Tree Impacts After Windstorms: How Forensic Engineers Scope the True Extent of Damage

When a tree hits a building, causation is usually obvious: the wind blew, the tree fell, the structure was struck. The harder part is everything that follows: separating superficial damage from structural damage, defining a defensible scope of repair, and distinguishing new impacts from pre-existing conditions. That’s where a forensic structural assessment earns its keep.

In the Pacific Northwest, our busiest season for tree impacts runs October through March. Storms normally blow in from the west; when we get a strong system from an unusual direction—say, a sustained east wind—trees that have “grown up” resisting different predominant loads can fail in large numbers. It’s not unusual to see thousands of properties affected in a single event.

Below is the approach I use on insurance and legal assignments to scope the true extent of damage after a tree strike.

Step 1: Confirm Causation

The first task is making sure the reported damage was truly caused by the recent storm, not a long-standing condition. That requires close field forensics:

• Fracture surfaces – Bright, sharp breaks usually indicate new damage; dull, dirty, or painted-over cracks often pre-date the storm.
• Debris in cracks – Dust, dirt, or paint suggests a condition that existed before impact.
• Patterns of distress – Tree loads are sudden and directional; pre-existing settlement or shrinkage looks very different under inspection.

Step 2: Trace Load Paths

A tree impact rarely stops where the trunk or limb makes contact. Energy moves through the building, sometimes damaging framing members far from the visible strike.

• Roof trusses can push forces into adjoining walls.
• Rafters may deflect, cracking drywall or plaster several feet away.
• Connections between framing members often reveal whether structural continuity has been lost.

Step 3: Define the Scope of Damage

Once causation is clear, the next question is: repair or replace? This is where the engineer’s scope becomes critical to insurers, attorneys, and contractors.

• Documenting what’s repairable – Elements like siding or interior finishes may be restored.
• Identifying what must be replaced – Compromised framing, damaged connections, or shifted foundations often require full replacement.
• Separating cosmetic from structural – What looks like a small crack could be the marker of deeper structural displacement.

Older vs. Newer Homes: Different Failure Modes

Not all buildings respond the same way to tree strikes. Older homes were commonly built with denser lumber and often had more ventilation. They may be less “flexible,” but materials can be remarkably tough. Newer homes are tighter and more energy-efficient, but tight assemblies can trap moisture after an impact if drying is not managed.

Practically, we treat each structure on its merits: materials, connections, and load path continuity tell the story.

The Bottom Line

Tree impacts are high-frequency, high-stakes claims in wind-prone regions. The cause may be obvious, but the true extent of damage is not. A structured forensic assessment—grounded in load paths, envelope integrity, and field forensics to age damage—produces scopes that are accurate, defensible, and cost-effective.

If you’re working a storm file with widespread tree strikes and need clarity on what’s cosmetic versus structural, or what to repair versus replace, an objective engineering scope can keep the claim on track from day one.

About the Author

Nels R. Peterson, P.E., M.S.C.E. is a Consulting Engineer in our Seattle-Tacoma Office. Mr. Peterson provides consultation in the areas of structural analysis, scope of damage, cause of damage, and water intrusion investigation. You may contact Nels for your forensic engineering needs at npeterson@edtengineers.com or (253) 345-5187.

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