Why does rooftop equipment added over the years matter in a roof collapse?
Rooftop equipment added over the years matters because it is added dead load: rooftop units, ductwork, piping, screen walls, satellite dishes, solar arrays and their supporting frames all impose load the original roof design may not have contemplated.
Individually each rooftop addition is modest, which is precisely the problem: nobody assesses the additions, and they accumulate over decades. Reconstructing what was on the roof at the time of failure, and when each item arrived, is the first step in assessing added dead load, and permit records and dated aerial imagery are usually the most productive sources for that reconstruction.
How does rooftop equipment change the way load is distributed on a roof?
Beyond its own weight, rooftop equipment changes how load distributes on a roof by collecting drifting snow and, when the equipment is placed near mid-span of a joist, by concentrating load where the joist is least able to take it. A rooftop unit is an obstruction that collects drifting snow, and a screen wall around a rooftop unit more so.
Rooftop equipment placed near mid-span of a joist imposes concentrated load where the joist is least able to take it, and equipment support frames spanning between joists can deliver load to points never designed for it. The location of a rooftop addition frequently matters more than its weight.
What happens when roof joist members are cut to run ductwork, conduit or pipe?
An open-web roof joist depends on its web for shear capacity, so when web members are cut to pass ductwork, conduit or pipe, removing a diagonal web member can reduce the joist’s shear capacity dramatically at the location where it is removed. Open-web joist web members cut for services are a recurring and serious finding.
Cutting open-web joist members to route services is typically done by mechanical or electrical trades with no structural review, it is invisible above a ceiling, and it commonly comes to light only after a failure. Examining the recovered roof members for cut, notched or modified elements is a standard step after a roof collapse.
Why does a roof’s re-roofing history matter when the roof collapses?
A roof’s re-roofing history matters because re-roofing adds weight and can bury the roof’s drainage. Installing a new membrane over an existing one, or adding insulation and ballast, increases dead load across the entire roof simultaneously.
Re-roofing can also raise the roof surface relative to drain inlets and scuppers, reducing drainage capacity, and it can obscure the low points where water collects. The re-roofing history of a collapsed roof, including how many membrane layers are present, is directly relevant and is usually determinable from the debris itself.
Where does hidden deterioration in a roof structure usually concentrate?
Deterioration in a roof structure concentrates where water reaches it: below chronic leaks, at parapet and wall interfaces, near drains and beneath condensing equipment. That is where corrosion of steel joists and their bearing seats, decay in timber roof framing, and deterioration of connections develop.
Section loss at a bearing or a connection reduces a roof structure’s capacity far more than the same loss at mid-span. Measuring remaining section at connections and bearings, rather than at convenient locations, is what characterizes roof deterioration properly.
Why is a building’s roof leak history treated as structural evidence?
A roof leak history is structural evidence because a record of persistent leaks at a location is a record of water reaching the roof structure there over a period, and the mapping between leak locations and deterioration is usually close.
Maintenance logs, tenant complaints, repair invoices and insurance claims for water damage all establish where and for how long water was present on a roof structure. This leak documentation is frequently more precise about timing than any physical measurement of corrosion can be.
Can a change in a building’s use put loads on its roof structure that the design never assumed?
It can: a building repurposed from one use to another may carry loads its structure was never designed for, such as storage stacked against a roof structure, suspended loads hung from roof framing, or a mezzanine added below and supported from above.
Where a building’s occupancy changed without structural review, the design basis and the actual use have diverged, and that divergence is often the whole explanation for a roof failure.
How is it established when a roof structure stopped being adequate?
When a roof structure stopped being adequate is bracketed by sequencing alterations from permits and imagery and by estimating deterioration rates from measured section loss and known exposure. The result is an estimate with real uncertainty, and it should be presented as a range rather than a point.
After a roof collapse, the analytical question is usually not whether the roof structure was adequate at failure but when it stopped being adequate, because that determines who had the opportunity to find the problem.
What evidence should be preserved after a roof collapse involving alterations or deterioration?
After a roof collapse, the roof framing members should be preserved with all modifications, cuts and deterioration intact and located, and the connections and bearing points should be preserved specifically, since those are where a roof structure’s capacity is usually lost.
Roof assembly samples showing how many membrane layers were present should also be kept. So should the permit and alteration record, the maintenance and leak history, and dated aerial or satellite imagery, which frequently establishes when rooftop equipment appeared better than any document in the building’s file.
This guidance on roof collapses after alterations and deferred maintenance is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular roof collapse requires hands-on examination by a credentialed expert.