Tree impacts on houses can happen suddenly and without warning. They can be a result of major weather events such as hurricanes and tornadoes or a common summer storm. They can even occur during precipitation with little-to-no major wind gusts. The damage created by a fallen tree can range from isolated and easily repairable to catastrophic, depending on how the tree falls onto a house’s structure. Here we’ll discuss a few of the steps that VERTEX’s Forensic Engineers take when assessing a tree impact.
If the date of loss is provided, we can use weather data from the National Weather Service and other online resources to track the magnitude and direction of a specific storm. The speed of sustained winds or maximum gusts can tell us if a particular storm exhibited extraordinary forces on a tree or branch. The direction of these sustained winds or gusts can inform us of an approximate direction of the tree fall. This data can provide us a frame of reference for how and where the tree contacted the house or building.
VERTEX is often contacted after the tree(s) have been removed from the building and temporary tarps have been installed to enclose openings in roofs and walls. In this situation, we will often review available photos or videos taken by homeowners, contractors, or adjusters while the tree was still in contact with the building. These images can help us focus our investigation during the site visit. Our initial focus will be on any structural members or interior finishes within this impact zone that could have been directly impacted by the fallen tree. In addition, historical photographs of the property available from the homeowner or online can inform us of details of the pre-loss condition. It can be advantageous to research photos of the whole tree prior to the incident as well. Some trees have tall slender trunks, whereas others have multiple trunks, and others have secondary branches. Depending on the size of the tree and the size of the house, the trunk could make primary contact with the rear wall while secondary branches and twigs could stretch over the ridge of the roof and make contact with the front eave or wall. Greater diameter and/or proximity of the trunk of the tree can in turn create greater momentum as the mass of the tree falls towards the house or building.
The age and/or type of construction of a house can also affect its performance during a tree impact. Older houses and buildings were typically constructed with more interior walls and designed with conventional (stick) framed roofs that used larger framing members. Comparatively, modern/newer buildings tend to be constructed with more open (great room) floor plans with less robust roof framing members.
For example, a house constructed in the 1950s-1970s could feature roof framing consisting of ¾-inch thick wood plank roof decking over 2×6 or 2×10 rafters spaced at about 16 inches.
In comparison, a more modern house (the 1990s to current) would more likely be constructed with 7/16-inch thick plywood roof decking over prefabricated wood trusses (comprised of 2×4 web members) spaced approximately 24 inches apart.
A benefit of prefabricated wood trusses is their ability to span greater distances, which allows for more open floor plans with fewer interior walls for homeowners and material and labor savings for residential contractors. However, the combination of larger spans, fewer interior walls, wider spacing between framing members, thinner roof decking, and/or relatively smaller wood truss components, can result in less robust protection from fallen tree impact for homeowners.
Tree Impact Repair Protocols
In addition to the obvious repairs to the roof decking and interior finishes, the repair protocols for older conventional (stick) framed construction often involves a relatively basic replacement or sistering of damaged rafters, ceiling joists, wall studs, etc. In modern houses, repairs to fractured prefabricated wood trusses can be much more complicated. Initially, the repairing of even a single damaged web member within a truss could require shoring of the entire truss as to “zero out” the loads in all of the truss members.
The removal and replacement of a truss might sound simple in theory. However, depending on the size and geometry of the replacement truss, large trucks may be needed for delivery and cranes may be needed to hoist this truss up to the roof. In addition, a modern house was most likely originally constructed in an open construction lot. After a tree impact, repair contractors may have to navigate a finished property that could include landscaping, curbs, sidewalks, neighbors, or other potential obstacles.
It should be noted that truss replacement can also include secondary repair items from one or more non-structural trades (mechanical, plumbing, gas, electrical). Often pipes, ducts, and/or conduits have been installed in between trusses and web members throughout the attic space and these utilities may require adjustment or repair due to removal and replacement of one or more roof trusses.
How Can VERTEX Help?
VERTEX’s Forensic Division can provide our clients with assistance in the aftermath of these incidents regarding Cause & Origin and/or Damage Assessment. We can also provide Repair Recommendations based on our observations. Together with our team of on-staff estimators, we can also calculate an approximate cost to restore a damaged house or building back to its pre-loss condition.
Joseph Bednarz, PE
Forensic Structural Engineer