Sagging Rafters, Leaning Walls, Collapsing Roofs

Causes, effects, & cures for sagging roofs
Bent & Folded Buildings

Bending Buildings Leading to Collapse

Let's look a bit more at the collapsing roof and walls of the Amenia New York Barn as shown below. This is a gable-end view of the same bent sagging building shown at the top of this page.

The downwards load on the roof structure is combined of dead loads: the weight of framing, roof covering, and other things that might be mounted onto a roof surface, plus the live loads of snow, wind, water, and an occasional worker or animal clambering around on the structure.

These forces press downwards or in the case of strong winds, the force may combine both horizontal and downwards forces on the roof surface. How these forces are carried down to the foundation and ultimately to earth determines what happens to the roof and to the rest of the building structure.

Even without doing the engineering or the math, we can see in photos below what happens to a building wall and roof when its support is incomplete, lost, damaged or missing.

Let's press our imaginary thumb down on the roof of an old barn whose rafter ties have been cut out and carried off by someone who's decorating their Los Angeles apartment.

Increased Horizontal Thrust on Low-Slope Roofs

This horizontal load - or thrust - can be considerable, especially on a low-pitched roof.

To resist thrust, the IRC calls for a structural ridge (required for any roof with a roof pitch less than 3/12) or for each pair of rafters to be securely connected to each other [at the lower end of the rafter] by a continuous joist. (R802.3, 2006 IRC).

Code does allow joists [serving as rafter ties] to be installed above the top plate, but only under certain conditions.

Previous building codes permitted rafter ties to be placed as high above the plate as two-thirds the distance between the top plate and the ridge, but the 2006 IRC now limits this height to one-third the distance between the plate and the ridge (see footnote A, Table R802.5.1, 2006 IRC). (Truesdell 2008)

As the ridge sags down and the centers of the front and rear walls push outwards at the wall top, the photo above provides a graphic illustration of direction of the forces at work.

1. The gable ends of the structure want to hold the two ends of the ridge up.
   
   That's why most of the ridge sagging occurs in the center of the roof (page top photo).
2. As the wall tops are pushed outwards at their center, either the entire wall leans over or, as the wall is held up at the building corners (gable ends), the wall center may break and bend over as in my photo above.
3. Ultimately as the roof collapses downwards starting at the center of the ridge line, the tops of the gable end walls are pushed inwards (photo just above).

   More examples of the direction of forces that push down on roofs and out on walls for low slope roofs are found in this article series

   at COMPRESSION BRACING for RAFTERS (Canada).

   Just below we trace a building's failure history through sagging roof to ultimate and total collapse.

Without the proper support of rafter ties or a structural ridge, a typical gable or sloped roof will sag downwards while pushing the building walls outwards towards a catastrophe.

Stages in the Sagging Collapse of a Roof Without Rafter Ties

What happens if rafter ties are omitted and the builder simply frames rafters butting against a ridge board with or without collar ties but with no rafter ties nor ceiling joists?

Unless the building included a structural ridge beam the downwards loads on the roof rafters will cause the building walls to bulge outwards, most noticeably at the center of the walls, and the roof rafters will sag.

Our photo above shows a collapsing building at a farm viewed from the Southbound Taconic State Parkway in New York State, just south of I84. Today (2024) this building has collapsed and its debris removed completely, leaving only a cow pasture.

This structure is part of a study that I [DF] watched for years as the structure slowly settled to earth. More photos of the successive collapse of this house are given below.

Our sketch above illustrates that as the ridge sags the rafters, either also sagging or even if relatively-straight, will push out the top of the wall.

This modest farm house served as a home to farm workers probably from the 1930's until around 1960. I watched its condition from 1969 until its disappearance a few years ago.

As the roof ridge sagged downwards (red line and arrow), the centers of the front and rear walls of this little house bulged outwards, telegraphing that movement into the outwards lean of the front porch as the front walls of the home also pushed the porch roof outwards (blue line and arrow). Above in snow is the sagging roof and bulging wall story of this home in 2003.

By 2007 the front porch had collapsed, the roof had sagged further and the front wall leaned outwards precariously. In fact it's remarkable how plastic building materials can be when bent over a long timer.

By 2011 the roof and most of the structure had collapsed (below).

And the next year, in 2012 this little house finally bit the dust, or I could say snow-dust (below).

Today all traces of the building have been removed and the cows seem to have wandered off as well.

Folding Building Structure Failures

While most of the photos shown above on this page show how wood frame buildings and some other structures can deform by bending slowly, usually into an arced or curved shape, in some cases the building roof folds in upon itself, as we see in this collapsing house photo from Craryville, NY (August 2024).

We call building collapses like the one above "folded buildings" or "folding collapses" because the roof has folded inwards at and collapsed downwards from the roof's ridge.

Often a folding collapse of a structure happens over a much shorter time interval, sometimes as a single event.

Looking over a portion of this building complex that was still standing we see several construction details that might contribute to a building's collapse, including

• Discontinuouse wall-sheathing not connecting wall studs: (pink arrows)
  
  The wall sheathing was made of scraps of whatever was available, providing little continuous connections between joists
  
  We saw the disastrous results of discontinuous structural sheathing across wall sections when inspecting building failures in Los Angeles following the 1994 Northridge earthquake that left many people dead in collapsed buildings. Some of those details can be seen
  
  at EARTHQUAKE DAMAGE JOURNAL 
• Stacked cripple walls, no structural connectors: (yellow arrows)
  
  The walls were framed using a cripple wall structure that adds an additional bending point in the bottom third of the wall - exacerbated when there are also no structural connectors and discontinuous structural wall sheathing.
• Concrete blocks set loosely between the bottom of the framed wall and the foundation top: (white arrow)
  
  apparently omitting structural connectors as well.

Really? Above we describe visual clues suggesting questionable building practices that might contribute to a collapse by looking at a portion of the building that has not collapsed - it's still upright.

It's more-difficult to find these clues in the rubble of the building remains at this site at which the building folded inwards upon itself.

Building structures may fold inwards (as at the roof above) or outwards - un-folding (as shown below).

Below: This small barn has collapsed, folding-out, or if you prefer, "un-folding" its roof structure that then fell atop the remains of the building walls and foundation.

I think that this building's walls failed before the roof structure, perhaps due to a combination of snow loading and rotted or damaged wall or foundation, then the roof "un-folded" and spread out as it fell to the bround.

At ROOF SLOPE DEFINITIONS we comment that this photograph illustrates a roof whose slope has become irrelevant after the building collapsed.

I suspect the very low-slope roof in the photo above was a bit steeper before the building fell in, and that the connections of its rafter ties to the rafter ends were inadequate or failed from rot or insect damage to the structure.

Rapid Total Collapse of a Snow Loaded Roof

The snow loaded roof collapse that occurred in Duluth in 2023 presents a third example, different from slow bending or even fast folding: here was total rapid catastrophic collapse.

In March 2023, a section of roof collapsed on the Miller Hill Mall, Duluth MN, as was widely reported in the state.

The photo below, adapted from a new report by Corin Hoggard from Fox 9 News on 14 March 2023 illustrates a snow load collapse.

OPINION: Considering that this was a very specific and neatly-rectangular section of roof that collapsed out of a much larger flat roofed shopping mall, one might expert forensic structural collapse experts to find contributors to this sudden collapse such as errors or omissions in connections or support in this area, or some other factor that caused higher weight of snow loading in this roof section than its neighbors.

Watch out: when there is deep snow accumulation on a building roof, particularly if followed by rain, the combination of the added weight of water held on the roof as now wet snow may significantly increase the risk of a collapse.

Research: Roof Failures due to Snow Loading

• Hoggard, Corin, Duluth Miller Hill Mall roof collapse spotlights potential statewide dangers, Fox 9 News, 14 March 2023, retrieved 2023/03/19, original source: https://www.fox9.com/news/duluth-miller-hill-mall-roof-collapse-spotlights-potential-statewide-dangers
  
  Excerpt: The city got more than a foot of snow over the weekend, adding to their near record total this winter and the mall’s wasn’t the first roof to cave in. ... [Duluth fire chief Shawn Krizaj] says they’ve had a few roof collapses in the area this winter while accumulating almost ten feet of snow.

Also see these studies of roof failures due to snow loading

• Abé, Masato, Kazuki Nakamura, and Makoto Shimamura. "Structural failures due to snow in Japan." Proceedings of the Institution of Civil Engineers-Forensic Engineering 171, no. 4 (2019): 163-170.
• Aksoylu, Ceyhun, Yasin Onuralp Özkılıç, and Musa Hakan Arslan. "Damages on prefabricated concrete dapped-end purlins due to snow loads and a novel reinforcement detail." Engineering Structures 225 (2020): 111225
• Alemdar, Zeynep Fırat, and Fatih Alemdar. "Progressive collapse of a steel structure under expected snow loads." Engineering failure analysis 125 (2021): 105378.
• Bean, Brennan L.; Maguire, Marc; Sun, Yan; Wagstaff, Jadon; Al-Rubaye, Salam; Wheeler, Jesse; Jarman, Scout; and Rogers, Miranda, THE 2020 NATIONAL SNOW LOAD STUDY [PDF] (2021). Mathematics and Statistics Faculty Publications. Paper 276.
  https://digitalcommons.usu.edu/mathsci_facpub/276
  
  Abstract

  The United States has a rich history of snow load studies at the state and national level. The current ASCE 7 snow loads are based on studies performed at the Cold Regions Research and Engineering Laboratory (CRREL) ca. 1980 and updated ca. 1993.
  
  The map includes large regions where a site-specific case study is required to establish the load. Many state reports attempt to address the "case-study regions" designated in the current ASCE 7 design snow load requirements.
  
  The independently developed state-specific requirements vary in approach, which can lead to discrepancies in requirements at state boundaries. In addition, there has been great interest to develop site-specific reliability-targeted loads that replace the current load and importance factors applied to 50-year snow load events as defined in ASCE 7-16.
  
  This interest stems from the fact that the relative variability in extreme snow load events is not constant across the country, leading to a non-constant probability of failure for a given design scenario.
  
  This report describes the creation of a modern, universal, and reproducible approach for estimating reliability-targeted design ground snow loads for the conterminous United States.
  
  This new approach significantly reduces the size of case-study regions as currently designated in ASCE 7-16 and resolves discrepancies in design snow load requirements that currently exist along western state boundaries.
  
  Excerpt from Project Aims:
  
  The final product of this project is a modern, universal, and reproducible approach for generating design ground snow loads for the conterminous United States.

• Buska, James, and Wayne Tobiasson. MINIMIZING THE ADVERSE EFFECTS OF SNOW AND ICE ON ROOFS [PDF] In International Conference on Building Envelope Systems and Technologies. 2001. Retrieved 2019/11/01 original source: https://www.erdc.usace.army.mil/
  
  Abstract excerpt:
  
  Snow load design criteria in the United States are established in ASCE Standard 7, “Minimum Design Loads for Buildings and Other Structures.” The information in that standard documents how dramatically the geometry of a building can influence snow loads on its roof.
• Çeribaşı, Seyit. "Reliability of steel truss roof Systems under variable snow load profiles." International Journal of Steel Structures 20, no. 2 (2020): 567-582.
• Croce, Pietro, Paolo Formichi, and Filippo Landi. "Extreme ground snow loads in Europe from 1951 to 2100." Climate 9, no. 9 (2021): 133.
• Yates, Janet K., and Edward E. Lockley. DOCUMENTING AND ANALYZING CONSTRUCTION FAILURES [PDF] Journal of construction Engineering and management 128, no. 1 (2002): 8-17.
  Abstract:
  
  A research project was conducted to explore construction failure investigation techniques and processes to determine whether they were adequate and to develop failure investigation guidelines.
  
  Data was collected on failures and failure investigation techniques from surveying 115 members of the engineering and construction industry.
  
  Construction failure case studies were created using documentation provided by the federal Occupational Safety and Health Administration, state offices of safety and health, and forensic engineers.
  
  The construction failure case studies were analyzed to determine how these organizations conduct their investigations and to develop guidelines that can be used for construction failure investigations. This article provides:
  
  -1 description of the methods used for the research;
  
  -2 results obtained from the industry survey;
  
  -3 summary of the results of an investigation into case studies on construction failures;
  
  -4 analysis of the results;
  
  -5 discussion on construction failure investigative techniques;
  
  -6 guidelines developed during the research project for investigating and documenting failures; and ~
  
  - 7 recommended format for reporting the findings of failure investigations.
  

Risk of Roof Collapse Due to Supporting Column Failure

As our editor Amy's photo illustrates below, a leaning column whose job was to support a porch roof and possibly porch floor structure as well may risk a serious structural collapse at this home in Great Falls, Montana (August 2024 - A.C.).

The porch column has leaned badly to the right.

We don't have a view of the porch and porch roof structure, but we do have a clue that may help explain why this porch column base is leaning so terribly to the right.

Notice the long downspout extension running off to the right in our photo?

Spilling roof runoff can undermine a column pier or footing, leading to the leaning disaster coming from the conditions in this photo.

More about the worry of leaning structural columns is

at SETTLING or MISSING COLUMN FOOTINGS or Piers