Roof Assessment, Structural Evaluation, and Load Basics

Why Roof Assessment Comes First

Before a single bracket is bolted down, you must know whether the roof can safely carry the added weight and forces of a PV array. A rooftop solar installation imposes new loads on a structure that was designed years or decades ago for other purposes. Skipping a thorough roof assessment is one of the most common causes of failed inspections, callbacks, and — in worst cases — structural damage. This lesson teaches you to gather the information that drives every attachment decision.

Personal Protective Equipment and Jobsite Safety

Even on a mock roof section at floor level, build the habit now. On an actual job, roof work requires a hard hat, non-slip footwear, and gloves as a minimum. Working at height adds fall-protection requirements (harness, anchor, lanyard) and compliance with OSHA standards for residential roofing. Never walk on a roof section without confirming it can hold your weight. Identify all skylights and fragile surfaces before stepping anywhere. Keep tools tethered or in a bag — a dropped tool from a roof is a falling-object hazard for anyone below.

Locating Rafters

Rafters (or roof trusses) are the structural members that carry the roof deck and everything placed on it. Lag bolts for racking must land in rafters, not just in the sheathing. Sheathing alone — typically OSB or plywood — cannot carry the tensile pullout load that wind uplift creates.

Use a stud finder set to the deepest scan mode and run it along the underside of the sheathing (from inside the attic if accessible) or across the top surface. Mark each hit with chalk or tape. Confirm with a probe or a small-diameter finish nail: the nail should meet solid resistance at the rafter center. Measure and record the on-center (OC) spacing — most residential roofs use 16-inch or 24-inch OC framing, but older or custom homes can vary. Measure at least three consecutive spaces and average them; do not assume uniform spacing.

Assessing Sheathing Condition

Once rafters are located, examine the sheathing between them. Good sheathing feels firm and does not deflect noticeably underfoot. Soft spots, dark staining, crumbling edges, or visible rot indicate moisture damage. Damaged sheathing must be repaired before any racking is attached; a lag bolt in rotted OSB will not hold design loads. Probe the sheathing at suspect areas and record your findings on the site assessment form. Note the estimated thickness (typically ½ inch to ¾ inch for residential OSB or plywood).

Identifying Roofing Material

The roofing material determines which attachment hardware and flashing method you use. The most common residential types are:

• Asphalt shingles — most common; lag-bolt-through-flashing method is standard.
• Concrete or clay tile — tiles must be removed and replaced around flashings; specialized tile hooks are available.
• Standing-seam metal — clamps attach directly to the seams without penetrating the roof membrane.
• Low-slope membrane (TPO, EPDM, modified bitumen) — ballasted or adhered systems avoid most penetrations.

Record the material, its apparent age and condition, and any visible cracking, curling, or granule loss. An aging roof nearing end of life should be flagged for the customer — it is far better to reroof before the array goes up than to remove and reinstall panels later.

Measuring Roof Pitch

Pitch describes slope as rise over run — how many inches the roof rises for every 12 inches of horizontal distance. A 6:12 pitch rises 6 inches per foot. Use a pitch gauge (a tool with a swinging indicator) placed flat against the roof surface for a direct reading. Pitch affects drainage, worker safety, and snow-load calculations. Record pitch as a ratio (e.g., 6:12).

Understanding Structural Loads

Four load types matter for PV attachment:

• Dead load — the constant weight of the roof structure, sheathing, roofing material, and the PV array itself. Modules, racking, and wiring add roughly 2–5 pounds per square foot depending on system design.
• Live load — temporary loads such as workers and equipment during installation or maintenance.
• Snow load — accumulated snow weight, governed by geographic location and roof pitch. Steeper pitches shed snow faster and carry less accumulated load.
• Wind uplift — negative pressure (suction) that tries to peel the array off the roof. This is often the governing load for attachment design and varies with wind speed, roof zone (field, edge, corner), and roof height.

Manufacturers publish load tables that combine these factors. Given a roof pitch, design wind speed, and rafter span, the table tells you the maximum allowed spacing between attachment points (lag bolts into rafters). You do not need to perform structural engineering calculations yourself — that is the engineer of record's job — but you must be able to read the table correctly and apply its requirements in the field.

Using the Manufacturer's Load Table

Load tables are organized by variables such as wind speed (mph), roof pitch range, and rafter or purlin span. Find the row matching your wind speed and pitch, then read across to your rafter span column. The intersection gives you maximum attachment spacing in inches or feet. If your calculated spacing exceeds the table maximum, you must add more attachment points. Always use the most conservative (smallest) value when your conditions fall between table entries. Record the table version, page number, and all inputs on the site assessment form so the Authority Having Jurisdiction (AHJ) can verify your work.

Completing the Site Assessment Form

A complete form is a legal and professional document. Every field must be filled in: rafter OC spacing (measured, not assumed), pitch, sheathing thickness and condition, roofing material and age estimate, and the load table attachment spacing calculation with inputs shown. Incomplete forms delay permits and inspections. Get it right the first time.