1.2 Site Survey and Roof Condition Assessment

Site Survey and Roof Condition Assessment

A rigorous site audit is the cornerstone of safe, code-compliant, and durable photovoltaic installations. The site survey bridges conceptual system sizing and physical field installation, confirming that the existing roof structure, building envelope, and electrical infrastructure can safely host the planned PV array for its 25-plus year operating lifespan.

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1. Comprehensive Site Audit Workflow and Tooling

Before mounting hardware or pulling electrical permits, an installation professional conducts an on-site evaluation covering structural, architectural, environmental, and electrical parameters.

Pre-Survey Preparation

• Review property satellite imagery, tax assessor property records, and permit history.
• Obtain utility rate schedules, past 12-month interval electricity consumption data, and existing electrical utility meter numbers.
• Check local building and electrical code editions adopted by the Authority Having Jurisdiction (AHJ), including local design wind speeds, ground snow loads, and seismic design categories.

Essential Field Survey Tools

• Safety & Fall Protection: Personal fall arrest systems (harness, shock-absorbing lanyard, temporary roof anchor), non-slip roofing boots, and OSHA-compliant fiberglass extension ladders.
• Structural & Geometry Measurement: Laser distance meters, 100-foot tape measures, digital inclinometer/pitch finder, stud finder, and deep-scan wood moisture meter.
• Electrical Verification: True-RMS digital multimeter (rated CAT III 1000V / CAT IV 600V), clamp-on ammeter, non-contact voltage detector, and caliper for conductor diameter measurement.
• Solar Resource Tools: Sighting compass with adjustable magnetic declination, panoramic digital solar assessment camera or optical pathfinder.

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2. Roof Substrate and Membrane Assessment

A photovoltaic system has an anticipated operating lifespan exceeding 25 to 30 years. If an existing roof covering has fewer than 10 to 15 years of remaining service life, installing a PV array over it creates a severe financial liability: removing and reinstalling the solar array to accommodate an imminent reroofing project adds substantial labor costs and voids warranties.

Asphalt Composition Shingle (Standard Sloped Roof)

• Lifespan: Standard 3-tab shingles last 15–20 years; architectural/laminate dimensional shingles last 25–30 years.
• Inspection Points: Examine for surface granule loss in gutters, curling or cupping edges, clawing, thermal cracking, brittle texture, or missing tabs. Inspect roof flashing around plumbing vents, chimneys, and dormer sidewalls.
• Rule of Thumb: If asphalt shingles are older than 12–15 years or show noticeable granule shedding, recommend homeowner reroofing prior to solar attachment.

Standing Seam Metal Roofs

• Lifespan: 50-plus years (ideal solar mounting substrate).
• Attachment Advantage: Clamps (such as S-5! standing seam clamps) grip the raised vertical seam mechanically using round-point setscrews without puncturing the roof sheet. This preserves the manufacturer's weather-tight warranty and eliminates water penetration risks.
• Inspection Points: Verify seam profile (snap-lock vs. standing mechanical double-lock), gauge of steel or aluminum (minimum 24-gauge or 26-gauge steel), and ensure substrate fastening is intact.

Concrete and Clay Tile Roofs

• Lifespan: Tiles last 50–100 years, but the underlying waterproofing underlayment (felt or synthetic) typically degrades within 20–25 years.
• Inspection Points: Check for cracked, broken, or displaced tiles. Walk only on the lower third of overlapping tiles to avoid breakage.
• Mounting Strategy: Use tile replacement mounts (metal flashing that replaces an entire tile) or heavy-duty stainless steel tile hooks bolted directly to framing rafters, avoiding point contact on fragile tile edges.

Low-Slope and Flat Commercial Roofs (TPO, EPDM, PVC, Built-Up)

• Membranes: Thermoplastic Polyolefin (TPO), Ethylene Propylene Diene Monomer (EPDM), Polyvinyl Chloride (PVC), and multi-ply asphalt built-up roofing (BUR).
• Inspection Points: Inspect seams for delamination, blistering, ponding water (areas retaining water 48 hours after rain), puncture wounds, and perimeter parapet flashing.
• Mounting Method: Unpenetrated ballasted racking systems utilizing concrete paver blocks, or mechanically attached systems with heat-welded flashing boots matching the roof membrane chemistry.

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3. Structural Framing and Load Path Evaluation

Every rooftop solar installation adds dead load and alters wind uplift forces. A physical attic and structural framing audit is necessary to confirm that structural members can support these applied loads.

Rafter Framing (Stick-Built) vs. Engineered Truss Systems

• Stick-Built Rafters: Individual dimensional lumber members (typically 2x4, 2x6, 2x8, or 2x10). Installers measure actual member dimensions, unsupported horizontal spans between the ridge and exterior bearing walls, and member condition (checking for dry rot, insect damage, or bowing).
• Engineered Wood Trusses: Factory-manufactured triangular assemblies joined by stamped metal gang-nail connector plates. Trusses rely on precise tension and compression forces in web members. Building codes strictly prohibit drilling, notching, cutting, or field-altering roof truss chords or webs without a stamped engineering repair design from a licensed Professional Engineer (PE).

On-Center (OC) Spacing and Fastener Engagement

• Standard residential framing is spaced either 16 inches on-center or 24 inches on-center.
• Mounting brackets must lag into the center of rafters or truss top chords. Standard lag bolts (5/16" diameter) require pre-drilling a pilot hole (typically 3/16" for a 5/16" lag) to prevent splitting the lumber, and must achieve a minimum structural embedment depth of 2.5 inches into solid wood, excluding roof sheathing thickness.

Roof Sheathing (Decking)

• Inspect roof decking from inside the attic: typically 7/16-inch or 1/2-inch Oriented Strand Board (OSB) or 1/2-inch to 5/8-inch CDX plywood.
• Check for water stains, mold, white efflorescence, or structural sag between rafters. Fasteners driven solely into sheathing without rafter engagement (unless using specialized, PE-certified decking anchors) have minimal pullout resistance and risk blow-off under design wind loads.

Structural Load Considerations

• Dead Load: Weight of solar modules, racking, hardware, and ballast. Flush-mounted residential systems add approximately 2.5 to 4.0 pounds per square foot (psf). Ballasted commercial systems typically add about 3 to 8 psf across the array, with heavier ballast in edge and corner wind zones.
• Live Load: Temporary loads imposed during construction and maintenance by workers and equipment (typically 20 psf).
• Snow Load: Regional ground snow load values must be adjusted for roof slope and exposure.
• Wind Uplift Load: Calculated in accordance with ASCE 7 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures).

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4. Roof Pitch, Tilt Angles, and Conversions

Roof slope in the United States construction industry is conventionally expressed as pitch: the ratio of vertical rise in inches per 12 inches of horizontal run (e.g., 4:12, 6:12, 8:12).

Mathematical Conversion from Pitch to Degrees

To convert roof rise-over-run pitch into an angular tilt ($\theta$) in degrees:

$\theta = \arctan\left(\frac{\text{Rise}}{\text{Run}}\right) \times \left(\frac{180}{\pi}\right) = \arctan\left(\frac{\text{Rise}}{12}\right) \times 57.2958$

Roof Pitch to Angle Conversion Table

Note on Building Code Classifications: The International Building Code (IBC) defines a low-slope roof as one with a slope less than 2:12 ($9.46^\circ$); a slope of 2:12 or greater is a steep-slope roof. Asphalt shingles are permitted only on slopes of 2:12 or greater, and slopes from 2:12 up to (but not including) 4:12 require double underlayment (IRC R905.2.2).

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5. Compass Declination: Correcting for True Solar South

A common and costly error during site audits is confusing Magnetic South with True Solar South. Solar geometry calculations rely exclusively on True South, which points directly toward the Earth's geographic South Pole.

Magnetic Declination Explained

A magnetic compass tracks the Earth's geomagnetic field lines, which converge at the Magnetic North and South Poles. The angular difference between Magnetic North and True Geographic North at any specific geographic location is known as magnetic declination (or magnetic variation):

• Westerly Declination (Negative): Common in the eastern United States. Magnetic north points west of true north.
• Easterly Declination (Positive): Common in the western United States. Magnetic north points east of true north.
• Agonic Line: The irregular line of $0^\circ$ declination that crosses the central United States. It drifts westward over time, so it now lies well west of where older textbooks draw it. Always look up the current declination for the site (for example, with NOAA's online magnetic field calculator) rather than relying on a printed map.

Rules for Compass Correction

When standing on a roof facing south to orient an array:

1. In regions with Westerly Declination, True South lies to the right (clockwise) of magnetic south on the compass dial: $\text{Magnetic Bearing to True South} = 180^\circ + |\text{Declination}|$ Example: In Boston, MA (Declination $\approx 14^\circ\text{W}$), True South is at $180^\circ + 14^\circ = 194^\circ$ magnetic.
2. In regions with Easterly Declination, True South lies to the left (counter-clockwise) of magnetic south: $\text{Magnetic Bearing to True South} = 180^\circ - \text{Declination}$ Example: In Seattle, WA (Declination $\approx 15^\circ\text{E}$), True South is at $180^\circ - 15^\circ = 165^\circ$ magnetic.

Regional Magnetic Declination Reference Table

Best Practice: Always adjust the declination setting screw on the housing of a professional sighting compass (such as a Suunto or Brunton surveyor compass) before beginning the field survey.

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6. Electrical Service Entrance and Panel Audit

The site survey must establish how and where solar generation connects to the premises wiring system in compliance with National Electrical Code (NEC) requirements.

Service Equipment Inspection Checklist

1. Main Service Panel (MSP) Location and Clearances: Confirm working space compliance under NEC 110.26 (minimum 30 inches wide, 36 inches deep, and 6.5 feet headroom of clear, unobstructed working space in front of the electrical panel).
2. Utility Meter & Service Rating: Read utility meter base data plate (e.g., 200A Class 200) and verify whether service entrance conductors are overhead service drop or underground service lateral.
3. Main Disconnect Rating ($I_{\text{main}}$): Note the ampacity rating of the main service circuit breaker (e.g., 100A, 150A, 200A).
4. Busbar Ampacity Rating ($I_{\text{bus}}$): Inspect the panel label to find the manufacturer's rated busbar ampacity. Often, a 200A rated panel has a 200A busbar, but some 200A panels feature a 225A busbar, which offers substantially more capacity for load-side solar backfeeding under the NEC 705.12(B) 120% rule.
5. Physical Breaker Space: Count available open breaker spaces. If the panel is fully populated, identify whether tandem breakers are listed on the panel directory label, or plan for a supply-side tap or service panel upgrade.
6. Existing Grounding Electrode System (GES): Identify driven ground rods, metallic underground water pipe bonding, and any concrete-encased electrode (Ufer ground), and check bonding jumper integrity. Under NEC 250.53(A)(2)–(A)(3), a single rod must be supplemented by a second electrode unless it measures 25 ohms or less to earth, and a supplemental rod must be at least 6 feet from the first.