claims scoping, balloon-framed structures frequently suffer total attic and roof destruction from relatively minor basement electrical fires. In remediation, building codes often require the installation of fire stops (solid 2-inch blocking) whenever these wall assemblies are exposed.

2. Platform Framing (Modern Standard)

• Structural Anatomy: In platform (or western) framing, each story is constructed as an independent, self-contained box or platform. The first-story wall studs rest on the first-floor deck and terminate at a double top plate. The second-story floor joists are placed directly on top of this double plate, and a subfloor deck is installed to create the next working platform before second-story wall studs are erected.
• Fire Stopping Advantage: The solid wood sole plates and double top plates naturally seal the top and bottom of each wall cavity, providing inherent horizontal fire blocking that confines flame spread to a single room or floor until burn-through occurs.

Key Structural Framing Members

When writing a line-item estimate in Xactimate or Symbility, public adjusters must accurately specify framing components:

• Mud Sill (Sill Plate): The bottom horizontal lumber member (typically preservative-treated 2x6) anchored directly to the top of the foundation wall with 1/2-inch steel anchor bolts embedded at least 7 inches into the concrete. A closed-cell foam sill seal gasket prevents air and moisture infiltration.
• Wall Studs: Vertical load-bearing members spaced either 16 inches on center (OC) (standard construction supporting multi-story loads) or 24 inches OC (non-bearing interior partitions or engineered advanced framing). Standard dimensional framing utilizes nominal 2x4 (actual 1-1/2" x 3-1/2") or 2x6 (actual 1-1/2" x 5-1/2") lumber.
• Floor Joists: Horizontal repetitive structural members supporting floor decking. Common materials include dimensional 2x10/2x12 lumber, engineered wood I-joists (with OSB webs and solid flanges), or laminated veneer lumber (LVL).
• Headers (Lintels): Heavy horizontal beams placed above door and window openings to transfer vertical roof and floor loads around the opening down through jack studs (trimmer studs) to the foundation.
• Rafters vs. Engineered Roof Trusses:
  • Conventional Rafters: Stick-built dimensional lumber sloping upward from the exterior wall top plates to a central horizontal ridge board, stabilized by ceiling joists or collar ties.
  • Engineered Light-Frame Wood Trusses: Factory-prefabricated planar structural triangular assemblies composed of top chords, bottom chords, and web members connected at joints by light-gauge stamped-metal connector plates (gusset plates or "gang nails").

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Foundation Systems and Forensic Structural Movement Analysis

The foundation system transfers the dead loads (building weight) and live loads (occupants, furniture, snow, wind) safely to the supporting subgrade soil. Public adjusters frequently investigate cracks and structural movement following heavy storms, water main breaks, or blasting.

Foundation Types Common in Illinois

1. Full Basement: Poured concrete or concrete masonry unit (CMU / cinder block) walls typically 8 to 10 inches thick, resting on continuous concrete footings located below the local frost depth (typically 42 to 48 inches in northern and central Illinois). Provides living, mechanical, and storage space.
2. Crawlspace: Unfinished space between the ground and first floor, enclosed by low foundation walls. Requires ground vapor barriers (minimum 6-mil polyethylene) and adequate mechanical or passive ventilation to prevent severe microbial mold colonization.
3. Slab-on-Grade: A monolithic concrete slab poured directly over compacted gravel and a continuous vapor retarder. In northern Illinois, floating slabs must incorporate perimeter frost-footings extending below the frost line to prevent catastrophic winter frost heave.

Settlement vs. Hydrostatic Lateral Pressure Cracking

A critical distinction on property claims is determining whether foundation cracking is caused by long-term non-covered soil settlement or sudden, storm-induced lateral hydrostatic pressure.

     VERTICAL FOUNDATION SETTLEMENT             LATERAL HYDROSTATIC SOIL PRESSURE

      ┌─────────────────────────┐                 ┌─────────────────────────┐
      │   CMU / Block Wall      │                 │  CMU or Poured Wall     │
      │                         │                 │     Saturated Soil ────►│
      │         ┌───┐           │                 │     High Water Table ──►│
      │     ┌───┘   └───┐       │  ◄── Stair-Step │═════════════════════════│  ◄── HORIZONTAL
      │ ┌───┘ Diagonal  └───┐   │      Mortar     │     Inward Bowing /     │      FRACTURE AT
      │─┘     Shear Crack   └───│      Fracture   │     Shear Deflection    │      MID-HEIGHT
      └─────────────────────────┘                 └─────────────────────────┘
      [ Uneven Footing Settlement ]               [ Extreme Lateral Earth Thrust ]

• Differential Foundation Settlement (Vertical Movement):
  • Mechanisms: Soil consolidation, inadequate subgrade compaction, desiccation of reactive expansive clay during severe summer droughts, or soil washout beneath a footing.
  • Crack Pattern: Manifests as diagonal shear cracks in poured concrete walls or distinct stair-step cracks following horizontal and vertical mortar joints in concrete block walls. Cracks are wider at the top and taper toward the bottom, indicating downward movement of one section of the footing relative to the remainder. Associated physical indicators include racked door frames, sticking windows, and sloping interior floors.
• Hydrostatic Lateral Pressure (Horizontal Deflection):
  • Mechanisms: Torrential rainstorms or failed perimeter drain tiles cause the water table to rise rapidly around the exterior basement perimeter. The soil becomes fully saturated, exerting immense horizontal hydrostatic thrust against the exterior wall face.
  • Crack Pattern: Manifests as a continuous horizontal crack traversing the length of the wall, typically located between the third and fifth block courses (roughly at the mid-height of the basement wall where lateral bending moment is maximized). The wall exhibits noticeable inward bowing (bulging) and horizontal shear displacement along the crack plane. If unmitigated, sudden catastrophic inward wall collapse occurs.

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Steep-Slope Roofing Assemblies and Damage Assessment

Roofing assemblies are categorized by slope: steep-slope roofs have an incline greater than 3:12 (or 4:12 depending on code classification), shedding water primarily via gravity, whereas low-slope roofs (incline ≤ 2:12 or 3:12) require completely watertight membrane systems.

Asphalt Composition Shingles

Asphalt shingles cover the vast majority of residential structures in Illinois. They consist of an internal reinforcing mat coated on both sides with mineral-stabilized asphalt waterproofing and surfaced on the weather-exposed face with ceramic-coated mineral granules.

1. Reinforcing Mat Types:
   • Fiberglass Mat: Non-woven glass fibers bonded with urea-formaldehyde resin. Highly fire-resistant (Class A fire rating), dimensionally stable, and resistant to rotting. Forms the backbone of virtually all modern residential shingles.
   • Organic Mat: Cellulose fibers derived from recycled paper, wood fiber, and rags. Absorbs moisture, prone to thermal degradation and premature curling. Phased out of production by major manufacturers in the late 2000s; when encountered on older roofs, organic shingles frequently exhibit severe brittleness and extensive physical damage.
2. Shingle Configuration:
   • 3-Tab Shingles: Flat, single-layer strip shingles with two cutouts creating three individual tabs. Uniform thickness (approx. 1/8 inch), lower wind resistance ratings (typically 60 mph), and shorter design lifespans (20–25 years).
   • Architectural (Laminated/Dimensional) Shingles: Multi-layered shingles constructed by laminating two or more asphalt-fiberglass sheets together. Lacks cutouts, provides a dimensional shadow-line appearance, features higher wind ratings (110–130 mph), and greater resistance to hail puncture.
3. Mineral Granules: Ceramic-coated crushed rock granules serve three vital functions: protecting the underlying asphalt waterproofing from ultraviolet (UV) solar degradation, providing aesthetic coloration, and conferring flame-retardant surface resistance.
4. Self-Sealing Adhesive Strip: A factory-applied thermal sealant strip (modified asphalt or SBS adhesive) located on the face or underside of the shingle. When heated by ambient solar radiation (temperatures exceeding 100°F–120°F at the roof deck), the strip activates and bonds the overlapping shingle course firmly in place to resist wind uplift.

Forensic Hail vs. Wind Damage Assessment

Insurance carriers and public adjusters frequently dispute whether roof damage is covered storm damage or non-covered natural weathering and manufacturing defects.

               FORENSIC ASPHALT SHINGLE DAMAGE PROFILES

     WIND UPLIFT & CREASING                    HAIL IMPACT BRUISE
   ┌──────────────────────────┐             ┌──────────────────────────┐
   │  Top Half (Covered)      │             │  Scattered Granules      │
   │                          │             │  Dislodged               │
===│══════════════════════════│=== CREASE   │        ▼                 │
   │  Broken Seal Strip       │             │     ( ░░░ ) ◄── Granule Loss│
   │  Shingle Tabs Flutter    │             │    ( ▓▓▓▓▓ ) ◄── Fractured   │
   │  Horizontal Stress Line  │             │     ( ░░░ )     Subsurface   │
   │  Across Upper Shingle    │             │                 Fiberglass   │
   └──────────────────────────┘             └──────────────────────────┘

     MANUFACTURING BLISTER                     MECHANICAL / FOOT TRAFFIC
   ┌──────────────────────────┐             ┌──────────────────────────┐
   │                          │             │                          │
   │          ╭───╮           │             │     ╲╲╲╲╲╲               │
   │         │     │          │             │      ╲╲╲╲╲╲ ◄── Scrape Mark│
   │    Dome Punctured        │             │                          │
   │    Hollow Cavity Beneath │             │    Granules Rubbed Off   │
   │    Mat Undamaged & Intact│             │    Intact Flat Mat       │
   └──────────────────────────┘             └──────────────────────────┘

1. Forensic Hail Damage Indicators

Under established industry forensic standards (including Haag Engineering and ASTM standards), functional hail damage to an asphalt shingle must diminish the water-shedding capability or long-term service life of the roofing material. True hail damage exhibits all of the following criteria:

• Localized Granule Loss: Ceramic granules are forcibly dislodged by direct kinetic energy impact, exposing the dark asphalt coating beneath.
• Fractured Reinforcing Mat: The underlying fiberglass mat suffers a distinct tear, puncture, or circular micro-fracture directly beneath the impact point.
• Tactile Soft Bruise: Pressing the thumb into the impact depression reveals a soft, spongy, localized loss of structural integrity (loss of mat bond) compared to the surrounding firm substrate.
• Random Directional Pattern: Spatter marks and bruises are randomly distributed across hail-facing roof slopes and correspond with directional collateral damage (dented metal gutters, downspouts, aluminum siding, AC condenser fins, and soft metal roof vents).

2. Differentiating Hail from Blisters and Scuffs

• Manufacturing Blisters: Raised, circular domes caused by volatile gases or trapped moisture expanding within the asphalt during manufacturing or extreme summer heat. When a blister pops, it leaves a crater with a small ring of exposed asphalt, but the underlying fiberglass mat remains completely intact and unruptured, with no soft bruise.
• Mechanical Scuffs / Foot Traffic: Abrasions caused by shoes, tools, or dragged ladders. They produce horizontal or directional scrape lines where granules are sheared off surface asphalt, without circular depression or subsurface mat puncture.
• Bird Droppings (Spatter Mimics): Whitish, localized surface spots that mimic oxidation or hail spatter but wash away with water without granule loss.

3. Wind Damage Indicators

• Sealant Failure: High aerodynamic negative pressure (uplift) breaks the thermal bond of the sealant strip along the eaves, rakes, or main slope.
• Horizontal Creasing: Once unsealed, wind-driven fluttering bends the shingle tab back and forth, creating a permanent horizontal fracture or dark creasing line across the top of the exposed tab where surface granules fracture off.
• Missing Tabs: Complete tearing of shingle tabs along the fastener line, leaving exposed nail heads and compromised underlayment.

Other Steep-Slope Coverages

• Wood Shakes vs. Wood Shingles: Wood shingles are machine-sawn with smooth, uniform tapered surfaces; wood shakes are hand- or machine-split, resulting in a thick, textured, highly irregular surface. Hail strikes split aged shakes along the natural grain line; fresh storm splits exhibit light, unoxidized wood beneath the fracture line, whereas pre-existing weathering cracks show dark, weathered, oxidized wood.
• Clay and Concrete Tile: Heavy dead load requiring reinforced roof framing. Susceptible to impact shatter and corner breaks from large hail or foot traffic.

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Low-Slope Commercial Roofing Assemblies

Low-slope commercial roofs (found on flat-roof retail centers, industrial warehouses, and multi-family structures throughout Illinois) utilize continuous watertight membranes.

1. Built-Up Roofing (BUR)

• Anatomy: Commonly called "tar and gravel," BUR consists of 3 to 5 alternating layers (plies) of asphalt- or coal-tar-saturated organic or fiberglass roofing felts laminated with coats of hot bitumen. Surfaced with aggregate gravel embedded in flood-coat asphalt to provide UV shielding and fire resistance.
• Damage Assessment: Highly resistant to small hail. Severe hail fractures the top bitumen flood coat, creating ring fractures. Freeze-thaw cycling accelerates water intrusion into saturated insulation beneath.

2. Modified Bitumen Systems

Asphalt sheets modified with specialized polymers to impart elasticity and thermal performance:

• SBS (Styrene-Butadiene-Styrene): Modified with synthetic rubber. Exhibits exceptional cold-weather flexibility and elasticity, resisting fatigue from thermal shock during extreme Illinois winter temperature swings. Applied using hot asphalt mop, cold elastomeric adhesive, or self-adhering backing.
• APP (Atactic Polypropylene): Modified with thermoplastic plasticizer. Imparts a plastic-like quality, high softening point, and excellent UV resistance. Typically torch-applied (open propane flame melting the asphalt backing to bond to the substrate).

3. Single-Ply Synthetic Membranes

Factory-manufactured elastomeric or thermoplastic sheets installed in large single-layer widths:

• EPDM (Ethylene Propylene Diene Monomer): A thermoset synthetic rubber membrane (typically 45 to 60 mils thick), black in color. Seams are joined using specialized primer and butyl-based seam tapes. Vulnerable to seam oxidation and shrinkage over time.
• TPO (Thermoplastic Polyolefin) & PVC (Polyvinyl Chloride): Thermoplastic membranes (typically white for high solar reflectivity / Cool Roof compliance). Seams are fused using robotic hot-air welding equipment, creating a monolithic, molecular bond that is structurally stronger than the membrane itself. Highly resistant to chemical degradation, grease (PVC), and punctures.

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Flashing Systems and Critical Waterproofing Details

Many roof leaks occur at intersections, penetrations, and transitions where flashing is installed. Proper scoping requires verifying the integrity of all flashing components:

• Step Flashing: Individual L-shaped metal pieces (typically 4"x4"x8" aluminum, copper, or galvanized steel) bent at a 90-degree angle, interleaved with each successive course of shingles along a sloped roof-to-vertical-sidewall transition.
• Counter-Flashing (Cap Flashing): Metal flashing installed over base flashing or step flashing, with its top edge embedded into a saw-cut groove (reglet) in masonry brick or concrete walls, sealed with elastomeric polyurethane sealant.
• Valley Flashing: Located at the internal V-shaped intersection of two sloping roof planes. Can be constructed as open valleys (lined with preformed W-profile heavy metal flashing) or closed valleys (closed-cut or woven shingles overlapping across the valley centerline).
• Drip Edge Flashing: L-shaped or T-shaped corrosion-resistant metal installed along eaves beneath the underlayment and along rakes over the underlayment, directing shedding rainwater away from the wood fascia and into gutters.

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Illinois Building Code Considerations: IRC Ice Barrier Requirements

Illinois has no single statewide residential building code for most homes; municipalities and counties adopt and amend model codes such as the International Building Code (IBC) and International Residential Code (IRC), so always confirm the local edition. In the Midwest, severe winter weather and repeated freeze-thaw cycles create severe ice damming.

Physics of Ice Damming

1. Snow accumulates on the roof deck.
2. Inadequate attic insulation or air leakage allows interior building heat to warm the upper roof sheathing above 32°F (0°C), melting the bottom snow layer.
3. Water trickles down the roof slope beneath the snow blanket until it reaches the cold, unheated eave overhang (which remains below 32°F).
4. The water refreezes at the cold eave, forming an expanding ridge of solid ice—an ice dam.
5. Continuing meltwater pools behind the ice dam. Because asphalt shingles are designed to shed gravity-fed downward water, the pooled standing water flows backward and upward under the shingle laps, saturating roof decking, wall cavities, and finished ceilings.

                 CROSS-SECTION: EAVE ICE BARRIER CODE REQUIREMENT

                              /\ Snow Pack
                             /  \  (Melts over warm attic)
                            /    \  Water Pools Behind Dam ◄── Lifts Shingles
                           /  /
                          /  /  ◄── Ice Barrier Membrane (Self-Adhering Bitumen)
                         /  /       MUST EXTEND MINIMUM 24" INSIDE INTERIOR WALL
                        /  / ◄───[ Ice Dam Forms at Cold Eave ]
                       /  / 
  ════════════════════╪══╪═════════════════════════════════════════ Roof Deck
                      │  │
                      │  │ ◄─────── Minimum 24 Inches ────────►
                      │  │                                    │
                 ┌────┴──┴────┐                          ┌────┴────────┐
                 │   Fascia   │                          │ Interior    │
                 │   & Eave   │                          │ Exterior    │
                 │  Overhang  │                          │ Wall Line   │
                 └────────────┘                          └─────────────┘

Mandatory IRC Code Provision (IRC R905.1.2 / R905.2.7)

Under the IRC, where a jurisdiction has adopted it and designates a history of ice forming along eaves causing water backup, an ice barrier is required:

• The ice barrier must consist of at least two layers of underlayment cemented together or a self-adhering polymer-modified bitumen membrane.
• The membrane must extend from the lowest eave edge to a point at least 24 inches inside the interior line of the exterior wall of the building.